BNG requirements for planning: A 2026 guide to costs, exemptions, and the 10% rule

Back in 2021, word came through that every development proposal in the UK would soon need to prove something new. Big or small. Whatever the state of the existing plot. The rule would apply across the board: each scheme would have to demonstrate a measurable uplift in the biodiversity it supported.

Fast forward to today, and that future has landed. Biodiversity Net Gain (BNG) is now a live planning requirement, and the rules have just been refined again following a significant government consultation in April 2026. 

Smaller sites are being offered more breathing room. Nationally Significant Infrastructure Projects are being brought into the fold. And the cost, timing, and coordination demands of BNG are now firmly part of every viability conversation.

For anyone weighing up a building project, the questions stack up quickly. And for those tackling more complex schemes, they multiply.

What does biodiversity actually mean in this context? Are there exceptions? How is it measured, and whose job is it to do the measuring? What is the cost likely to be? And how long will the council be watching to make sure the net gain is delivered?

There is plenty to unpack. So let us start with the headlines.

Before we get into the detail, here is the shortlist. The headline rules, the 2026 changes, and the points worth holding on to as you read on.

With the headlines in mind, the rest of this article unpacks what each of these rules means for real projects, where the costs land, and where coordination between architects, planners, and ecologists makes the difference between a clean planning route and a scramble. Starting with the foundations is the right place to begin.

Biodiversity Net Gain (BNG) is a mandatory approach in England that requires most new developments to leave the natural environment in a measurably better state than before.

Introduced under the Environment Act 2021, BNG obliges developers to deliver a minimum 10% uplift in biodiversity value, calculated using the Statutory Biodiversity Metric and secured for at least 30 years through a Section 106 agreement or conservation covenant.

Within planning policy, the defining feature of BNG is that it is measurable. Not aspirational. Not a gesture. Measurable. As the Local Government Association frames it, the requirement is that biodiversity value is quantified before development, quantified after, and the difference demonstrably positive.

The 10% requirement applies to most planning applications submitted on or after 12 February 2024 for major developments and 2 April 2024 for small sites. Nationally Significant Infrastructure Projects (NSIPs) follow from 2 November 2026.

Local planning authorities have been moving in step with the policy ever since it was announced in 2021. By March 2023, 40% of councils had BNG policies written into their adopted or emerging local plans, up from 28% the year before. A meaningful number have gone further still. Fourteen councils now require a 20% gain on some or all new development projects, well above the statutory floor.

That picture may shift under the draft NPPF 2025. Published in December 2025 for consultation, the draft proposes to limit the circumstances in which local plans can require BNG contributions above the statutory 10%, with exceptions reserved for specific, justified allocations. If adopted, councils currently pushing for 20% gain would need stronger evidence to keep that bar in place.

Biodiversity refers to the variety of living species on Earth. Plants, animals, bacteria, fungi. The full cast. Both natural and built habitats contribute to it, and that diversity is what makes human life possible and keeps entire economic industries ticking.

Public agencies like Natural England have spent years pressing for stronger, more consistent policy around BNG. The reasoning is simple: development causes environmental damage, and the rules need to reflect that.

Biodiversity loss in the UK is serious. The BNG requirement is designed to push back against that trend by hard-wiring biodiversity gains into the planning process, so that development contributes to the environment rather than chipping away at it.

One of the headline items in that Bill is a reduction target for fine particulate matter (PM2.5), the pollutant most harmful to human health. Unsafe PM2.5 levels affect everyone. Yet there is also a social justice dimension that cannot be ignored.

In 2020, a landmark ruling by the London Inner South Coroner's Court found that air pollution had caused the death of nine-year-old Ella Roberta Adoo Kissi-Debrah, who lived in the London Borough of Lewisham. Throughout her short life, particulate matter concentrations in Lewisham consistently breached WHO guidelines, EU limits, and national standards.

Underlying health conditions made Ella more vulnerable to airborne toxins. Her case also exposed something else. Air pollution tends to settle more heavily over less advantaged areas, often because of their proximity to industrial sites and busy roads.

