Biodiversity Net Gain Plan in Hillingdon

Biodiversity Net Gain is a principle now written into English planning law: development should leave the natural environment in a measurably better state than it was before. Rather than the old approach of simply avoiding or minimising harm to wildlife, BNG requires a positive, quantified improvement. The measure is biodiversity value, expressed in standardised 'biodiversity units', and the required improvement is a minimum of ten per cent — so a site must end up, on paper and in the ground, worth at least 110% of its pre-development biodiversity value.

The word to hold onto is 'measurable'. What makes BNG different from earlier environmental duties is that it is calculated. A qualified assessor surveys the habitats on a site, classifies each of them, and enters their type, size, condition and location into a government-issued spreadsheet — the statutory biodiversity metric — which converts them into a number of biodiversity units. The same metric is then used to calculate the units that the proposed scheme will create or enhance. If the 'after' figure is at least 10% higher than the 'before' figure, the scheme meets its BNG obligation; if it is not, the shortfall has to be made up somewhere.

The Biodiversity Net Gain plan is the document that records all of this. At application stage it usually takes the form of a BNG assessment or statement — setting out the baseline, the metric calculation, and how the 10% will be achieved — so the council can judge the scheme. After permission is granted, the statutory biodiversity gain plan proper has to be submitted and approved before development can lawfully begin, because BNG is enforced through a pre-commencement planning condition deemed to be attached to almost every permission. In practice the two are closely related: the assessment done well at application stage becomes the backbone of the gain plan approved later.

For a residential scheme in Hillingdon, BNG is not an optional extra bolted on at the end. It shapes the design. Because the requirement is measured from the site's baseline habitats, the trees, hedges, grassland, garden and any watercourse frontage that exist before you build are worth real units — and how you treat them, retain them, or replace them determines whether your 10% is easy and cheap to hit or difficult and expensive. That is why BNG belongs in the conversation from the first feasibility sketch, not the final submission.

Mandatory Biodiversity Net Gain comes from the Environment Act 2021, which inserted a new Schedule 7A into the Town and Country Planning Act 1990 and, with it, a 'general biodiversity gain condition' that is now deemed to be attached to almost every grant of planning permission in England. The effect is that permission is granted on the basis that development may not begin until a biodiversity gain plan has been submitted to, and approved by, the local planning authority — in Hillingdon's case, the London Borough of Hillingdon as local planning authority. This is what turns BNG from good practice into a legal precondition of building.

The core obligation set by the Act is the 'biodiversity gain objective': the biodiversity value attributable to the development must exceed the pre-development biodiversity value of the on-site habitat by at least the relevant percentage, which the Government set at 10%. The value is measured using the statutory biodiversity metric published by the Secretary of State, and the post-development gain — whether delivered on-site, off-site or through statutory credits — must be maintained for a minimum of thirty years. Those three numbers — 10% gain, one metric, thirty years — are the spine of the whole regime.

The requirement was switched on in stages. Mandatory BNG applied to major development from 12 February 2024, and to most minor (small-site) development from 2 April 2024. 'Major' broadly means residential schemes of ten or more dwellings, or sites of half a hectare or more; 'minor' captures the smaller householder-and-below-ten-homes projects that make up so much of Hillingdon's application list. Nationally significant infrastructure projects follow their own, later timetable. For the overwhelming majority of Hillingdon residential work — extensions above the exemption thresholds, new houses, small infill sites and larger developments alike — the mandatory regime is now fully in force.

Alongside the primary law sits a body of secondary legislation and Government guidance that fills in the detail: the Biodiversity Gain Requirements (Exemptions) Regulations and the Biodiversity Gain (Town and Country Planning) (Consequential Amendments) Regulations, the statutory metric and its user guide, and the online planning practice guidance on Biodiversity Net Gain. These set out exactly what the gain plan must contain, which developments are exempt, how off-site gains and statutory credits work, and how the thirty-year obligation is secured and registered. We work to this framework as it currently stands, and check it at the time of each application because BNG is still a young and evolving regime.

The starting assumption is that mandatory BNG applies to your Hillingdon application. The general biodiversity gain condition is deemed to be attached to grants of planning permission for the development of land, which sweeps in the great majority of residential schemes — new dwellings, replacement dwellings, infill and backland plots, changes of use that involve development, and larger extensions. If BNG applies, you must submit the information the council needs to judge net gain with the application, and the biodiversity gain plan must be approved before you start on site.

