Sustainable Drainage Strategy in Lewisham

A Sustainable Drainage Strategy is a technical planning document that explains and proves how a development will manage the rainwater that falls on it. It goes by several names — SuDS report, drainage strategy, surface water drainage strategy, drainage statement — but the substance is the same: it describes the existing drainage of the site, sets out the sustainable drainage systems the scheme will use, and demonstrates with calculations that after development the site will discharge surface water no faster, and ideally more slowly, than it did before. It is the evidence that your scheme has taken flooding — its own and everyone else's — seriously.

The core idea behind sustainable drainage is simple. When you cover ground with buildings, patios, driveways and roads, rain that used to soak into the soil instead runs straight off hard surfaces into gullies, pipes and sewers, arriving in the river or the drain in a sudden rush rather than a gentle trickle. Multiply that across thousands of developments in a river catchment and you get the modern problem of surface water flooding: sewers surcharge, rivers spike, and low-lying properties flood. SuDS reverse this by managing water where it lands — capturing it on green roofs, letting it infiltrate through permeable paving, holding it in rain gardens and basins, storing it in tanks and releasing it slowly. The strategy is the document that specifies which techniques you will use and proves they work.

Crucially, a drainage strategy is not the same as a Flood Risk Assessment, although the two are closely related and are often produced together. A Flood Risk Assessment looks at the risk of flooding to and from your development from all sources — rivers, tidal water, surface water, groundwater and sewers — and shows how the development is safe and does not increase risk elsewhere. A drainage strategy focuses specifically on how the development manages its own surface water run-off sustainably. On many Lewisham sites — especially near the Ravensbourne, Quaggy or Pool — you will need both, and they should tell one consistent story.

The strategy is a live design tool, not a formality written at the end. The best drainage strategies are shaped from the first sketch, because the SuDS features — the green roof, the permeable drive, the rain garden, the attenuation tank under the garden — take up space, add cost and change the levels of a scheme. Design them in early and they cost little and read beautifully; leave them until after the layout is fixed and you end up shoehorning an expensive underground tank into whatever space is left, or worse, discovering the scheme cannot drain and having to start again. That is why Crown prepares the drainage strategy alongside the architecture rather than sub-contracting it as an afterthought.

The clearest trigger is scale. For major development — defined as ten or more dwellings, or a site of 0.5 hectares or more, or 1,000 square metres or more of floorspace — a sustainable drainage strategy is effectively always required, and the lead local flood authority (a function of Lewisham Council itself) is a statutory consultee on the application. The National Planning Policy Framework requires major development to incorporate SuDS unless there is clear evidence it would be inappropriate, and the London Plan reinforces this. If you are building a block of flats, a small housing scheme, or converting a large site, plan for a full drainage strategy from the outset.

But the threshold is not the whole story in Lewisham, and this is where local knowledge matters. Because so much of the borough sits in the river valleys of the Ravensbourne and Quaggy, and because parts of Deptford, Lewisham, Catford and Ladywell fall within Environment Agency flood zones 2 and 3, the council routinely expects drainage information on schemes well below the major threshold. A site-specific Flood Risk Assessment — with which the drainage strategy is usually paired — is required for any development in flood zone 2 or 3, for sites of one hectare or more even in flood zone 1, and where the council identifies a surface water or groundwater flood risk. Lewisham's own validation checklists and Strategic Flood Risk Assessment set out these expectations.

Even for a householder — an extension, a loft with a rear addition, a new garden room, a replacement dwelling or a large area of new paving — surface water drainage is a live issue. Permitted development rights for hard surfaces in a front garden are conditional on the surface being permeable or on the run-off draining to a permeable area, precisely to avoid adding to the sewer load. On planning applications for extensions the council will expect run-off to be managed on site, and on a full application for a new house or a replacement dwelling near a river, expect to demonstrate SuDS and, frequently, to produce a formal drainage strategy. A common surprise is a homeowner assuming drainage is 'a building regs matter for later' and then finding it has become a planning condition or, worse, a reason for refusal.

The safest approach is to establish the drainage requirement at the very start, alongside the flood-risk question, because the two together determine both what documents you need and how the scheme has to be designed. At feasibility we check the Environment Agency flood maps, the borough's surface water flood mapping and Strategic Flood Risk Assessment, the ground conditions, and the proximity to the Ravensbourne, Quaggy, Pool or Deptford Creek, and we tell you plainly whether your scheme needs a full drainage strategy, a lighter drainage statement, or simply a sensible SuDS approach designed into the plans — before any expensive drawing work begins.

