Biodiversity Net Gain Assessment in Bromley

A Biodiversity Net Gain Assessment is the body of work that shows a development will improve the natural environment of its site — that it will leave the land in a measurably better state for wildlife than it was before. In practice it has three parts. First, a survey of the existing habitats on the site, so their biodiversity value can be measured: what is growing there, in what condition, over what area. Second, a calculation using the statutory biodiversity metric, which converts habitats into standardised 'biodiversity units' and works out both the baseline value and the value the completed scheme would deliver. Third, a set of proposals and documents — the Biodiversity Gain Plan, drawings, and a management plan — that demonstrate how the required gain is achieved and maintained.

The headline number is 10 per cent. The completed development must deliver a biodiversity value at least 10 per cent higher than the pre-development baseline, measured in the metric's units. That is the minimum; it can be more, and Bromley's own policy and the London Plan encourage schemes to go further where they can. The gain has to be genuine and lasting, which is why the rules attach a thirty-year management requirement to it: creating a habitat is not enough if it is allowed to fail, so the habitats that deliver the gain have to be looked after and monitored for at least three decades.

It is important to see BNG as separate from — and additional to — the existing protections for wildlife. A scheme still has to comply with the law on protected species (bats, great crested newts, nesting birds, badgers and so on), with the protection of designated sites, and with the local plan's own biodiversity policies. BNG sits on top of all of that as a positive duty to leave the site richer. A site can be perfectly compliant with protected-species law and still fail its BNG obligation if it does not deliver the 10 per cent gain — and vice versa. A good assessment addresses both.

For an applicant, the practical value of getting the assessment right early is enormous. BNG is checked at the validation stage, which means the local planning authority can refuse even to register an application that does not carry the required BNG material. It is then a condition of the permission, which means the detailed gain plan has to be approved before development can lawfully begin. A scheme where the biodiversity gain has been designed in from the start sails through both stages; a scheme where it has been ignored can be held up at validation, at the discharge of conditions, or find itself having to buy expensive off-site units to make up a shortfall that better design would have avoided.

The legal basis for mandatory Biodiversity Net Gain is the Environment Act 2021. Schedule 14 of that Act inserted a new Schedule 7A into the Town and Country Planning Act 1990, which makes the delivery of biodiversity gain a condition of most grants of planning permission in England. The mechanism is elegant: rather than banning harm, the law deems a general 'biodiversity gain condition' to be attached to permissions, and that condition is only discharged when the authority approves a Biodiversity Gain Plan showing that the development will deliver at least the required percentage of gain. The required percentage is set at 10 per cent.

The requirement was switched on in two stages. It became mandatory for major development — broadly, residential schemes of ten or more dwellings, or sites of half a hectare or more — for applications made on or after 12 February 2024. It was then extended to small sites, meaning minor development below those thresholds, for applications made on or after 2 April 2024. So since spring 2024, the default position for almost every planning application in England, from a single new house down to many householder-scale schemes, has been that it must demonstrate a 10 per cent biodiversity gain unless a specific exemption applies.

Two further pieces of national policy reinforce the statutory duty. The National Planning Policy Framework requires development to provide biodiversity net gains and to pursue opportunities for improving the natural environment, and Planning Practice Guidance explains how BNG works in practice — the metric, the gain plan, the register of off-site gains, and the exemptions. And because Bromley is a London borough, the London Plan applies too: Policy G6 (Biodiversity and access to nature) expects development to secure net biodiversity gain, and Policy G5 (Urban Greening Factor) requires major schemes to design in a measured amount of greening. BNG, the London Plan and Bromley's own Local Plan therefore all point the same way.

The upshot for a Bromley applicant is that biodiversity gain is now a mainstream, unavoidable part of the planning process rather than a specialist add-on. It is not confined to large or rural schemes: it reaches ordinary residential development across the borough. The right response is not to treat it as a hurdle to be cleared at the last minute, but to build it into the design from the outset — which is both cheaper and more likely to produce a genuinely good scheme. That is exactly how we approach it.