There is, however, a defined and deliberately narrow set of exemptions. The most relevant for householder and small-scale work in Hillingdon is the de minimis exemption: development is exempt where it does not impact a priority habitat and impacts less than 25 square metres of on-site habitat that has biodiversity value (or 5 metres of linear habitat such as hedgerow). Householder applications — the extensions, loft conversions and outbuildings that dominate Hillingdon's suburban streets — are also exempt from mandatory BNG as a category, as are self-build and custom-build developments meeting the statutory definition and certain small-scale changes.

Other exemptions cover development granted permission by a development order (including many permitted-development and prior-approval schemes), development whose sole purpose is to conserve or enhance biodiversity, and certain urgent Crown and high-speed rail works. It is important to be precise here, because the exemptions are drawn tightly and the applicant has to justify and often evidence a claim to one. A common mistake is to assume a modest scheme is exempt when it in fact crosses the 25-square-metre threshold or touches a priority habitat — which resets the position entirely.

For Hillingdon specifically, the geography makes the exemptions worth reading carefully. Because so much of the borough contains genuinely valuable habitat — the Colne Valley river corridors, Ruislip Woods, the green belt grassland and hedgerows, mature suburban trees and gardens — the 'priority habitat' carve-out and the small-area thresholds bite more often here than in a wholly built-up borough. A site that looks like an ordinary garden plot may include priority habitat or a valuable hedge that removes the exemption and brings the full BNG obligation into play. We check the baseline and the exemption position at feasibility, before design, precisely because it determines what the application has to carry.

At the heart of every BNG plan is the statutory biodiversity metric — a Government-issued calculation tool, published by Natural England and the Department for Environment, Food and Rural Affairs, that converts habitats into a comparable number of 'biodiversity units'. It exists so that a hedgerow, a lawn, a woodland edge and a wildflower meadow can all be measured on a single, consistent scale, and so that gains and losses can be added up and compared. Every mandatory BNG scheme in Hillingdon has to use this statutory metric (or, for qualifying small sites, the simplified small sites metric).

The metric works from a handful of factors. Each parcel of habitat is classified by type (its 'distinctiveness' — a species-rich hedgerow scores far higher than an amenity lawn), by its condition (assessed against defined criteria, from poor to good), by its size in hectares, and by its strategic significance (whether it lies in an area identified as locally important for nature). These factors combine into a unit score for the baseline. The proposed scheme is then scored the same way, with additional 'risk' multipliers applied to habitats that are created or enhanced — because a newly planted meadow is not worth as much on day one as an established one, and creating high-value habitat is harder and riskier than keeping what is already there.

Those risk multipliers are one of the most important — and least understood — features of the metric, and they explain why retaining existing habitat is almost always more valuable than replacing it. Habitats take time to reach target condition, so the metric discounts created and enhanced habitats for the time-to-target and the difficulty of creation. It also applies a 'spatial risk' discount to off-site gains that are far from the development. The upshot is that a mature tree, an old hedge or established grassland kept in place carries units that are expensive and slow to recreate — which is why good BNG design starts with what you can keep.

There are separate strands within the metric for area habitats (grassland, woodland, scrub, gardens), hedgerows and lines of trees, and watercourses — the last of which matters in Hillingdon given the Colne, Frays, Pinn, Yeading Brook and the Grand Union Canal. A scheme touching a river bank or a canal edge has a watercourse baseline to measure and, potentially, a watercourse gain to design. The metric is technical, and the version and publication date used must be recorded in the gain plan, because the figures depend on which version you ran. We use a qualified ecologist and the correct current version so that the calculation stands up to the council's scrutiny.

A great deal of Hillingdon's residential development is small: a single replacement dwelling, a pair of infill houses, a modest backland scheme of a few units. For qualifying minor development, the Government produced a simplified version of the metric — the small sites metric — designed to make BNG proportionate for schemes with limited habitat and limited budgets. It has been mandatory for qualifying minor development since 2 April 2024, and it can substantially reduce the survey and calculation burden compared with the full metric.

The small sites metric is available for minor development that is not otherwise excluded — broadly, residential development of between one and nine dwellings on a site of less than one hectare (or where the number of homes is not known, a site under half a hectare), and small commercial schemes below defined floorspace and area thresholds. Crucially, the small sites metric cannot be used where the site contains or is likely to contain priority habitat, or protected sites and species that trigger the full assessment. In those cases — not unusual on Hillingdon's greener plots — the full statutory metric applies even to a small scheme.