You cannot understand drainage policy in Lewisham without understanding its rivers. The River Ravensbourne rises from springs near Keston in Bromley and flows roughly north for around eleven miles, entering Lewisham near Catford and running through the heart of the borough — past Ladywell, through Lewisham town centre where it is joined by the Quaggy, and on to Deptford, where its tidal, Thames-connected reach becomes Deptford Creek. Its principal tributaries within the borough are the River Quaggy, which flows in from the east through Mottingham and Sundridge, the River Pool, which joins near Catford, and the Honor Oak Stream, also known as Chudleigh Brook. Together these watercourses drain a large, densely built catchment, and every hard surface added within it sheds more water into them.

The borough's flood history is long and serious. Flooding from the Ravensbourne in the Lewisham and Catford area has been documented since 1809. There were very serious floods in 1814 as the last of the Little Ice Age's ice melted, damaging events in 1853, flooding after heavy rain on Christmas Eve 1876, and floods in 1878 described at the time as the worst in living memory. The defining modern event, though, was September 1968: on the 15th and 16th of that month, south-east London received around 150 millimetres of rain — roughly three months' worth — in forty-eight hours, and the Ravensbourne, Quaggy and Pool all overtopped their banks, flooding several hundred homes and businesses. Less severe river flooding followed in 1977, 1992 and 1993. The Quaggy's tendency to flood was serious enough to be debated in Parliament as early as 1965.

The twentieth-century response was hard engineering. Through the 1920s and 30s and again in the 1960s and 70s, the borough's rivers were straightened, deepened and confined in concrete channels and underground culverts to hurry floodwater through the urban area — the classic 'grey' approach. In recent decades the thinking has reversed: Lewisham has become a national exemplar of river restoration and 'daylighting', bringing culverted rivers back to the surface and giving them room to flood safely. The restoration of the Quaggy at Sutcliffe Park and Chinbrook Meadows, championed by the Quaggy Waterways Action Group, turned engineered channels back into meandering rivers with floodplain storage, and the Ravensbourne has been opened up and naturalised through Ladywell Fields and Cornmill Gardens. The Environment Agency's Lewisham and Catford flood alleviation work sits within this same shift towards working with water rather than against it.

For anyone developing in Lewisham, this history is the reason the drainage bar is set high — and it is also the design context. The council and the Environment Agency will look hard at any scheme that could add run-off to an already-stressed catchment, and they positively welcome schemes that reduce run-off, create green infrastructure, or enhance a river corridor. A drainage strategy that treats SuDS as a grudging tick-box misses the opportunity; one that embraces green roofs, rain gardens, permeable surfaces and — on riverside sites — genuine river-corridor enhancement reads as exactly the kind of development Lewisham wants. Understanding which watercourse your site drains to, whether it lies in a flood zone or a surface-water hotspot, and how it connects to the borough's blue-green network is the foundation of a strategy that succeeds.

The heart of a London drainage strategy is the drainage hierarchy set out in London Plan Policy SI 13. It is not enough to show that your scheme drains; you have to show that you have followed the hierarchy — working down from the most sustainable option to the least, and only moving to a lower tier where you can justify that the tier above is not reasonably practicable. The council and its flood authority will read your strategy tier by tier, so understanding the hierarchy is understanding how to write a strategy that passes.

The first and most preferred tier is rainwater use — capturing rainwater and using it as a resource. This means rainwater harvesting for garden irrigation or, at larger scale, for toilet flushing and other non-potable uses, and 'blue roofs' that store water for later use or controlled release. The second tier is infiltration to ground at or close to source — letting rain soak into the earth through permeable paving, soakaways, infiltration basins and rain gardens, mimicking the way undeveloped ground absorbs rainfall. The third tier is attenuation in green features for gradual release — holding water back in green roofs, rain gardens, swales, ponds and basins and letting it out slowly.

Where those upper, green tiers cannot fully manage the run-off, the hierarchy moves to grey and piped options, in strict order of preference. The fourth tier is discharge direct to a watercourse, where one is available and it is appropriate — releasing at a controlled, restricted rate. The fifth tier is controlled discharge to a surface water sewer or drain. The sixth and least preferred tier of all is controlled discharge to a combined sewer — the option most likely to contribute to sewer flooding and pollution, and the one the whole policy is designed to move development away from. Every step down the hierarchy has to be justified, not assumed.

In practice, most real schemes use a combination of tiers — a green roof and permeable paving to soak up and slow the first flush, a rain garden or planter to attenuate, and a restricted connection to the sewer or watercourse for the overflow in the biggest storms. The skill in writing a strategy is demonstrating, honestly and with evidence, that you have genuinely tried the higher tiers before relying on the lower ones. On a Lewisham site where infiltration works well, a strategy that leaps straight to an underground tank and a sewer connection will be challenged; on a site with impermeable London clay and a high water table, infiltration may not be feasible and the strategy has to explain that with permeability test results. Getting the hierarchy narrative right is where strategies are won or lost.