Bromley is the largest of London's thirty-two boroughs by area — around 150 square kilometres — and comfortably its greenest. The majority of the borough south of the A232–A21 line, between West Wickham and Pratt's Bottom, is Metropolitan Green Belt, and the borough has the highest proportion of farmland of any London borough, around 30 per cent of its land. It contains over a hundred parks and open spaces, from Crystal Palace Park in the north to genuine open countryside and country parks in the south. This is not incidental to BNG: the borough's exceptional greenness means its habitats are among the richest in London, and the way BNG bites on a Bromley site depends heavily on what habitat is already there.

The borough is nationally important for woodland. Bromley contains something in the order of a third of all London's ancient woodland — woodland that has existed continuously since at least the year 1600 — and nearly a quarter of its Green Belt is wooded. Ancient woodland is an irreplaceable habitat, which the statutory metric and national policy treat with great caution: it cannot simply be 'recreated' elsewhere, and its loss weighs very heavily. Alongside the woodland, Bromley's chalk soils in the south support species-rich chalk grassland — one of England's most biodiverse habitats, home to rare orchids — as well as acid and neutral grassland, lowland heath, veteran trees, wetlands and valley mire.

That richness is formally recognised. Bromley has 93 Sites of Importance for Nature Conservation (SINCs), six Sites of Special Scientific Interest (SSSIs) — three of them owned by the council, at Crofton Woods, Keston & Hayes Commons, and Downe Bank & High Elms — and five Local Nature Reserves. High Elms is an SSSI precisely because its chalky soil supports species-rich grassland with rare orchids, and its ancient woodland shelters dormice; the adjoining Downe Bank, owned by the Kent Wildlife Trust, is part of the same designation. The Thames tributaries of the Cray, Ravensbourne and Beck rivers thread through the borough, carrying wet woodland and connecting Bromley's habitats to the wider London network. The Darwin landscape around Downe, Cudham, High Elms and Keston — where Charles Darwin walked and observed nature for decades — sits at the heart of it.

For a Biodiversity Net Gain Assessment, this context is the raw material. A site in the leafy, chalk-influenced south of the borough may carry high-distinctiveness habitat that the metric scores heavily, so retaining and enhancing it is usually far better than clearing it. A more urban site in the north — a suburban garden in Beckenham, Penge or Bromley town centre — will typically have a lower baseline of amenity grassland, ornamental planting and hard surfacing, where designing in native planting, a green roof or a species-rich lawn can create genuine gain relatively easily. Knowing exactly what habitat a specific plot holds, and which designations and priority habitats apply, is the first thing we establish, because it shapes the whole BNG strategy.

The starting assumption is broad: most planning applications for development in England now need to address Biodiversity Net Gain, and Bromley expects the relevant BNG material with the great majority of applications. If you are applying for planning permission for anything that will change the habitats on a site — clearing vegetation, building on garden or open land, extending over a lawn, or creating hardstanding — the default is that BNG applies and the application must show at least a 10 per cent gain, unless a defined exemption removes the duty.

For major development — broadly ten or more homes, or a site of half a hectare or more — BNG has applied since 12 February 2024, and a full statutory biodiversity metric calculation and a Biodiversity Gain Plan are expected. For minor and small-scale residential development — a single new house, a small infill scheme, a conversion, or a modest development below the major thresholds — BNG has applied since 2 April 2024, and a simplified route using the small sites metric has been available to make the calculation proportionate. Householder applications — extensions and works within the curtilage of an existing home — are treated as an exempt category, which we explain below.

There are important exemptions that can take a scheme out of the BNG requirement entirely, and the list changed materially in 2026. The long-standing exemptions include householder development, development below a de minimis threshold (impacting less than 25 square metres of on-site habitat and less than 5 metres of on-site linear habitat such as hedgerow), self-build and custom-build housing of a limited scale, biodiversity gain sites themselves, and certain nationally significant and permitted-development schemes. Whether any of these applies to your project is a technical question that the assessment answers precisely — and getting it right matters, because claiming an exemption you are not entitled to invalidates the application, while overlooking one you could use wastes money.

In every case, the safest course is to establish the BNG position at the very start of the project, before the design is fixed. We check whether BNG applies to your specific Bromley scheme, whether an exemption is available, and — if BNG does apply — what the baseline is and how the 10 per cent can be delivered. That early answer shapes the design, the budget and the programme, and it prevents the classic problem of a scheme designed without regard to BNG that then cannot validate or has to buy expensive off-site units to make up the difference.