For a typical Hillingdon minor scheme on a suburban plot of amenity grassland, ornamental planting and a fence line, the small sites metric makes BNG genuinely manageable: the baseline is modest, the 10% gain is often achievable through better landscaping, native planting, a new hedge, and features such as bird and bat boxes and small wildlife-friendly areas, and the whole thing can be evidenced without a disproportionate survey programme. The skill is in designing the landscape so the gain falls out of it naturally rather than being retro-fitted.

Where the small sites metric does not apply — because the plot includes a valuable hedge, mature trees of ecological value, priority habitat, or a watercourse frontage — the honest advice is to say so early, because the assessment, the survey seasonality and the cost all change. We establish at feasibility which metric your Hillingdon site falls under, so the BNG strategy, the survey scope and the budget are all set correctly from the outset rather than discovered halfway through.

Hillingdon is unusual among London boroughs in how much genuine nature it contains. It is the second-largest London borough by area, and beyond the built-up cores of Uxbridge, Hayes, Ruislip and West Drayton it opens out into a landscape of river valleys, ancient woodland, green belt farmland and canal corridors. This is a borough where Biodiversity Net Gain is not an abstraction: the habitats it measures — woodland, hedgerow, grassland, wetland and river — are genuinely present across large parts of the area, which makes getting the BNG baseline right both more consequential and more rewarding here than in a wholly urban borough.

Water defines much of the borough's ecology. The River Colne runs down its western edge, joined by the Frays, and the Pinn, the Yeading Brook and the Wraysbury flow through it too, alongside roughly twenty kilometres of the Grand Union Canal. These waterways, and the wet meadows, marginal habitats and former gravel workings around them, form a network of ecological corridors — much of it within the Colne Valley Regional Park, a protected swathe of countryside, lakes and waterways on London's north-west edge. For any scheme near these corridors, the watercourse and wetland habitats they support carry real biodiversity value and need careful handling in the metric.

To the north, Ruislip Woods is a National Nature Reserve — one of the largest surviving blocks of ancient semi-natural woodland in Greater London — with the neighbouring Ruislip Lido a much-loved landmark. Across the borough, non-statutory sites are designated as Sites of Importance for Nature Conservation (SINCs), graded Metropolitan, Borough or Local, and there are statutory sites and priority habitats besides. This layered map of protected and valuable habitat is exactly why Hillingdon takes biodiversity seriously in its planning decisions — and why a credible BNG plan here has to start with a proper understanding of what is actually on and around the site.

The borough's history reinforces the point. Uxbridge grew as a market town serving the corn trade, with flour mills strung along the Colne and Frays; the green belt farmland around Harefield and Ickenham is working countryside with hedgerows and grassland of real ecological interest; and even the suburban streets of Ruislip and Northwood carry mature street trees and generous gardens that add up to significant urban habitat. Hillingdon has long balanced growth against a landscape it values, from its green belt designation to the Colne Valley Regional Park and its Biodiversity Action Plan. BNG is the latest, and most quantified, expression of that long-standing balance — and a well-prepared plan is how a residential scheme fits into it.

Mandatory BNG is a national statutory requirement, but it does not sit in isolation — it operates alongside Hillingdon's own biodiversity policies and the London Plan, and a good BNG plan engages with both. Hillingdon's development plan is in two parts. The Local Plan Part 1 (Strategic Policies, adopted 2012) sets the borough's overall direction, and the Local Plan Part 2 (Development Management Policies) contains the detailed policies applications are tested against, with the Local Plan under ongoing review.

The key strategic biodiversity policy is Local Plan Part 1 Policy EM7 (Biodiversity and Geological Conservation). Under it, where development is proposed on or near a site with features of ecological or geological value, applicants must submit appropriate surveys and assessments to show the proposal will not have unacceptable effects, and the development must make a positive contribution to protecting and enhancing the site or feature of ecological value. That expectation of a positive contribution sits very comfortably alongside mandatory BNG, and a BNG plan is now the natural vehicle for demonstrating it. Policy EM7 also underpins the borough's approach to its Sites of Importance for Nature Conservation, graded Metropolitan, Borough and Local.

The green belt and open-land policies matter too, because so much of the borough's habitat lies within them. Local Plan Part 1 Policy EM2 protects the green belt, Metropolitan Open Land and Green Chains, and the borough's Biodiversity Action Plan identifies opportunities for nature conservation and, where possible, the development of wildlife corridors. A scheme on the rural fringe or near a river corridor has to respect these designations in principle, and its BNG plan should be designed to reinforce rather than cut across the ecological network they protect — for instance by strengthening a hedgerow or riverside habitat that forms part of a green chain.