The quantitative heart of a drainage strategy is the run-off rate: how fast, in litres per second, water leaves the site. The policy benchmark is the greenfield run-off rate — the rate at which rainwater would run off the site if it were undeveloped countryside. London Plan Policy SI 13 asks development to aim for greenfield run-off rates, and Lewisham's Local Plan and flood-risk documents expect exactly that: post-development discharge restricted to greenfield rates on greenfield sites, and demonstrable betterment on previously developed land. The whole strategy exists to prove, with calculation, that this target is met.

The greenfield rate is estimated using established hydrological methods — commonly the IH124 method or the FEH/FSR approach — which derive the site's mean annual flood flow, known as QBAR, from the site area, soil type, rainfall and geographical factors, and then factor it for different storm return periods. For most lowland England sites the greenfield QBAR works out at roughly 2 to 5 litres per second per hectare, which is strikingly low compared with the torrent that pours off an unmanaged modern development. That gap — between how fast a hard site sheds water and the trickle a greenfield site releases — is the volume your SuDS have to store and hold back. Free tools such as the UK SuDS greenfield run-off estimator are used to derive a defensible figure, which the flood authority will check.

The design must then be shown to work not just for an average shower but for the extreme storm, with a margin for a changing climate. DEFRA's National Standards for Sustainable Drainage Systems — substantially updated in June 2025 — require that the peak run-off rate for the 1-in-1-year, 1-in-30-year and 1-in-100-year events never exceeds the greenfield rate for the same event. Lewisham, following national and London guidance, expects the drainage to cater for up to the 1-in-100-year rainfall event with a climate-change allowance added — typically a 40% uplift on rainfall intensity for residential schemes with a long design life. The attenuation storage — the tank, the basin, the blue roof — is sized to hold the difference between what falls and what may be released, so that even in that extreme, climate-adjusted storm, the site discharges at the restricted rate and nothing floods.

Previously developed sites — which is most of Lewisham — are treated a little differently, because a brownfield plot may already shed water fast from existing buildings and hard surfaces. The 2025 national standards expect brownfield redevelopment to discharge at no more than five times the greenfield rate, and only where evidence shows greenfield rates genuinely cannot be achieved, agreed with the approving body; in practice the flood authority pushes hard for greenfield rates or the closest achievable, and will expect meaningful betterment — often a 50% reduction on the existing 1-in-1-year run-off as a minimum starting point. The strategy has to state the existing run-off, the proposed restricted rate, the storage volume that achieves it, and the calculations behind every figure. Vague assurances do not pass; numbers do.

A drainage strategy is only as good as the SuDS it specifies, and the right combination depends entirely on the site: its size, its ground conditions, its levels, and how much green space it has. On the constrained urban plots typical of Lewisham — infill sites, backland gardens, terraced-house extensions, small blocks of flats — the toolkit has to be chosen carefully to fit real space. The point of the strategy is to blend techniques so the scheme captures, slows and stores water through the hierarchy rather than relying on a single feature.

Green roofs are one of the most effective techniques for dense sites, because they use space that would otherwise do nothing: a planted roof soaks up rainfall, slows what runs off, and delivers biodiversity and insulation benefits on top. Blue roofs — a controlled storage layer on a flat roof — attenuate water and can feed rainwater reuse. At ground level, permeable paving for drives, patios and parking lets water infiltrate rather than run off, and is often the single most useful move on a house or small development; rain gardens and planted swales collect run-off from roofs and hard surfaces into shallow, planted depressions that soak away and hold back water attractively. Water butts and larger rainwater-harvesting systems capture roof water for garden use, sitting at the top of the hierarchy.

Where above-ground green features cannot store enough — often the case on tight plots draining a large roof — underground attenuation provides the extra volume: geocellular crates or oversized pipes buried beneath a garden or driveway, holding storm water and releasing it slowly through a flow-control device such as a Hydro-Brake or an orifice plate set to the restricted discharge rate. On larger schemes, detention basins and ponds provide storage and amenity together. The flow control is the quiet hero of most strategies: it is the small device that guarantees, whatever the storm, that water leaves the site only at the greenfield rate.

Ground conditions decide what is possible, and in Lewisham this needs real testing rather than assumption. Much of London sits on London Clay, which infiltrates poorly, and parts of the river valleys have a high water table — both of which can rule out soakaways and infiltration and push a scheme towards attenuation-and-restricted-discharge instead. That is why a proper strategy is underpinned by site-specific infiltration (permeability) testing and a knowledge of groundwater levels; the flood authority will not accept infiltration SuDS on faith. Where infiltration works, it should be used; where it does not, the strategy must show that and design accordingly. Choosing the right techniques for the actual ground is the difference between a strategy that is approved and built and one that fails on site.