The ruling became a rallying point for environmental justice campaigners and organisations like Impact on Urban Health, whose work tackles the uneven distribution of environmental health risks across London.

So the thinking behind BNG runs deeper than ecology alone. It speaks to climate action, public health, and wider social wellbeing. Protecting biodiversity feeds directly into other environmental goals, from cleaner air to halting species decline.

The development industry is often cast as the villain in the climate story. The planning system tells a more nuanced tale, and it can play a serious part in the solution.

Alongside the strong ecological protections that come with designations like Areas of Outstanding Natural Beauty and Sites of Special Scientific Interest, current planning policy actively encourages biodiversity improvements wherever they can be delivered, locally and nationally.

Natural England makes the wider point that BNG can work in developers' favour too. Bake an effective BNG plan into a proposal at the early stages, and the scheme is far less likely to draw objections on grounds of nature conservation or ecological harm.

Achieving a 10% BNG is a two-stage process. Developers must first work through the mitigation hierarchy, then deliver the gain through one or more of three statutory routes.

BNG does not replace existing environmental protections. It sits on top of the mitigation hierarchy set out in the NPPF, which has four steps applied in order. First, avoid biodiversity losses. Second, minimise any impacts that cannot be avoided. Third, mitigate the harm that remains. Fourth, compensate only for losses that none of the first three steps can address.

The mandatory 10% gain is then added on top of this sequence. Designated sites, irreplaceable habitats, and protected species retain their existing layers of legal protection.

With the mitigation hierarchy worked through, attention turns to how the 10% gain itself is delivered. BNG is delivered through three routes, ranked in order of preference under the statutory biodiversity gain hierarchy.

Developers can combine routes, but the hierarchy must be worked through in this order.

That order has just shifted for smaller schemes. Following the 2026 reforms, off-site gains now sit on equal footing with on-site habitat creation and enhancement for minor developments. Smaller developers can therefore reach the off-site market without exhausting on-site options first.

One further point sits beneath all three routes and shapes how the 10% target is calculated in the first place. The biodiversity baseline is set by the condition of the site at the moment of survey, not by its historic condition. Sites left to scrub up over years can carry a higher pre-development biodiversity score than expected, which raises the bar on the 10% gain. Survey timing matters, and so does how the site is managed in the months before the survey is carried out.

On-site biodiversity gains must be maintained for at least 30 years from the moment development completes. The legal lock-in usually comes through a Section 106 agreement or a conservation covenant.

The sticking point is enforcement. The specific mechanisms by which councils will monitor that 30-year obligation are still being worked through. For now, the practical question of how local authorities will keep tabs on long-term maintenance remains an open one.

Years of policy work have given us a useful concept to lean on: 'habitat distinctiveness'. The term captures the richness, diversity, and rarity of species on a given site, along with the degree to which a habitat uniquely supports species that are scarcely found elsewhere.

Where habitat replacement is proposed as the route to BNG, the logic follows on from there. Replacement should be done with a habitat of equal or higher distinctiveness. 

Leeds City Council, for instance, requires replacement to be "like for like or a realistic increase of one step upwards". Translated into practice, a low-distinctiveness habitat can be swapped for a medium-distinctiveness habitat, but not for a high-distinctiveness one.

There is a further wrinkle worth flagging. 

Any new high-distinctiveness habitat will only be acceptable if the land is transferred to a specialist nature conservation body, or if long-term management is delivered by an accredited ecologist. Reaching for high distinctiveness is therefore not always the right ambition. The aftercare obligations rarely suit the scale of most schemes.

Stripped back to essentials, the toolkit is well-established. Protect mature trees and existing nesting sites. Plant native species. Carve in ponds and wetlands. Install bat boxes and bird feeders. Cut back on pesticides. Let some areas grow wild.

The point worth making is that these moves only add up to a coherent strategy when they are designed in, not bolted on. That is a function of the design team scoping the right specialists at the right RIBA stages, defining the brief tightly, and then translating the ecologist's findings into a layout that actually delivers the units.