Over the top of the local plan sits the London Plan (2021), which applies across all London boroughs including Hillingdon. Policy G6 (Biodiversity and access to nature) requires development proposals to manage impacts on biodiversity and secure net gains, and Policy G5 (Urban Greening Factor) requires major development to calculate an Urban Greening Factor, with an indicative target of 0.4 for predominantly residential schemes and 0.3 for predominantly commercial ones. In London, urban greening and BNG are closely linked, and a well-designed landscape can serve both the Urban Greening Factor and the BNG metric at once. Because policy references evolve as Hillingdon reviews its Local Plan, the exact numbers should always be confirmed against the current adopted plan and validation checklist when you apply — but the direction of travel is settled: protect what is valuable, and leave the site measurably better for nature.

The statutory biodiversity gain plan is a defined document with a defined content, and the council cannot approve it unless it addresses each required element. At its core it must set out the pre-development biodiversity value of the on-site habitat — the baseline — measured on the date of the planning application (or an earlier agreed date), together with the completed statutory metric calculation showing the workings, the version of the metric used and its publication date. That baseline figure is the reference point against which the whole 10% obligation is judged, so it has to be robust and properly evidenced.

The plan must then show the post-development biodiversity value and demonstrate that the biodiversity gain objective — at least a 10% increase over the baseline — is met. It has to explain how that gain is achieved: what habitats are retained, enhanced or created on site; what, if anything, is delivered off site; and whether any statutory biodiversity credits are being purchased to make up a shortfall. Where habitat is created or enhanced, the plan sets out the target habitat types and conditions and the timescales for reaching them, which flows straight into the management and monitoring commitment.

Two particular statements are required and often overlooked. First, a degradation statement: the plan must confirm whether the biodiversity value of the on-site habitat is lower on the application date than at an earlier date because of activities carried out on the site — deliberately clearing habitat to lower the baseline before applying is caught by anti-avoidance rules, and if degradation has occurred the earlier, higher value must be used and evidenced. Second, the plan must demonstrate that the biodiversity gain hierarchy and the mitigation hierarchy have been followed — that harm was avoided and minimised before any gain was designed to compensate for it.

Finally, the plan must show how the gain will be secured and maintained for at least thirty years. For on-site gains this is typically through a planning obligation (a section 106 agreement) or a planning condition tied to a habitat management and monitoring plan; for off-site gains it is through a conservation covenant or section 106 with the land registered on the national Biodiversity Gain Sites Register; and statutory credits are evidenced by the purchase from the Government. The exact package depends on the scheme, but the principle is constant: the units on paper have to be locked in on the ground for three decades. We assemble all of these elements so the plan is complete, internally consistent, and capable of being approved to discharge the pre-commencement condition.

BNG is not a licence to bulldoze a site and buy the difference in credits. It sits on top of the long-standing mitigation hierarchy — avoid, then minimise, then restore, then compensate — and adds its own biodiversity gain hierarchy that sets the order in which gains should be delivered. The strong preference is for biodiversity value to be retained and enhanced on the site itself; only where on-site gain is genuinely not possible or not sufficient should off-site gains be used; and statutory biodiversity credits, bought from the Government, are the last resort of all.

The reason the hierarchy matters so much in practice is that it shapes both the design and the cost. On-site gain — keeping the mature trees, retaining and strengthening the hedge, turning surplus amenity lawn into species-rich grassland, adding native planting, green roofs and wildlife features — is usually the cheapest and most defensible way to hit 10%, and it is what the council and the metric both prefer. Reaching for off-site units or statutory credits before genuinely exhausting on-site options is both more expensive and harder to justify to a case officer working to the hierarchy.

That said, the hierarchy has been adjusted for smaller schemes. For minor development that is not exempt, the Government has treated off-site biodiversity gains as equally preferable to on-site enhancement or creation, recognising that a small plot often cannot deliver much habitat within its own boundary. This is a helpful proportionality for many Hillingdon infill and single-plot schemes, where the site is too tight for meaningful on-site gain and a registered off-site unit is a sensible, lawful solution.