A drainage strategy that passes validation and satisfies the flood authority follows a recognisable structure, and each part carries weight. It opens with a description of the site and the proposal — location, area, existing and proposed use, and a plan showing existing and proposed impermeable and permeable surfaces, because the change in hard surfacing is what drives the whole calculation. It sets out the existing drainage: how the site currently drains, to which sewer or watercourse, and at what estimated rate, establishing the baseline the scheme has to improve upon or at least not worsen.

The strategy then works through the drainage hierarchy explicitly, showing for each tier — rainwater use, infiltration, green attenuation, watercourse, surface water sewer, combined sewer — whether it has been adopted and, where a tier has been discounted, why. This hierarchy narrative is not optional padding; it is the specific thing Policy SI 13 asks for, and a strategy that jumps to a solution without walking down the hierarchy invites a refusal or a request for more information. Where infiltration is discounted, the permeability test results and groundwater data are cited to justify it.

At the technical core sit the calculations and the proposed system. The strategy states the greenfield run-off rate and the method used to derive it, the existing and proposed run-off rates, the proposed restricted discharge rate, and the attenuation storage volume required to meet that rate up to the 1-in-100-year event including the climate-change allowance — with the modelling (often MicroDrainage or equivalent) and the assumptions shown. It is accompanied by an annotated drainage layout drawing showing the location and dimensions of every SuDS element — green roof, permeable paving, rain garden, tank, flow control — and how they connect to the wider network, plus long sections or levels where they matter.

Finally, a complete strategy addresses the future, not just the design day. It sets out water-quality treatment where required (SuDS also clean run-off before it reaches a watercourse), exceedance and overland-flow routes for storms larger than the design event so that water floods safely rather than into buildings, and a maintenance plan naming who will maintain each SuDS feature, how often, and for how long — because a SuDS system that is not maintained silts up and stops working. On major schemes this maintenance and management regime is secured by condition or Section 106. In London, all of this is drawn together and summarised in the London Sustainable Drainage Proforma, which Lewisham uses, and which we complete as part of the package.

London has a standardised way of presenting a drainage strategy: the London Sustainable Drainage Proforma, produced by the Greater London Authority to give boroughs a consistent summary of a scheme's surface water drainage. It is a structured spreadsheet that records the site details, the existing and proposed run-off rates, the SuDS features proposed, the drainage-hierarchy position, and the storage volumes, so a case officer or flood-risk officer can see at a glance whether the scheme complies with Policy SI 13. Lewisham publishes its own version of the proforma, and completing it accurately is part of preparing a strategy for the borough. It does not replace the full strategy and calculations — it summarises them — but a scheme that omits it looks incomplete.

Lewisham's validation checklists set out when drainage and flood-risk information is required and what form it must take. Broadly, the borough requires a site-specific Flood Risk Assessment for development in flood zones 2 and 3, for sites of one hectare or more, and where a surface water or other flood risk is identified; that FRA has to identify the sources of flooding, assess the risk to and from the development, and show how it will be managed and mitigated. For the drainage element the borough expects the proposed drainage to have regard to the SuDS hierarchy in the London Plan, plans and specifications of the SuDS proposed, suitable infiltration testing where infiltration is relied upon, and calculated post-development run-off rates restricted to greenfield rates for greenfield sites.

The lead local flood authority is central to the process. Since 2015, upper-tier and unitary councils act as lead local flood authorities, and the LLFA is a statutory consultee on all major planning applications for their surface water drainage. In Lewisham, that function sits within the council, and its flood-risk officers will review the drainage strategy on major schemes and advise the planning department whether the surface water proposals are acceptable. A strategy that satisfies the case officer on planning grounds but fails to convince the flood authority on drainage grounds will not succeed — so the document has to speak to both audiences, marrying good design with sound hydrology.

Getting validation right saves weeks. If the required drainage strategy, FRA or proforma is missing or plainly inadequate, the council can refuse to validate the application, and the clock does not even start until the paperwork is complete. If it validates but the strategy is weak, the flood authority raises objections or asks for further information, stalling the determination and eating into the eight-, thirteen- or sixteen-week target. The efficient route is to prepare a complete, calculation-backed strategy with the proforma from the outset, pitched to Lewisham's specific expectations, so the application sails through validation and gives the flood authority no reason to object.

Sustainable drainage sits on a layered framework of national law, national planning policy, London policy and Lewisham policy — and a good strategy shows awareness of all four. At the national level, the Flood and Water Management Act 2010 created the concept of lead local flood authorities and contains, in its still-unenacted Schedule 3, a mandatory SuDS approval regime; because Schedule 3 has never been switched on in England, SuDS are currently secured through the planning system and planning conditions rather than a separate statutory approval, but the direction of travel is clear and the government has signalled its intention to strengthen the regime.