Biodiversity Net Gain is measured, not judged. The measuring tool is the statutory biodiversity metric, published by Natural England and Defra and in force since February 2024 (based on the earlier Biodiversity Metric 4.0). The metric converts the habitats on a site into a common currency of 'biodiversity units', so that the value before development and the value after can be compared and the percentage change calculated. It removes the guesswork: the gain is a number, produced by a defined method, that the local planning authority can check.

The metric works by scoring each parcel of habitat. The number of units a habitat is worth is derived from its area (or, for hedgerows and rivers, its length) multiplied by a set of scores: distinctiveness (how ecologically valuable and rare the habitat type is — species-rich chalk grassland scores far higher than amenity lawn), condition (its ecological quality, assessed by survey against defined criteria), and strategic significance (a higher score where the habitat sits in an area identified as a priority by the relevant nature-recovery strategy). Area habitats, hedgerow (linear) habitats and watercourse habitats are counted as three separate types of unit.

A crucial rule follows from that: the three unit types cannot be added together or traded against each other, and the 10 per cent gain has to be met for each type present. If a scheme removes hedgerow, it must replace the hedgerow units — it cannot make up the loss with extra area habitat elsewhere. The metric also applies 'trading rules' that require lost habitat to be replaced on a like-for-like or like-for-better basis: high-distinctiveness habitat that is lost has to be compensated with the same habitat type, not with something easier to create. And it applies risk multipliers that penalise habitats which are difficult to create, slow to mature, or located far from the impact — which is precisely why retaining existing good habitat almost always beats destroying and recreating it.

For a Bromley scheme, the metric is where the borough's rich habitats show their weight. A mature tree, a species-rich verge, a length of native hedgerow or a patch of chalk grassland can carry a surprising number of baseline units, and losing them creates a large deficit that is expensive and slow to replace. The metric therefore rewards a design that works with what is there. We run the metric as an integral part of the design process — testing different layouts and landscape strategies against the numbers — rather than as a one-off calculation at the end, so the scheme is shaped to deliver the gain efficiently.

A great deal of Bromley's residential development is small: a single new house on an infill plot, a pair of dwellings, a conversion, a modest backland scheme. BNG applies to these minor developments — it has done since 2 April 2024 — but the government recognised that requiring the full statutory metric on a small plot would be disproportionate, so a simplified small sites metric (SSM) was made available for developments that fall below the major-development thresholds. The SSM asks for less detailed habitat data and is designed to be usable on the kinds of small residential sites that make up much of the borough's development.

For these schemes, the practical reality is that the 10 per cent gain is very often achievable on-site with sensible design. A typical small Bromley plot — a suburban garden, an infill within a residential street — usually has a modest baseline dominated by amenity grass, ornamental shrubs, fences and hard surfacing, none of which scores highly. Replacing some of that with native or wildlife-friendly planting, a species-rich lawn or wildflower area, native hedgerow, a green roof on an extension or outbuilding, and retaining existing trees can generate enough units to meet and exceed the target without any need to look off-site. The trick is to design the landscape as part of the scheme rather than as an afterthought.

The picture is different where a small site carries good existing habitat. An infill plot that happens to hold a mature tree, an established native hedge, or a fragment of species-rich grassland starts from a higher baseline, and clearing it creates a deficit that a small plot may struggle to make up within its own boundaries. On sites like these — which are more common in Bromley's greener south — the design has to be more careful: retain the valuable habitat, work the building around it, and enhance what remains. An early metric run tells us whether the baseline is a help or a hindrance and shapes the layout accordingly.

One further point on small sites is timing. The rules for minor and small-scale development have been under active review, and 2026 brought significant changes to how BNG applies to the smallest schemes (covered in the next section). Because the position has been moving, we always check the current requirement for a specific small-site application at the point it is submitted, rather than relying on a general assumption — the difference between a scheme that needs a full assessment and one that is exempt can be substantial for a small project's budget and programme.

Not every development has to deliver BNG. The regulations set out a list of exemptions, and getting the exemption analysis right is one of the most valuable parts of an assessment — because an exemption can remove the cost and delay of BNG entirely, while a wrongly claimed exemption invalidates the application. Where an exemption is relied on, Bromley expects an Exemption Statement to be submitted explaining clearly why it applies, and for certain exemptions a specific proforma has to accompany the application.