In every case, though, the first move is to avoid unnecessary harm. A design that needlessly removes a valuable hedge or a mature tree not only damages nature but inflates the biodiversity units the scheme has to replace — units that, thanks to the metric's time-and-risk discounts, are expensive and slow to recreate. Good BNG design in Hillingdon therefore begins by mapping what is worth keeping and building the scheme around it, so the gain hierarchy is satisfied and the 10% is reached with the least cost and the greatest ecological benefit.

There are three ways to deliver the required biodiversity gain, and most Hillingdon schemes use one or a combination. On-site gain is habitat retained, enhanced or created within the red line of the development — the retained trees, the new native hedge, the wildflower verge, the green roof, the pond. It is the preferred route under the hierarchy, it keeps the ecological benefit local, and it is usually the most cost-effective. For many Hillingdon schemes with a garden or landscaped setting, thoughtful landscape design delivers all or most of the 10% on site.

Off-site gain is used where the site cannot deliver enough on its own. Here the developer secures biodiversity units on another piece of land — a habitat bank or a landowner's site that has created or enhanced habitat — which is registered on the Biodiversity Gain Sites Register maintained nationally. The units are allocated to the development, secured for thirty years by a conservation covenant or section 106 agreement, and recorded on the register so they cannot be double-counted. Off-site units are increasingly available through habitat banks, though price and the metric's spatial-risk discount for distant sites both matter to the sums.

Statutory biodiversity credits are the backstop. Where a developer genuinely cannot deliver enough gain on site or secure suitable off-site units, they can buy statutory credits from the Government as a last resort. The credits are deliberately priced to be less attractive than real habitat creation, precisely so that they are used only when the other routes are exhausted — which keeps the incentive on delivering actual habitat. Relying on credits also has to be justified against the hierarchy, so it is not a shortcut around genuine effort on and off site.

For a typical Hillingdon residential project the practical picture is this: a suburban plot with a decent garden often meets 10% on site through good landscaping; a tight infill plot may need a modest off-site unit purchase; and a larger green belt or Colne Valley scheme with significant habitat may combine substantial on-site enhancement with off-site units for any residual shortfall. We model the options at design stage — testing how much on-site gain the scheme can carry before deciding whether off-site units or credits are needed — so the cheapest compliant route is chosen deliberately rather than by default.

The single feature that catches people out about BNG is that it does not end at completion. The biodiversity gain has to be maintained for a minimum of thirty years, and that means the created and enhanced habitats have to be managed and monitored across that period so that they actually reach and hold the condition the metric assumed. A wildflower meadow that is scored as 'good condition' in the calculation but then mown as a lawn delivers no real gain — which is why the thirty-year management and monitoring obligation is enforced, not aspirational.

This is captured in a Habitat Management and Monitoring Plan (an HMMP), which sits alongside or within the biodiversity gain plan. It sets out, habitat by habitat, what management is needed and when — how the new grassland is cut and when, how the hedge and trees are maintained, how a pond or wetland is looked after, how invasive species are controlled — and it sets out a monitoring regime, typically periodic surveys through the thirty years, to check the habitats are on track to their target condition, with remedial action if they are not. It also identifies who is responsible and how the work is funded across the decades.

How this is secured depends on where the gain is. On-site gains are usually tied down by a section 106 planning obligation or by a planning condition requiring the HMMP to be implemented, so the duty runs with the land and binds future owners — a genuinely important point for a Hillingdon homeowner, because the thirty-year commitment does not simply evaporate if the property changes hands. Off-site gains are secured by conservation covenant or section 106 and registered, with the management funded through the arrangement with the habitat provider.

For most Hillingdon residential schemes the management is proportionate: keeping a native hedge trimmed appropriately, cutting a wildflower area once or twice a year and removing the arisings, maintaining trees and a pond, and topping up planting where needed. The key is that it is written down, costed, and honestly deliverable — because a management plan that assumes an intensity of work no ordinary household will sustain is worse than useless. We design the on-site habitats and their management together, so the ongoing commitment is realistic for whoever ends up living there, and the gain the council approved is the gain that actually persists.

Because the whole calculation depends on the baseline, the survey that establishes it is the foundation of the BNG plan — and it has to be done properly. For most schemes this means a suitably qualified ecologist carrying out a habitat survey to classify and condition-assess every parcel of habitat on the site, following the UK Habitat Classification that the metric uses. On greener Hillingdon sites this often sits within a wider Preliminary Ecological Appraisal that also flags protected species and designated sites, so the ecology and the BNG baseline are established together.