The National Planning Policy Framework is the pivotal national planning document. It requires that major development incorporates sustainable drainage systems unless there is clear evidence this would be inappropriate, that the systems are designed to take account of maintenance and climate change, and that they provide multifunctional benefits wherever possible. It also embeds the sequential and exception tests for flood risk, steering development away from the highest-risk land, and makes flood risk a material consideration on every relevant application. DEFRA's National Standards for Sustainable Drainage Systems — significantly updated on 19 June 2025 for the first time in a decade — set the technical design bar, including the peak-flow controls to greenfield rates and the climate-change allowances described above.

In London, the London Plan 2021 is the strategic development plan, and Policy SI 13 (Sustainable drainage) is the controlling policy for surface water. It requires development to aim to achieve greenfield run-off rates, to manage surface water run-off as close to its source as possible, and to follow the drainage hierarchy that prefers green over grey. It works alongside Policy SI 12 on flood risk management and the Mayor's London Sustainable Drainage Action Plan, which drives the retrofit and daylighting of drainage across the capital — the same philosophy Lewisham has pursued on its own rivers.

Locally, the Lewisham Local Plan 2020-2040, adopted on 16 July 2025, carries the borough's own climate-change and flood-risk policies, requiring proposals to accord with the sustainable drainage hierarchy and to provide sufficient capacity for up to the 1-in-100-year rainfall event including the latest climate-change guidance, with infiltration SuDS supported by site-specific permeability testing. The Plan is underpinned by the borough's Strategic Flood Risk Assessment, and the council's Local Flood Risk Management Strategy (most recently the 2022-2027 strategy) sets out how Lewisham manages surface water, groundwater and ordinary watercourse flooding as lead local flood authority. Because policy numbers and thresholds can change with plan reviews and new national standards, we confirm the exact current references for your scheme before we rely on them — but the underlying requirements have been stable and demanding for years.

We start with the site, not the template. The first step is to establish the drainage context: we check the Environment Agency flood maps, Lewisham's Strategic Flood Risk Assessment and surface-water flood mapping, the underlying geology and any known groundwater levels, and which watercourse or sewer the site drains to — Ravensbourne, Quaggy, Pool or the public network. We measure the existing site: the roof and hard-surface areas, the levels, the trees and constraints. From that we can tell you immediately whether a full drainage strategy, a lighter drainage statement, or a designed-in SuDS approach is needed, and what the likely shape of the solution is.

Because Crown designs the building and the drainage together, the SuDS are woven into the architecture from the first sketch. We locate the permeable surfaces, the rain gardens, the green roof and the attenuation where they work with the plan and the levels, rather than forcing them into leftover space at the end. Where infiltration might be viable, we arrange site-specific permeability testing early, so the strategy can either rely on infiltration with evidence or rule it out honestly. This integrated approach is the single biggest reason our strategies are both approvable and buildable — the drainage and the design agree with each other.

We then do the calculations properly. We derive the greenfield run-off rate for the site, model the existing and proposed run-off, size the attenuation storage for the 1-in-100-year event with the climate-change allowance, and set the flow control to the restricted discharge rate. We work through the drainage hierarchy tier by tier in the written strategy, justifying every step, and we produce the annotated drainage layout, the exceedance routes and the maintenance plan. Everything is set out clearly enough for the case officer and rigorously enough for the flood-risk officer, and the London SuDS proforma is completed to summarise it.

Finally, we manage the strategy through the process. We submit it as part of a coordinated application, respond to any questions from the flood authority or the case officer, and adjust the design where a reasonable point is raised. Where the borough attaches a drainage condition — often requiring detailed design and a maintenance regime to be approved before construction — we discharge it. And because we hold the architecture and the drainage in the same practice, any change to one is reflected in the other without the gaps and contradictions that appear when drawings and drainage come from firms who never speak to each other. The result is a strategy that gets your Lewisham scheme permitted and actually works when it is built.

A drainage strategy never travels alone. It is one document in a coordinated application, and its most important relationship is with the Flood Risk Assessment. On any Lewisham site in a flood zone, or of one hectare or more, or with an identified flood risk, the FRA and the drainage strategy are usually produced together and cross-reference each other: the FRA establishes that the development is safe from flooding and does not increase risk elsewhere, and the drainage strategy proves the surface-water element of that — that run-off is managed and restricted. If the two documents disagree, both are undermined, so we author them to tell one consistent story.

The strategy also connects to the sustainability and energy story. SuDS deliver multiple benefits — green roofs and rain gardens contribute to biodiversity, urban cooling and amenity — so the drainage strategy dovetails with the energy and sustainability statement, the urban greening factor that the London Plan expects, and, on residential schemes, the biodiversity considerations. A green roof specified for drainage also earns credit for greening and biodiversity; designed in isolation, those opportunities are missed. On larger schemes the drainage strategy interacts with the landscape design, which is where much of the visible SuDS lives.