The established exemptions include: development that does not impact a priority habitat and impacts less than 25 square metres of on-site habitat and less than 5 metres of on-site linear habitat (the 'de minimis' exemption, which catches many very small works); householder development, meaning extensions and alterations within the curtilage of an existing dwelling; small-scale self-build and custom-build development of a limited size; development on a biodiversity gain site itself; and various nationally significant infrastructure, permitted-development and high-speed-rail categories that sit outside the ordinary residential picture. Whether a given Bromley scheme falls within one of these is a matter of precise fact and measurement.

In 2026 the government reformed BNG for smaller schemes to reduce cost and delay. On 15 April 2026 Defra published its response to the consultation on improving BNG for minor, medium and brownfield development, confirming a package of changes to be brought in by secondary legislation, expected around the end of July 2026. The headline change is a new area-based exemption for sites of less than 0.2 hectares, which is expected to remove the BNG requirement from a large share of the smallest residential planning applications. Alongside it, the small-scale self-build and custom-build exemption is being removed — on the basis that most single self-build homes will instead fall within the new 0.2-hectare exemption — and temporary permissions of up to five years are to be exempted. The biodiversity gain hierarchy for minor development is also being amended so that off-site gains are given the same preference as on-site creation or enhancement.

For a Bromley homeowner or small developer, these reforms are significant and genuinely helpful: many small plots that would previously have needed a full BNG assessment may now be exempt on area grounds. But the changes are staged and depend on the exact date an application is made and the regulations in force at the time, so they have to be applied carefully rather than assumed. We keep the exemption analysis current, confirm precisely which rules apply to your scheme on the day it is submitted, and prepare the correct Exemption Statement or proforma where an exemption is claimed — so the application validates and the exemption stands.

Once a scheme needs BNG, the gain can be delivered in three ways, arranged as a hierarchy that expresses a clear preference for delivering nature where the development happens. The first and preferred route is on-site: creating or enhancing habitat within the development itself — new native planting, a wildflower meadow or species-rich lawn, retained and enhanced trees and hedgerows, green roofs, sustainable drainage features that double as wet habitat, and so on. On most residential schemes in Bromley, and especially on smaller ones, on-site delivery is both achievable and by far the most straightforward and cost-effective way to meet the 10 per cent.

The second route is off-site: where a scheme cannot deliver all of its gain within its own boundaries, it can create or enhance habitat on other land — either land the applicant controls, or units bought from a habitat bank or a landowner who has registered biodiversity units on the national biodiversity gain sites register. Off-site delivery is common on larger, tighter or brownfield schemes where there is simply not enough room to create the required habitat within the site. The 2026 reforms adjusted the hierarchy for minor development so that, for those smaller schemes, off-site gains are treated on an equal footing with on-site creation, giving small developers more flexibility.

The third and last-resort route is statutory biodiversity credits: units bought from the government where on-site and off-site options are genuinely not available. Credits are deliberately priced to be the most expensive option, precisely so that they remain a fallback rather than a first choice, and their revenue is used to invest in habitat. For most residential schemes this route is never reached — but it exists so that no scheme is ever made undeliverable by an inability to find habitat, and the metric contains a simplified route allowing very small residual requirements (below a defined threshold) to be met with credits without exhaustive evidence of off-site searching.

The financial logic of the hierarchy is worth understanding, because it drives good design. On-site habitat is usually the cheapest way to meet BNG; off-site units cost real money per unit; and statutory credits cost the most of all. A scheme that thoughtlessly clears a valuable habitat can find that it has manufactured an expensive off-site or credit liability that careful design would have avoided entirely. That is why we treat the gain hierarchy as a design tool: keep what is valuable, create gain on-site where the plot allows, and only look off-site when the site genuinely cannot accommodate the required gain.

A defining feature of Biodiversity Net Gain is that it is not a one-off gesture. The habitats that deliver the gain — whether on-site or off-site — must be managed and maintained for a minimum of thirty years from completion of the development. The logic is simple: a newly created wildflower meadow or hedgerow only delivers its promised biodiversity value if it is looked after and allowed to mature, so the law requires that value to be secured for a full generation. This thirty-year obligation is one of the things that distinguishes real BNG from a token landscaping scheme.

