What is biodiversity net gain?

Biodiversity net gain is an approach to development that leaves biodiversity in a measurably better state than it was beforehand.

In England, most development subject to mandatory biodiversity net gain must deliver at least a 10% increase in biodiversity value. The gain is calculated using the statutory biodiversity metric and significant habitat improvements must generally be secured and managed for at least 30 years.

The required gain can be delivered within the development site, through registered off-site habitat or, as a last resort, by purchasing statutory biodiversity credits.

How does biodiversity net gain work?

Biodiversity net gain, commonly shortened to BNG, requires the biodiversity value of a site to be measured before and after development.

The statutory biodiversity metric converts habitats into biodiversity units. It takes account of matters including the type, size, condition and distinctiveness of habitat, as well as factors such as its location and the time and difficulty involved in creating or enhancing it.

Where mandatory BNG applies, the development must normally provide at least 10% more biodiversity units than the pre-development baseline.

Ten per cent is the statutory minimum. A local planning policy may seek a higher level of BNG where that requirement is properly justified and relevant to the development.

BNG does not replace the need to avoid or reduce ecological harm. Development should first avoid adverse effects where possible and mitigate those that cannot be avoided. BNG then deals with the remaining habitat impacts and the additional biodiversity gain required.

When did mandatory biodiversity net gain take effect?

Mandatory BNG applies to most planning permissions in England where the planning application was made on or after 12 February 2024.

It was extended to most non-major development from 2 April 2024.

Transitional arrangements remain important. For example, a Section 73 application to vary an earlier permission will generally remain outside mandatory BNG where the original application was made before the statutory regime applied.

Separate provisions will make BNG mandatory for nationally significant infrastructure projects in England from 2 November 2026. The requirement will apply to relevant development consent order applications made on or after that date.

Which developments are currently exempt?

The principal exemptions currently include:

householder development, such as an extension to an existing home;
development granted through permitted development rights;
development on an application site of no more than 0.2 hectares, where no onsite priority habitat is negatively affected;
development falling within the de minimis exemption;
certain temporary development permitted for no more than five years, where no onsite priority habitat is negatively affected;
urgent Crown development;
certain development undertaken to provide BNG for another development; and
development connected with the high-speed railway network.

The de minimis exemption applies where a development affects less than 25 square metres of onsite habitat and less than five metres of onsite linear habitat, such as a hedgerow, provided it does not negatively affect an onsite priority habitat.

BNG also does not apply to retrospective planning permissions granted under Section 73A of the Town and Country Planning Act 1990.

The detailed exemptions and transitional provisions can be important. Whether BNG applies may depend not only on the type of development but also on when the original application was made and whether an earlier permission was itself subject to BNG.

What changed on 6 August 2026?

A series of changes to the mandatory BNG regime came into force on 6 August 2026.

Development of sites of 0.2 hectares or less is exempt

Planning applications for development on sites within a red line boundary of no more than 0.2 hectares are now exempt from mandatory BNG.

This is an area-based exemption and applies to different forms of development, not only residential schemes.

It does not apply where the development negatively affects an onsite priority habitat.

The 0.2-hectare exemption is separate from the de minimis exemption. The former looks at the total area within the planning application boundary; the latter looks at the amount of habitat actually affected by development.

An exemption from mandatory BNG does not remove other ecological requirements. Development must still comply with national and local planning policies relating to biodiversity, protected sites, protected species and important habitats where relevant.

Certain temporary development is exempt

Temporary development may now be exempt where the whole development is temporary and planning permission requires the buildings, works or use to be removed or discontinued, and the land reinstated, within no more than five years.

Again, the exemption does not apply where an onsite priority habitat would be negatively affected.

There are detailed rules dealing with successive temporary permissions, so the exemption should not be assumed to apply simply because an individual application seeks a permission of five years or less.

The self build and custom build exemption has been removed

There is no longer a separate BNG exemption for new self-build and custom-build planning applications made on or after 6 August 2026.

Those schemes are instead considered under the same exemptions as other development.