There are practical connections too. The levels and finished floor levels in the architectural drawings have to match the drainage design and any flood-resilience measures; the SuDS features have to fit around the foundations, basements and services; and where the scheme discharges to or builds near a public sewer, a build-over agreement with Thames Water may be needed alongside the strategy. The drainage layout has to be consistent with the site plan, the landscape plan and the levels drawings submitted with the application — inconsistencies between documents are a classic trigger for a request for further information.

Because everything connects, the drainage strategy is best commissioned as part of the whole design rather than as a stand-alone report bought from a separate consultant late in the day. When the same practice holds the architecture, the levels, the landscape intent and the drainage, the documents agree, the SuDS pull their weight across drainage, greening and amenity, and the application reads as a single coherent proposal — which is exactly what a case officer and a flood-risk officer want to see. That coherence is not a luxury; it is often the difference between a smooth determination and a stalled one.

The most common failure is leaving drainage until the end. When the layout, levels and unit count are already fixed and only then does someone ask 'how does this drain?', the answer is often an expensive underground tank crammed into whatever space remains, or the unwelcome discovery that the site cannot achieve the required rates without redesign. Drainage designed in late is drainage designed badly — it costs more, works less well, and holds the application up. The fix is to establish the drainage strategy at feasibility, which is precisely how we work.

The second recurring mistake is skipping the hierarchy. Applicants and their engineers sometimes jump straight to the solution they know — a tank and a restricted sewer connection — without demonstrating that they have worked down from rainwater use and infiltration first. Policy SI 13 asks specifically for the hierarchy to be followed, so a strategy that does not walk through it, tier by tier, with reasons, invites a request for further information at best and a refusal at worst. Related to this is relying on infiltration without testing it: claiming soakaways will work on London clay without permeability results is a fast route to an objection from the flood authority.

Weak or missing calculations are the third failure. A strategy that asserts the scheme will 'manage surface water sustainably' without stating the greenfield rate, the existing and proposed run-off, the restricted discharge rate and the storage volume — with the modelling behind them — is not a strategy, it is a statement of intent, and the flood authority will treat it as such. So too is a strategy that designs only for an ordinary storm and forgets the 1-in-100-year event and the climate-change allowance; the numbers have to prove resilience to the extreme, adjusted for a warming climate, or they do not pass.

The fourth cluster of mistakes is about the future and about consistency. Strategies that ignore maintenance — who will keep the SuDS working, how often, and for how long — fail, because unmaintained SuDS silt up and stop functioning, and the council knows it. Strategies that omit exceedance routes leave no answer to the question of where water goes in a storm bigger than the design event. And strategies that contradict the site plan, the levels or the FRA undermine the whole application. Finally, forgetting the London SuDS proforma, or submitting one that does not match the strategy behind it, signals an incomplete application. Each of these is avoidable with a properly authored, coordinated strategy — which is what we deliver.

A sustainable drainage system is not a one-off installation; it is a piece of infrastructure that has to be maintained for the life of the development, and Lewisham — like every London borough — will want to know who will do that before it grants permission. Permeable paving has to be swept and kept from clogging; green roofs need their vegetation managed; rain gardens and swales need planting maintained and silt removed; attenuation tanks and flow controls have to be inspected and cleared. A SuDS scheme that is never maintained silts up, blocks and stops performing — at which point the flooding it was designed to prevent returns. That is why the maintenance plan is a required part of the strategy, not an optional extra.

The maintenance plan names each SuDS feature, states what maintenance it needs and how often, sets out who is responsible, and confirms how it will be funded over the design life of the scheme. On a single home, the owner maintains the SuDS as part of looking after the property. On a block of flats or a multi-home scheme, responsibility usually sits with a management company or the freeholder, funded through the service charge, and the arrangement is often secured by a planning condition or a Section 106 obligation so the council can be confident it will endure. The strategy has to make that arrangement explicit and credible.

Adoption is a separate question that sometimes arises. Because Schedule 3 of the Flood and Water Management Act — which would have created a SuDS approving body and clearer adoption routes — has not been enacted in England, there is no automatic public adoption of SuDS in most cases. Water companies adopt some sewers and drainage under their own agreements, and highway authorities maintain highway drainage, but many SuDS features remain in private ownership and are maintained privately under the management arrangements described above. The strategy should be clear about what, if anything, is proposed for adoption and what stays private, so there are no gaps in responsibility.