For every scheme, regardless of size or distinctiveness, ecology input should sit firmly inside RIBA Stage 3 and RIBA Stage 4. The consultant's feedback is what allows applicants to demonstrate that every reasonable on-site enhancement has been properly considered.

On a typical 12-unit residential scheme on a half-hectare site, that might mean retaining a mature oak as the landscape anchor, swapping amenity grass for a species-rich meadow strip along the southern boundary, threading a small wildlife pond into the SuDS strategy, and integrating bird and bat boxes into the elevations rather than as afterthoughts. None of these moves cost much. All of them count.

When on-site delivery cannot carry the full 10%, the next layer of the hierarchy comes into play.

Off-site delivery opens up a wider menu. It can include donating land for conservation, purchasing biodiversity credits, or backing habitat restoration projects. Some developers go further still and engage the local community in conservation work, which carries soft benefits at the planning application stage.

The policy term to know here is 'registered off-site biodiversity gain'. It refers to BNG delivered on land outside the development site itself. Whether the gain is achieved through a third-party agreement or by the applicant's own efforts, it counts towards the 10% target on one condition. It must be recorded on the biodiversity gains site register.

Natural England has been building this register as a publicly available record, capturing every site being used to deliver BNG. It identifies ownership and ties each site to the development whose BNG it is helping satisfy. 

The mechanism has a clear purpose: stopping applicants from 'gaming' the system. No single parcel of land can be claimed twice as the means of delivering BNG by separate proposals. Transparency by design.

Credits are the last resort, not a shortcut. To rely on them, developers must demonstrate that the 'mitigation hierarchy' (NPPF Paragraph 175) has been worked through, and that biodiversity loss cannot reasonably be addressed on-site or through a local off-site contribution.

This is where strategic input from a specialist consultant, properly briefed by the design team, makes a tangible difference. The right strategy depends on the character of nearby sites, local authority priorities, and the wider landscape strategy of the scheme itself.

The pricing is deliberately unfriendly. Credits for the lowest-distinctiveness habitats start at £42,000 each, and because two credits are needed to compensate for one biodiversity unit, the entry-level cost lands at £84,000 per unit. Medium and high distinctiveness habitats climb significantly from there. Hardly pocket change.

On the wider cost picture, ecologist fees vary with site size and complexity, off-site units on the private market typically range from £20,000 to £35,000 per unit, and statutory credits sit at the top end as the deliberate last resort. 

Factor the cost into early-stage feasibility, not the back end of the planning programme. The schemes that struggle are usually the ones where BNG is treated as a late-stage compliance item rather than a design and viability input from day one.

The exemptions regime has seen the most significant overhaul of any part of BNG in 2026. The government has used the reforms to ease teething pains, simplify life for local planning authorities, and lift unnecessary hurdles off SME developers and brownfield sites.

The headline change is an area-based exemption. From 31 July 2026, any development with a red line site area of 0.2 hectares or below is exempt from BNG, regardless of development type. 

The expected impact is significant. Around 50% of residential planning permissions that previously needed to deliver BNG will fall outside the regime altogether. There is one important caveat. The exemption falls away if on-site priority habitats are affected.

Sitting alongside this, a few longstanding exemptions remain in place. 

Developments granted temporary planning permission for a maximum of five years are exempt. Planning applications submitted before mandatory BNG kicked in on 12 February 2024 are also exempt, with transitional arrangements covering variations to existing permissions.

The 'de minimis' exemption continues to catch genuinely tiny interventions: developments that do not impact a priority habitat and affect less than 25 square metres of on-site habitat, or 5 metres of on-site linear habitat such as hedgerows. 

Householder applications, defined under article 2(1) of the Town and Country Planning (Development Management Procedure) (England) Order 2015, also remain outside scope. That covers most home extensions, conservatories, and loft conversions.

A handful of structural exemptions round out the list. 