A self-build proposal on a site of no more than 0.2 hectares may therefore be exempt under the area-based exemption, provided it does not negatively affect onsite priority habitat. A larger proposal is not exempt simply because the homes are self-build or custom-build.

Transitional provisions preserve the former exemption in some circumstances involving earlier applications and subsequent Section 73 applications.

Minor development has greater flexibility to use off site units

The biodiversity gain hierarchy has also changed for non-major development where the application was made on or after 6 August 2026.

For major development, the hierarchy continues to prioritise appropriate onsite habitat enhancement and creation before registered off-site biodiversity gains are used.

For non-major development, onsite habitat enhancement, onsite habitat creation and registered off-site biodiversity gains now sit at the same level of preference.

This means a minor scheme that remains subject to BNG can choose an off-site solution without first having to demonstrate that delivering the required units onsite is impracticable.

Avoiding and mitigating impacts to important onsite habitat remains part of the hierarchy, while statutory biodiversity credits remain the final option.

This additional flexibility can be significant on small or constrained sites, where accommodating habitat purely to satisfy BNG may otherwise compete with the development itself, landscaping, access or other planning requirements.

How can a development deliver biodiversity net gain?

A development may meet its BNG requirement through one or more of three routes.

Onsite habitat: existing habitat can be retained or enhanced and new habitat can be created within the development site. Early coordination between the ecologist, planner, landscape architect and design team can help integrate habitat into the development rather than trying to find space for BNG after the layout has already been fixed. Significant onsite habitat enhancement or creation must generally be secured and maintained for at least 30 years.

Registered off site biodiversity gains: a developer may create or enhance habitat on other land or purchase biodiversity units from a third-party provider. Land used to supply off-site biodiversity units must be registered on the biodiversity gain sites register and the relevant units must be allocated to the particular development. The habitat must also be legally secured and managed for at least 30 years. Location can affect how many units are required. The statutory metric includes a spatial risk multiplier for certain off-site habitat, which can penalise biodiversity gains delivered farther from the development site. The availability, type and location of suitable units can therefore have a direct effect on development cost and programme.

Statutory biodiversity credits: where the required gain cannot be achieved through the appropriate combination of onsite habitat and registered off-site units, statutory biodiversity credits can be purchased from the Government. Credits are deliberately intended as a last resort and are priced accordingly. They should not normally be treated as a substitute for developing a workable BNG strategy earlier in the project.

What must accompany a planning application?

Where mandatory BNG applies, a planning application must contain information establishing the pre-development biodiversity position.

This normally includes:

confirmation of whether the statutory biodiversity gain condition applies;
the site's pre-development biodiversity value;
the relevant statutory biodiversity metric information;
plans identifying existing onsite habitats;
details of any irreplaceable habitat; and
information about activities undertaken before the application that may have reduced the site's biodiversity value.

Where an applicant considers that the development is exempt, the application should identify the relevant exemption and provide sufficient information to demonstrate why it applies.

The local planning authority can seek further information where this is reasonably necessary to understand the biodiversity implications of the proposal.

What is a biodiversity gain plan?

The biodiversity gain condition is automatically applied to most planning permissions that fall within the mandatory regime.

After planning permission has been granted, and before development starts, the developer must submit a Biodiversity Gain Plan to the planning authority.

The plan demonstrates how the development will meet its BNG requirement. It will usually confirm matters including the final biodiversity metric calculation, onsite habitat provision, registered off-site allocations and any statutory biodiversity credits.

The local planning authority must approve the Biodiversity Gain Plan before the development subject to the condition begins.

For phased development there are additional requirements, with an overall Biodiversity Gain Plan and a separate plan for each relevant phase.

BNG therefore does not end when planning permission is granted. Its delivery can affect the discharge of conditions, legal agreements, land arrangements and the point at which development can lawfully commence.

How is irreplaceable habitat treated?

Irreplaceable habitats are dealt with differently because they cannot realistically be recreated, or would take an exceptionally long time to recover their ecological function.

They include habitats prescribed by regulations, such as certain ancient woodland and ancient or veteran trees.