The gain is secured through legal mechanisms. On-site gains are typically tied down by a planning condition — often a requirement to submit and then implement a habitat management and monitoring plan — and, for more significant gains, by a Section 106 planning obligation. Off-site gains are secured through a Section 106 agreement or a conservation covenant (a relatively new form of binding agreement introduced by the Environment Act), and are recorded on the national biodiversity gain sites register so they cannot be double-counted. Whatever the mechanism, the point is that the thirty-year commitment is enforceable, not merely promised.

The document that sets out how the gain will actually be delivered and looked after is the Habitat Management and Monitoring Plan (HMMP). It describes the baseline and target habitats, the works needed to create or enhance them, the management operations required year by year, the condition targets the habitats must reach, and the monitoring regime that will check progress across the thirty years. On a residential scheme the HMMP might cover, for example, how a new species-rich lawn is to be cut and managed, how new native hedgerow and trees are to be maintained and replaced if they fail, and who is responsible for doing it. It turns the metric's numbers into practical, enforceable commitments on the ground.

For a Bromley householder or small developer, the thirty-year obligation is manageable when it is designed sensibly. On a small on-site scheme, the management is often modest — the right cutting regime for a wildflower area, the care of new native planting, the retention of a green roof — and is captured in a proportionate management plan discharged by condition. We design the gain so that its ongoing management is realistic and affordable, and we prepare the HMMP and advise on the securing mechanism so the obligation is met without unpleasant surprises later. The aim is a gain that genuinely lasts and a commitment the owner can actually keep.

National BNG law sets the floor, but a Bromley application is determined against the development plan for the borough — the Bromley Local Plan (adopted 16 January 2019) together with the London Plan. The Local Plan's biodiversity policy sits in its 'Valued Environments' chapter, and the key policy is Policy 79 (Biodiversity and Access to Nature). It requires development to conserve, enhance and where possible extend the borough's biodiversity, protecting designated sites and priority habitats, and it expects new development to improve existing or create new habitats — through measures such as green roofs, living walls, native planting and features like nest boxes and 'bee bricks' — and to provide for their ongoing management. BNG now gives that policy a measurable, enforceable edge.

Policy 79 works alongside the rest of the Valued Environments chapter. The Local Plan protects the borough's Sites of Importance for Nature Conservation (of Metropolitan, Borough and Local importance), its Sites of Special Scientific Interest and Local Nature Reserves, and it gives particular attention to the habitats that define Bromley — chalk grassland, ancient woodland and lowland heath. Because most wildlife habitats are difficult to recreate, the plan treats the replacement or relocation of habitat as a last resort, echoing the metric's own strong preference for retention over recreation. Policy 77 (Landscape Quality and Character) and the plan's tree and green-belt policies add further protection for the borough's landscape and its trees.

The strategic context is the London Plan. Policy G6 (Biodiversity and access to nature) requires development to manage its impacts on biodiversity and to secure net biodiversity gain, and Policy G5 (Urban Greening Factor) requires major developments to include urban greening as a fundamental element, with a target Urban Greening Factor score of 0.4 for predominantly residential schemes (0.3 for commercial). Policy G1 (Green infrastructure) sets the wider expectation that development contributes to London's green network. These London-wide requirements sit on top of national BNG and Bromley's own policy, so a well-designed scheme is answering all three at once — which, done properly, they naturally do.

Bromley is also reviewing its Local Plan. The council consulted on a 'Direction of Travel' draft in 2025 setting out its preferred approach across green infrastructure, open space and biodiversity, including a commitment to plan positively for networks of biodiversity and green infrastructure. Policy can move, so we always work from the current adopted plan and the latest national and London-wide policy, and we check for emerging policy that carries weight. On any BNG scheme we make sure the assessment answers Policy 79, the London Plan and national BNG together, so the application is robust against the full policy framework the case officer will apply.

A complete BNG submission is a coordinated set of documents, and Bromley checks them at validation and again at the discharge of the biodiversity gain condition. The foundation is the habitat survey: a suitably qualified ecologist surveys the site, identifies and maps the habitats present, and assesses their condition against the metric's criteria. This baseline survey is the single most important input, because everything downstream depends on an accurate picture of what is on the site before development. It is best carried out in the appropriate season, which is why establishing the BNG position early in a project matters.