Seasonality is the practical trap. Habitat condition assessment, and especially botanical surveys to identify species-rich grassland or classify a hedgerow, are most reliable in the growing season — broadly spring to early autumn. A survey done in midwinter may under-record a site's value, which can distort the baseline and, ironically, make the 10% harder to reach if the true value emerges later. Protected-species surveys (bats, great crested newts, reptiles, breeding birds) have their own seasonal windows too. On Hillingdon's more natural sites — near the Colne Valley, close to Ruislip Woods, or on green belt land — these windows can determine the whole programme.

This is why BNG rewards early instruction. If we identify at feasibility that a site needs a growing-season survey, we can programme it so the ecology does not become the thing that holds up the application at the end. Leaving the survey until the drawings are finished, only to find it cannot be done until the following spring, is a genuinely avoidable and painful delay — and one we see repeatedly on schemes where BNG was treated as a late-stage box to tick rather than an early design input.

The anti-avoidance rules add a further reason to survey early and honestly. If habitat is cleared before the application in an attempt to lower the baseline, the higher pre-clearance value can be used, and the degradation statement in the gain plan has to address it. So the worst thing an owner can do is 'tidy up' a site by removing a hedge or grubbing out scrub before taking advice. We establish the baseline first, from a proper survey, so the scheme is designed against the site's genuine value — which is both the compliant approach and, usually, the cheaper one.

It helps to see how BNG lands on the kinds of project Crown does across Hillingdon. On a householder extension in Ruislip or Ickenham, the good news is usually that mandatory BNG does not apply at all — householder applications are exempt as a category — although designing in a few wildlife-friendly features and native planting is still worthwhile and supports the London Plan's greening aims. The exemption is one of the first things we confirm, because it can take the whole BNG question off the table for a straightforward extension.

On a single new dwelling or a replacement house — a common Hillingdon project, from an infill plot to a knock-down-and-rebuild — mandatory BNG generally applies, and the small sites metric usually governs. Here the 10% is typically achievable on site through the landscape design: retaining a mature tree or hedge, converting amenity lawn to species-rich grassland or a wildlife garden, adding native hedging, a green roof to an outbuilding, a small pond, and integrated features such as bird and bat boxes. The trick is to design the landscape so the gain is a natural consequence of it rather than an add-on.

On a small development of a few units — backland or infill of two to nine homes — BNG is more involved but still very manageable with early planning. The site may be tight, so we test how much on-site gain it can carry and, where it falls short, cost a modest off-site unit purchase. On these schemes the interaction with parking, drainage and amenity space is real, and BNG has to be designed alongside them so the landscape does several jobs at once — which is exactly the kind of coordination an integrated design service is built for.

On a larger scheme, or one on the green belt fringe or near a Colne Valley river corridor, BNG becomes a central design driver. A larger site can deliver significant, genuinely valuable habitat — strengthening a hedgerow network, creating wet grassland or a pond, enhancing a river margin — that both meets the metric and reinforces the ecological corridors Hillingdon's policies protect. Here BNG stops being a constraint and becomes an opportunity to make the scheme demonstrably better for nature, which strengthens the whole planning case. Whatever the scale, we tailor the BNG strategy to the project rather than applying a one-size-fits-all approach.

We prepare BNG as an integrated part of the design and planning service, not as a disconnected report. It starts at feasibility, where we establish the exemption position, identify what the site's baseline habitats are likely to be, work out which metric applies, and flag any protected sites, priority habitats or watercourse frontage that change the picture. This early diagnosis is what lets us set the BNG strategy — and the survey scope, budget and programme — correctly from the outset.

We then coordinate the ecological survey with a suitably qualified ecologist, timed to the right season, to establish the baseline properly and run the statutory metric (or small sites metric). With the baseline fixed, we design the scheme to deliver the gain — testing how much the landscape can carry on site, and modelling off-site units or credits only where a genuine shortfall remains. Because we are designing the building and the landscape together, the trees, hedges, grassland and features that generate the units are part of the architecture rather than a separate exercise bolted on afterwards.

The output at application stage is a BNG assessment or statement — the baseline, the metric calculation, the proposed gain, the degradation statement, and the demonstration that the gain hierarchy has been followed — presented so the case officer can see clearly that the scheme meets the 10% objective. This sits within a coordinated application pack whose drawings, landscape plan and statements are internally consistent, so the habitat shown on the landscape plan matches the units in the metric and the commitments in the plan.