For homeowners and small developers, the practical message is reassuring: on domestic-scale schemes the maintenance is modest and manageable — sweeping permeable paving, keeping a rain garden tidy, occasionally checking a soakaway or tank — and we set it out plainly so you know what you are taking on. On larger schemes we build a realistic, funded maintenance and management regime into the strategy from the start, so it satisfies the council and works in practice. Either way, thinking about the long term is part of doing SuDS properly, and it is part of every strategy we write.

A drainage strategy is usually one of several supporting documents, and knowing which others your Lewisham scheme needs avoids nasty surprises at validation. The closest companion is the Flood Risk Assessment, required for sites in flood zones 2 and 3, for sites of a hectare or more, and where a flood risk is identified; on most riverside or low-lying Lewisham sites the FRA and the drainage strategy are produced together as a pair. The two share data and must be consistent, which is a strong reason to have them authored by one team.

On residential development, an Energy and Sustainability Statement is commonly required, setting out how the scheme meets the London Plan's energy hierarchy (be lean, be clean, be green, be seen) and its sustainability policies; the SuDS in your drainage strategy contribute to that story through green roofs and urban greening. Larger residential schemes need to address the London Plan's urban greening factor and biodiversity — and green SuDS features count towards both, which is why a well-designed strategy pays back across several documents. Since February 2024, most development also has to deliver a minimum 10% Biodiversity Net Gain, and green drainage features can support that too.

Depending on the site, you may also need a Construction Method Statement or a Construction Environmental Management Plan setting out how surface water and pollution are managed during the build; a Basement Impact Assessment where a basement affects groundwater and drainage; an arboricultural report where trees interact with SuDS and soakaways; and a Transport Statement or travel plan on larger schemes. Where the scheme discharges to or builds near a public sewer, a build-over or discharge agreement with Thames Water sits alongside the planning documents.

The point of listing these is not to alarm but to plan. Every additional document is an opportunity for inconsistency if the reports are commissioned piecemeal from different consultants, and an opportunity for coherence if they are coordinated. Crown assembles the full validation set for a Lewisham application and makes sure the drainage strategy, the FRA, the energy and sustainability statement, the landscape and greening information and the drawings all agree — so the application is complete, consistent and ready to be validated first time.

The cost of a drainage strategy depends on the scale and complexity of the scheme, and the honest answer is that it ranges widely. For a domestic project — an extension, a new house, or a small development where SuDS can be designed in and the calculations are straightforward — a proportionate drainage strategy or drainage statement is a modest, well-defined piece of work. For a major scheme with a full FRA, extensive modelling, infiltration testing, attenuation design and a management regime, it is a larger commission reflecting the engineering involved. We give you a fixed, itemised fee at the outset so you know exactly what you are paying for, with no surprises.

There are also third-party costs to budget for. Site-specific infiltration (permeability) testing, where infiltration is being considered, involves a ground investigation and is usually a separate cost paid to a specialist. A topographical and drainage survey may be needed to establish levels and existing connections. If a CCTV survey of an existing drain or a connection investigation is required, that too is an additional cost. We identify all of these up front so the whole picture is clear before you commit, rather than letting costs creep as the project goes on.

It is worth seeing the strategy as an investment that reduces risk rather than a cost to be minimised. A weak or missing strategy that stalls an application, triggers a flood-authority objection, or leads to a refusal costs far more — in time, in re-submission fees, and in the value of a delayed project — than a properly done strategy ever would. And designing SuDS in early is cheaper than retrofitting them late; the underground tank forced into a fixed layout at the eleventh hour is one of the most expensive ways to drain a site. Getting the strategy right first time is the economical route.

Because Crown holds the architecture, engineering and drainage under one roof, we can often deliver the drainage strategy more efficiently than a stand-alone consultant, because we already hold the survey, the drawings and the levels and we design the SuDS into the scheme rather than reverse-engineering them. That coordination saves both fees and abortive work. Tell us the site and the scheme and we will give you a clear, fixed price for the drainage strategy — and, if you want it, for the full application package around it.

The journey begins with feasibility, and on any scheme where drainage or flood risk is in play we start there. We check the flood maps, the borough's Strategic Flood Risk Assessment and surface-water mapping, the ground conditions and the receiving watercourse or sewer, and we design the scheme with SuDS built in. For anything of scale, or anything sensitive, pre-application advice from Lewisham is often worthwhile: it lets the council's officers — including, where relevant, the flood-risk team — comment on the drainage approach before the application is made, which reduces the risk of a late objection and can smooth the whole determination.

When the design is settled we prepare the full package: the drainage strategy and calculations, the FRA where required, the annotated drainage layout, the completed London SuDS proforma, and the architectural drawings and other supporting documents, all consistent with each other. We submit through the Planning Portal, pay the fee, and the council checks the application against its validation checklist. A complete, well-prepared submission validates quickly; a deficient one is held back until the missing information is provided, so getting validation right is the first practical hurdle and one we take seriously.