From the survey, the statutory biodiversity metric (or the small sites metric for minor development) is completed. The metric calculation produces the baseline biodiversity units, models the proposed habitats, and demonstrates the percentage gain — showing that the scheme achieves at least 10 per cent for each type of unit present. The completed metric calculation tool, stating its version, accompanies the application. Alongside it, drawings show the existing and proposed habitats — a habitat plan or biodiversity plan that ties the metric's numbers to real areas on the site, so the case officer can see exactly where the retained, enhanced and created habitats sit.

The Biodiversity Gain Plan is the statutory document that has to be approved to discharge the biodiversity gain condition. It sets out the pre-development biodiversity value, the steps taken to minimise habitat loss, the on-site and any off-site gains, and how the required percentage is met. Where the gain relies on habitats being created or enhanced and maintained, a Habitat Management and Monitoring Plan sets out the thirty-year management and monitoring regime. Where an exemption is claimed instead, an Exemption Statement (and any required proforma, such as Bromley's self/custom-build BNG proforma) is prepared to demonstrate that the exemption applies.

We coordinate all of this with the architecture, working with ecologists so the survey, metric, drawings and management plan are consistent with the design and with each other. A BNG submission that is internally coherent — where the landscape shown on the drawings matches the habitats in the metric, and the management plan reflects what is actually being created — validates smoothly and is straightforward for Bromley to approve. A submission assembled from separately produced, inconsistent documents is where applications get held up, and we are careful to avoid exactly that.

The most important thing we do on a BNG scheme is design the gain in from the beginning. BNG rewards a small number of design decisions taken early: what to keep, what to create, and how to arrange the building so the two work together. Retaining existing valuable habitat — a mature tree, an established hedgerow, a patch of good grassland — protects the baseline and avoids a deficit; creating new habitat within the scheme delivers the gain; and arranging the building and hardstanding to leave room for both is what makes it possible. Done at concept stage, this costs little; done at the end, it can be expensive or impossible.

On a typical Bromley residential plot the palette of on-site measures is generous. Native and pollinator-friendly planting, a species-rich lawn or wildflower area in place of amenity grass, new native hedgerow along boundaries, retained and newly planted trees, green or brown roofs on extensions and outbuildings, living walls, and sustainable drainage features such as rain gardens or swales that double as wet habitat all generate biodiversity units. Small wildlife features — bird and bat boxes, bee bricks, hedgehog gaps — support the borough's Policy 79 expectations and, while modest in metric terms, reinforce a genuinely nature-friendly scheme.

The design also has to respect what makes a Bromley site special. On a chalk-influenced plot in the south, the right response might be to enhance towards species-rich calcareous grassland, echoing the borough's nationally important habitat; on a suburban plot in the north, native woodland-edge planting, hedgerow and a wildflower lawn may fit better. Aligning the created habitat with the borough's priority habitats and with any local nature-recovery priorities not only strengthens the scheme ecologically but can score better in the metric through strategic significance. We design habitat that belongs in its part of Bromley, not a generic landscape.

Crucially, we run the metric alongside the design rather than after it. As the layout develops we test it against the numbers — does this arrangement retain enough baseline, does this landscape strategy deliver the 10 per cent, could a green roof close the gap — so that the design and the BNG outcome are resolved together. That iterative approach is what produces a scheme that meets BNG efficiently, on-site, without a late scramble for off-site units, and it usually produces a better, greener home into the bargain.

A BNG assessment rarely travels alone. It sits within the wider suite of documents a planning application needs, and it is closely related to several of them, so producing it in coordination with the rest of the submission saves duplication and avoids contradictions. The most closely linked is the ecological work more broadly: a preliminary ecological appraisal or protected-species survey often shares the same site visit as the BNG baseline survey, and the findings on bats, nesting birds, reptiles or great crested newts sit alongside the BNG story. Protected-species compliance and BNG are separate duties, but they are best handled together.