Sites being developed primarily to fulfil a BNG planning condition for another scheme are exempt, as are developments forming part of, or ancillary to, the high-speed rail network. Urgent Crown developments and schemes granted permission by a development order, including permitted development rights, also sit outside the framework.

The self-build and custom build exemption has been removed as part of the 2026 reforms. The reasoning is straightforward. The new 0.2 hectare exemption is expected to capture the vast majority of small-scale single dwellings anyway, making the bespoke self-build carve-out redundant.

Brownfield is the next item on the policy agenda. The government is currently consulting on a targeted exemption for residential brownfield development, with a potential cut-off at sites up to 2.5 hectares. If introduced, this would represent a meaningful shift for urban regeneration projects. Further exemptions covering biodiversity-focused developments, parks, playing fields, and public gardens are expected later in 2026.

For anyone considering an urban infill or brownfield redevelopment, the message is to watch this space carefully. The eventual shape of the brownfield exemption could materially change project viability, and design strategies that quietly bake biodiversity into the scheme now will only strengthen the position later.

While the government is loosening the grip on small sites, it is tightening it at the other end of the scale. From 2 November 2026, BNG becomes mandatory for all Nationally Significant Infrastructure Projects (NSIPs).

The NSIP regime will be more front-loaded than the existing system. Major infrastructure developers will be expected to weigh biodiversity impacts during pre-application, not at the back end. Given the sheer scale of NSIP footprints, and a more relaxed approach to 'spatial risk' (the rules governing the distance between impact and compensation), industry observers expect NSIPs to absorb a substantial share of the off-site BNG unit market.

That has implications well beyond the NSIP cohort itself. Mid-sized developers planning schemes in regions where NSIPs are coming forward should think carefully about securing off-site units early. Waiting it out is unlikely to be the cheaper option.

The 2026 announcements are best read as a refinement, not a retreat. 

The 10% net gain target is intact, and the government has restated its commitment to nature-positive development. What has changed is the calibration: easier on the smallest schemes, sharper on the largest, with a pragmatic clean-up of teething issues across the board.

A new digital biodiversity metric tool is also on the way later in 2026, designed to replace the current Excel-based system and make assessments quicker and more accessible, particularly for smaller developments where consultant input has been disproportionately costly relative to scheme value.

For the development industry, the strategic implications are clear. Small projects gain breathing room. Mid-sized residential schemes face the same 10% obligation but with sharper scrutiny on how it is delivered. Large projects need BNG running through every stage of the design programme from day one.

The thread connecting all of this is coordination. BNG is not a planning condition you can drop on an ecologist and walk away from. It sits at the intersection of design, planning policy, ecology, landscape, and long-term land management. 

Pulling it together properly takes a design team that knows how to scope the right specialists, brief them with precision, and weave their findings into a coherent scheme.

Done well, the BNG strategy strengthens the architectural proposition rather than competing with it. Get that coordination right, and BNG becomes an asset to your application rather than a hurdle to clear.

At Urbanist Architecture, we are a multidisciplinary team of architects, planners, and project managers, with decades of combined experience across the most challenging corners of the UK planning system. 

Green Belt, Grey Belt, conservation areas, listed buildings, brownfield infill, and complex BNG-driven schemes all sit firmly inside our wheelhouse.

We have collaborated with dozens of ecology consultants across hundreds of projects where BNG, protected species, or habitat regulations have shaped the planning strategy. That experience matters in two practical ways.

First, we know which consultants are best suited to which sites, ecological conditions, and local authority preferences. Your project gets matched to the right specialist from day one rather than the first name on a list.

Second, we know how to brief them, sequence their work alongside the design programme, and translate their findings into a planning case the LPA can sign off on.

The result is schemes where biodiversity, design, and planning strategy speak the same language, meeting LPA expectations and exceeding our clients' ambitions.

If you are weighing up a project where BNG could shape your viability, your design, or your planning strategy, we would be glad to talk it through. The earliest conversations tend to be the most useful, and the most cost-effective. Get in touch and let us know what you are working on.