Once validated, the application is assessed. The case officer consults neighbours and statutory and non-statutory consultees, and on major schemes the lead local flood authority reviews the drainage strategy and advises whether the surface-water proposals are acceptable. We respond to any queries from the case officer or the flood authority, provide clarification or adjust the design where a reasonable point is raised, and keep the application moving. Minor applications target eight weeks from validation; major applications target thirteen weeks, or sixteen where an environmental statement is involved, though in practice larger schemes take longer with committee and any Section 106.

A decision often comes with conditions, and drainage conditions are among the most common: the council frequently grants permission subject to the detailed drainage design, the final SuDS specification and the maintenance regime being submitted and approved before development begins. We discharge those conditions as part of the service, providing the detailed design and the maintenance plan the condition requires, so the permission converts into a scheme that can actually be built. From feasibility to discharging the last drainage condition, we run the drainage side of your Lewisham application end to end.

The single biggest reason to use Crown is integration. We are an architecture and engineering practice, and we design the building, the structure, the services and the drainage together, under one roof. On drainage that matters enormously, because SuDS take up space, change levels and interact with foundations, landscape and energy; designed in isolation by a consultant who never sees the drawings, they clash and compromise the scheme, whereas designed as part of the whole they read beautifully and pull their weight across drainage, greening and amenity. Your drainage strategy and your architecture will agree with each other because the same practice authored both.

The second reason is that we know Lewisham. We work across the borough — Deptford, New Cross, Brockley, Lewisham, Ladywell, Catford, Forest Hill, Sydenham and the residential suburbs — and we understand its rivers, its flood history, its Strategic Flood Risk Assessment, its Local Plan 2020-2040 policies and how its flood-risk officers think. We know which sites sit in the Ravensbourne and Quaggy valleys, where infiltration is unlikely to work on London clay, and how the council's appetite for river enhancement and green infrastructure can be turned to a scheme's advantage. That local fluency shows in strategies that anticipate the borough's concerns rather than colliding with them.

The third reason is rigour. A drainage strategy lives or dies on its numbers and its hierarchy narrative, and we do both properly: defensible greenfield-rate calculations, storage sized for the 1-in-100-year climate-adjusted storm, a hierarchy worked through tier by tier, permeability testing where infiltration is claimed, and a credible maintenance plan — all summarised in the London SuDS proforma the borough uses. We write for both audiences: clearly enough for the planning case officer, and rigorously enough for the flood-risk officer, so the strategy convinces everyone who has to sign it off.

The fourth reason is that we see it through. From the first feasibility check of the flood maps to discharging the final drainage condition after permission, we run the drainage side of your application end to end — coordinated with the rest of the submission, honest about what your site can and cannot do, and focused on getting you a permission that is also a scheme you can build. Send us your site and your idea, and we will tell you straight what drainage it needs and give you a clear, fixed fee to deliver it.

For a homeowner extending a Victorian terrace in Brockley or Ladywell, drainage may feel like the last thing on your mind — but it is often what tips a marginal application. Adding a rear extension and re-laying the garden increases hard surfacing, and in a borough this flood-conscious the council will expect that extra run-off to be managed on site: a permeable patio instead of a solid one, a water butt on the downpipe, a rain garden in the border, or a soakaway where the ground allows. Designing these in from the start costs little, satisfies the drainage expectation, and often improves the garden. Retrofitting them after refusal costs far more.

For someone building a new house or replacing a dwelling on an infill plot — common across Lewisham's suburban streets — a formal drainage strategy is frequently needed, especially near a watercourse or in a flood zone. The strategy shows the new house drains no faster than the plot did before, using permeable surfaces, a green or blue roof where appropriate, and attenuation with a restricted connection. On a self-build or custom-build project the drainage strategy is part of the package we prepare, integrated with the design so the house sits comfortably on its levels and drains sustainably.

For a small developer building a backland scheme, a mews, or a block of flats — the project types that dominate Lewisham's housing pipeline — the drainage strategy is a full, calculation-backed document and, on ten or more homes, a major-application requirement with the flood authority as statutory consultee. Here the SuDS have to store real volumes: green roofs on the flats, permeable surfaces to the parking and access, rain gardens in the landscape, and underground attenuation with flow control beneath the yard or garden. We size it, prove it, and coordinate it with the affordable-housing, density, energy and fire requirements that also bear on the scheme.

Whatever the scale, the common thread is that drainage in Lewisham is not a formality — it is a genuine test rooted in the borough's rivers and its flood history, and it is best answered by designing SuDS into the scheme from the first sketch rather than bolting them on at the end. That is how we work: the drainage strategy as part of the architecture, honest about the site, rigorous on the numbers, and pitched to what Lewisham actually expects. Get it right and the water side of your application simply works — which is exactly the point.