The landscape and planting proposals are effectively the same drawings that deliver the on-site gain, so the landscape design, the arboricultural report (tree survey and any tree protection), and the BNG habitat plan all have to align. A tree the arboricultural report recommends retaining is a tree carrying baseline units in the metric; a planting scheme in the landscape drawings is the created habitat in the gain plan. Where a scheme involves sustainable drainage, the drainage strategy and the BNG proposals overlap too, because features like swales and ponds can serve both.

The design and access statement (where one is needed) and the planning statement draw the threads together, explaining how the scheme responds to Bromley's Policy 79, the London Plan's greening and biodiversity policies, and the national BNG requirement. On larger schemes the Urban Greening Factor calculation required by London Plan Policy G5 is a further, related piece of quantification that has to be consistent with the BNG and landscape work. And where the site is near a designated site — a SINC, SSSI or ancient woodland, of which Bromley has many — additional assessment of effects on that designation will sit alongside the BNG assessment.

Because so many of these documents share the same underlying survey and design, the efficient way to produce them is together, from one coordinated understanding of the site. We prepare the architecture and coordinate the supporting documents so the BNG assessment, the ecology, the arboriculture, the landscape and the drainage all tell one consistent story. That coordination is what makes an application validate first time and gives the case officer a clear, coherent case to approve.

The most common BNG mistake is leaving it too late. BNG is checked at validation, so an application submitted without the required metric, gain plan or exemption statement can be refused registration before it is even considered — an immediate delay. Worse, if the design was fixed before BNG was thought about, the scheme may simply not be able to deliver the gain on-site, forcing a redesign or an expensive off-site purchase. Establishing the BNG position at the start of the project avoids both problems, which is why we always do so.

The second common mistake is destroying valuable habitat unnecessarily. Because the metric penalises the loss of high-distinctiveness or hard-to-create habitat and applies risk multipliers to recreation, clearing a mature tree, an established hedgerow or a patch of good grassland can create a deficit that is costly and slow to make up. Bromley's greener sites are especially exposed to this. A design that retains what is valuable starts from a much stronger position than one that clears the plot and then tries to rebuild the value.

A third problem is an inaccurate or ill-timed baseline survey. The whole assessment rests on the survey of existing habitats, and a survey done in the wrong season, or that misidentifies or under-records habitat, undermines everything downstream — and can be challenged by the case officer. Related to this is inconsistency between documents: a metric that does not match the landscape drawings, or a management plan that does not reflect the habitats being created, invites questions and delay. Coordinated, well-timed, internally consistent documents are the antidote.

Finally, exemptions are a frequent source of error in both directions — claiming one that does not apply (which invalidates the application) or missing one that does (which wastes money on an unnecessary assessment), a risk made sharper by the 2026 changes to the small-sites rules. And some schemes simply under-provide, submitting a metric that does not actually reach 10 per cent for every unit type, or that relies on off-site units without securing them. We anticipate all of these: we fix the BNG strategy early, survey properly and in season, keep the documents consistent, get the exemption analysis right for the rules in force, and make sure the metric genuinely delivers the required gain before anything is submitted.

The process begins with an early appraisal of the site and the BNG position. Before the design is fixed, we establish whether BNG applies to your scheme or whether an exemption is available, and — if it applies — we arrange the baseline habitat survey (ideally at the right time of year) and run an initial metric to understand the starting point. This early step is where the whole approach is set: it tells us what to keep, how much gain the site needs, and how it can realistically be delivered, and it lets us give you an honest view of cost and feasibility before design work runs ahead.

The design is then developed with BNG built in. We shape the layout and the landscape to retain valuable habitat and create the required gain on-site where possible, testing the design against the metric as it evolves, and coordinating the BNG work with the ecology, arboriculture, landscape and drawings. Where a scheme is significant or sensitive, pre-application advice from Bromley can be worthwhile to confirm the council's expectations on biodiversity before a full submission is committed — particularly near designated sites or where good habitat is affected.

The application is then assembled and submitted through the Planning Portal with the full BNG material: the completed statutory (or small sites) metric, the Biodiversity Gain Plan or Exemption Statement, the habitat and biodiversity plans, and the supporting statements. Bromley checks the BNG material at validation against its current validation list, and again during determination against Policy 79 and the London Plan. We manage the application through validation and determination, responding to the case officer and any consultees on biodiversity matters, so the BNG case is carried through to a decision.