After permission, we prepare and submit the statutory biodiversity gain plan and the Habitat Management and Monitoring Plan to discharge the pre-commencement condition, and we help put in place the securing mechanism — a section 106, condition or covenant — and any off-site unit purchase or registration. Because the same team carried the scheme from survey to consent, the transition to discharging the condition is smooth rather than a fresh scramble, and the gain that is approved is the gain that gets built and maintained.

The most common and most damaging mistake is leaving BNG to the end. Because the requirement is measured from the baseline, decisions taken early — how much habitat is kept, where the building sits, how the landscape is arranged — largely fix how achievable and how expensive the 10% is. A scheme designed with no regard to BNG, then handed to an ecologist to 'sort out the numbers', frequently finds it has needlessly destroyed valuable units and now has to claw them back through expensive off-site purchases. Designing with BNG in mind from the start avoids this entirely.

The second is the winter survey. A baseline established from a survey done in the wrong season can under-record a site's value and produce a calculation the council will not accept, or force a return visit that delays everything by months. Getting the survey properly scoped and seasonally timed at feasibility is the simplest way to keep the application on programme.

The third is the 'tidy-up' trap — clearing a hedge, felling a tree or grubbing out scrub before taking advice, in the mistaken belief it simplifies the site. It does the opposite: the anti-avoidance and degradation rules mean the higher pre-clearance value can still be used, and the gain plan has to explain the loss. The safe rule is to survey and take advice before touching anything on a site with any ecological value.

The remaining recurring problems are documentary: a metric calculation that does not record its version and date; a landscape plan that shows habitats the metric does not credit, or vice versa; a management plan that assumes an unrealistic intensity of work; an exemption claimed without the evidence to support it; or a gain plan submitted to discharge the pre-commencement condition that quietly contradicts the drawings approved with the permission. Each of these is avoidable with a coordinated, internally consistent package — which is exactly how we prepare BNG plans, so the numbers, the drawings and the commitments all tell the same story.

A Biodiversity Net Gain plan rarely travels alone. On most Hillingdon applications it is one of a family of supporting documents, and it has to be consistent with all of them. The closest relative is the ecology work: a Preliminary Ecological Appraisal or protected-species survey establishes what lives on the site and feeds the same baseline the BNG metric uses, so the two must agree. Where protected species are present, a species mitigation strategy (and, in some cases, a licence) sits alongside the BNG plan, and the habitats created for BNG and for species mitigation should be designed to reinforce one another.

The landscape plan is the BNG plan's twin. Every unit the metric claims has to be shown on the landscape and planting drawings — the retained trees, the new hedge, the species-rich grassland, the green roof, the pond — and the two documents have to describe the same scheme in the same terms. A landscape plan drawn without reference to the metric is one of the surest ways to end up with a BNG calculation the council cannot reconcile with the proposals.

In London, BNG also connects to the Urban Greening Factor required by London Plan Policy G5 on major schemes: a well-designed landscape can score for both the UGF and the BNG metric, and it is efficient to design them together. On sites near the Colne Valley or the borough's rivers, BNG intersects with flood risk and sustainable drainage — a rain garden or wetland feature can serve drainage, greening and biodiversity at once — so the flood-risk assessment and drainage strategy should be coordinated with the BNG plan rather than developed in isolation.

Finally, the Design and Access Statement (where one is required) and the planning statement should reference the BNG strategy and explain how the scheme responds to Hillingdon's biodiversity policies and the London Plan. When all of these documents are prepared as one coordinated pack — as we prepare them — the application reads as a single, consistent proposal, which is materially more likely to be validated and approved than a set of reports that contradict one another around the edges.

The cost of BNG on a Hillingdon scheme has several components, and it helps to separate them. First is our design and planning fee, which we scope to the specific project and quote as a clear fixed fee before any work begins. Second is the ecological survey and metric assessment by a qualified ecologist — modest for a small suburban plot under the small sites metric, larger for a greener site needing seasonal or protected-species surveys. Third, where the 10% cannot be met entirely on site, is the cost of off-site biodiversity units or, as a last resort, statutory credits.

The on-site delivery cost is really part of the landscape budget: the trees, hedges, grassland, green roofs, ponds and planting that generate the units are things a good scheme would often want anyway, so BNG frequently adds less to the build cost than people fear — particularly on a plot with room to landscape. Where the site is too tight for on-site gain, the off-site unit cost becomes the main BNG-specific expense, and it varies with the market for units and the metric's spatial-risk discount for distant sites. We model this at design stage so you can see the trade-off between designing in more on-site gain and buying off-site units.

There is also the thirty-year management to budget for — the ongoing cost of maintaining and monitoring the on-site habitats, or the price built into an off-site arrangement that includes management. For a typical Hillingdon home this is modest and proportionate (occasional cutting, hedge and tree maintenance, periodic monitoring), but it is real, and we set it out honestly so it is planned for rather than a surprise later.

On timescales, the critical path is usually the survey. Where a growing-season or protected-species survey is needed, it can add weeks or, if the season has passed, months — which is exactly why we scope it at feasibility. The metric calculation and gain plan themselves are quick once the baseline is fixed, and the application then runs against the council's normal targets. After permission, the statutory gain plan and HMMP are prepared to discharge the pre-commencement condition; because we prepare them from the same coordinated information, this stage is efficient rather than a fresh round of work. We give you a realistic end-to-end programme for your specific site at the outset.

The process starts with feasibility. We establish whether mandatory BNG applies to your scheme or whether an exemption is available, identify the likely baseline habitats and the correct metric, and flag any priority habitat, protected site, SINC or watercourse frontage that changes the approach. This is where we give you an honest early view of how achievable your 10% is, what it is likely to cost, and what surveys the site needs — before you commit to a full application.

Hillingdon offers pre-application advice, and on sites where BNG is finely balanced — a plot with valuable habitat, a green belt or Colne Valley setting, or a scheme relying on off-site units — an early written steer from the council can be worthwhile. It confirms the council's expectations on the baseline, the securing mechanism and the management plan while there is still time to adjust the design cheaply, and it shows the eventual case officer that the scheme has been developed carefully. We prepare and manage pre-application submissions where they add value.

We then prepare the coordinated application — drawings, landscape plan, BNG assessment and metric, and the supporting ecology and statements — and submit it to Hillingdon through the Planning Portal, checking it against the council's current validation checklist so the BNG information is complete on day one and does not stall validation. During determination we manage the application, respond to the case officer, and negotiate where that will secure approval, keeping the BNG evidence consistent with any design changes.

After permission is granted, the biodiversity gain condition still has to be discharged before you can start on site. We prepare and submit the statutory biodiversity gain plan and the Habitat Management and Monitoring Plan, help put the securing mechanism in place — a section 106, condition or conservation covenant — and arrange any off-site unit allocation and registration. Only when the council approves the gain plan can development lawfully commence, so we treat this as an integral final step rather than an afterthought, and we plan for it from the beginning.

Crown Architecture prepares Biodiversity Net Gain plans and assessments across Hillingdon as part of an integrated design and planning service — the architecture, the landscape, the planning strategy and the BNG evidence handled together. That integration is exactly what BNG needs, because the units that deliver your 10% come from the landscape and site design, and treating BNG as a separate report bolted on at the end is precisely how schemes end up with expensive, avoidable shortfalls. We design the gain into the scheme, not around it.

We know the Hillingdon context: a borough with genuinely valuable habitat — the Colne Valley river corridors, Ruislip Woods, green belt grassland and hedgerows, mature gardens and street trees — where the baseline matters, the priority-habitat carve-outs bite, and the council takes biodiversity seriously under Policy EM7 and the London Plan. We use that knowledge to set the right BNG strategy for your site, to confirm the exemption position honestly, and to design a scheme that satisfies both the metric and the borough's policies.

We are also straightforward to work with. We tell you early whether BNG applies, how achievable your 10% is, and what it will realistically cost — including the survey, any off-site units and the thirty-year management — so there are no surprises. We quote a clear fixed fee, coordinate the ecologist and the metric, and prepare a self-consistent application where the drawings, the landscape plan and the BNG numbers all agree, which is what gets schemes validated and approved rather than queried.

And we stay with the project beyond permission. The same coordinated information carries through to the statutory biodiversity gain plan and the Habitat Management and Monitoring Plan needed to discharge the pre-commencement condition, and to the securing mechanism that locks the gain in for thirty years. The aim is a single accountable point of contact from the first feasibility conversation to a scheme that is consented, buildable, and genuinely better for nature.

If you are planning a residential project in Hillingdon — a new house, a replacement dwelling, an infill or backland scheme, or a larger development on the green belt fringe or near the Colne Valley — send us the address and what you have in mind, and we will tell you honestly whether BNG applies, how achievable your 10% is, and how to build it into a scheme you can actually deliver.