Once permission is granted, the biodiversity gain condition has to be discharged before development lawfully begins: the approved Biodiversity Gain Plan (and any required habitat management and monitoring plan) must be in place, and off-site gains, where used, registered and secured. We see the scheme through that discharge and into delivery, so the gain designed on paper is actually created and secured on the ground — because BNG is only real once it is built and maintained. Pre-application advice and, on larger schemes, early engagement on biodiversity are available and often worthwhile.

The cost of dealing with BNG varies enormously with the scheme, and the honest answer is that it is usually modest when BNG is designed in early and much larger when it is not. The core costs are the ecological baseline survey, the metric assessment and the preparation of the gain plan and any management plan — proportionate work on a small site using the small sites metric, more involved on a major scheme with a detailed statutory metric. We scope this clearly for your project so you know the figure up front rather than discovering it late.

The bigger cost variable is how the gain is delivered. On-site gain, designed into the landscape of the scheme, is generally the cheapest way to meet BNG — it is largely a matter of specifying the right planting and features and managing them. Off-site units, bought where a site cannot accommodate the gain, are a real per-unit cost that can add up on tighter or larger schemes; and statutory credits, the last resort, are priced to be the most expensive option of all. The whole thrust of our approach — retain what is valuable, create gain on-site — is aimed at keeping schemes in the cheapest part of that range wherever the plot allows.

There is also the thirty-year management commitment to budget for. On a small on-site scheme this is usually light — the ongoing care of a wildflower area, new planting or a green roof — but it is a genuine long-term responsibility that should be understood at the outset, and it is captured in the management plan. Where gain is secured off-site, the cost of the units typically includes their long-term management, and where a Section 106 or conservation covenant is used there may be legal and monitoring costs. We set all of this out so the full picture is clear before you commit.

On timescales, the key sensitivity is the survey. Habitat surveys are best done in the growing season, so establishing the BNG position early avoids a wait for the right survey window later. With the survey in hand, the metric and gain plan can be prepared alongside the rest of the application, and BNG then adds little to the overall programme — provided it was designed in from the start. Left to the end, BNG can add weeks or months through a missed survey season, a validation refusal, or a redesign to find the gain. Getting ahead of it is the single best way to keep a Bromley project on time and on budget.

Crown Architecture prepares Biodiversity Net Gain assessments as part of a coordinated architectural and planning service for residential projects across Bromley — extensions and new houses, replacement dwellings, conversions, infill plots and small developments. Because we handle the design and coordinate the supporting documents together, the BNG work is built into the scheme from the first sketch rather than bolted on at the end, which is exactly what makes the gain achievable on-site, keeps the cost down, and gets the application through validation first time.

We understand how BNG actually works: the Environment Act 2021 duty and the mandatory 10 per cent, the statutory metric and its trading rules and risk multipliers, the small sites route, the thirty-year management obligation, and the fast-moving exemption rules — including the significant 2026 reforms for small sites. And we understand how it lands in Bromley specifically: the borough's exceptional greenness and its ancient woodland and chalk grassland, the way its Local Plan Policy 79 and the London Plan reinforce the national duty, and how the council checks BNG at validation and determination.

We are honest to work with. We establish the BNG position at the very start, tell you plainly whether an exemption applies or whether the scheme needs to deliver gain, and give you a realistic view of how the 10 per cent will be met and what it will cost — before design work runs ahead. Where a site carries valuable habitat, we say so and design around it; where an exemption is available, we make sure it is claimed correctly for the rules in force. We would rather resolve the BNG strategy at appraisal than have an application held up at validation.

And we stay with the project from the first appraisal to the discharge of the biodiversity gain condition. We arrange the survey, run the metric, design the gain, prepare the gain plan and management plan, coordinate the BNG work with the ecology, arboriculture, landscape and drawings, and carry the application through Bromley's validation, determination and condition discharge — so the gain designed on paper is genuinely created and secured on the ground. The aim is a single, accountable point of contact for a scheme that satisfies BNG cleanly and leaves your corner of Bromley measurably better for nature.

If you are planning a project in Bromley and want to know how Biodiversity Net Gain will affect it — whether it applies, whether you are exempt, and how the 10 per cent can be met — send us the site and what you have in mind, and we will give you an honest early view before any design work begins.