The normal 10% calculation is not used to determine compensation for the part of a site containing irreplaceable habitat.

Instead, harm should be avoided so far as possible and any permitted impact requires bespoke compensation agreed through the planning process.

Statutory biodiversity credits cannot be used as compensation for impacts on irreplaceable habitat.

BNG should therefore not be understood as providing a licence to develop important ecological assets simply because sufficient biodiversity units can be obtained elsewhere.

Why should BNG be considered early?

BNG can affect the developable area of a site, layout, landscaping, viability and development programme.

An ecological survey undertaken after the scheme has been substantially designed may identify habitat that should have been retained, reveal a larger biodiversity-unit requirement than anticipated or result in the need to acquire off-site units.

Those issues can require redesign, reduce development capacity or create additional cost and delay.

Early ecological surveys and an initial metric calculation can instead inform site acquisition, feasibility and design. They can show where habitat carries a high biodiversity value, where impacts would be particularly expensive to compensate and whether an onsite or off-site strategy is likely to be more appropriate.

The planning application boundary also requires care. Since the 0.2-hectare exemption is based on the area within the red line, the boundary may affect whether a development is subject to mandatory BNG.

However, an application boundary should still properly encompass the land needed to carry out the development. Artificially altering a red line simply to obtain an exemption may create separate planning, access, implementation or enforcement problems.

BNG is therefore best treated as an early development constraint and design consideration rather than a calculation to be completed shortly before an application is submitted.

Are further changes to BNG expected?

Yes. The Government has announced further reforms to the BNG regime which have not all yet been implemented.

These include proposed changes to the statutory biodiversity metric and further exemptions for some forms of development whose purpose is principally ecological enhancement or certain improvements to public green spaces.

The Government has also separately consulted on a possible targeted BNG exemption for some residential development on brownfield land.

That consultation closed in June 2026. As of September 2026, the Government is still analysing the responses and has not confirmed that a brownfield exemption will be introduced.

Developers should therefore continue to apply the current statutory regime unless and until further changes are brought into force.

Key points

Most qualifying development in England must provide at least 10% biodiversity net gain.
The gain is calculated using the statutory biodiversity metric.
Significant onsite gains and registered off-site habitat must generally be maintained for at least 30 years.
BNG can be delivered onsite, using registered off-site biodiversity units or, as a last resort, statutory biodiversity credits.
Applications made on or after 6 August 2026 on sites of no more than 0.2 hectares are generally exempt, provided no onsite priority habitat is negatively affected.
Certain temporary development is also exempt.
The separate self-build and custom-build exemption has been removed for new applications.
Non-major development now has greater flexibility to use registered off-site biodiversity units.
BNG will become mandatory for relevant nationally significant infrastructure projects from 2 November 2026.
Further reforms, including a possible brownfield residential exemption and changes to the statutory metric, should be treated as proposals until they are formally implemented.
BNG can materially affect development capacity, cost and programme, so it should be considered during site assessment and design rather than after a scheme has been fixed.

When should professional advice be sought?

Planning and ecological advice should be obtained early where a site contains valuable habitat, the development is close to an exemption threshold, off-site units may be required or the proposal involves phased development.

Advice may also be important when establishing the application boundary, agreeing the biodiversity baseline, assessing the implications for development capacity and deciding how long-term habitat delivery will be secured.

For acquisitions and early-stage feasibility work in particular, understanding the BNG position before a scheme is designed can help identify costs and constraints that may otherwise only become apparent later in the planning process.

Updated 10 September 2026

Knowledge journey

Article

Do the new BNG exemptions make smaller development easier to deliver examines how the 0.2ha exemption changes mandatory BNG requirements and what the new rules mean for development costs, application strategy and ecological risk.

Deeper reading

When starting again might be the better planning strategy considers whether applicants with live applications could benefit from withdrawing and resubmitting to take advantage of the new 0.2ha BNG exemption, and where the programme, cost and planning risks may outweigh that benefit.

In practice

BNG in practice: Meadgate Works, Nazeing see how Maddox Planning coordinated the biodiversity net gain discharge strategy at Meadgate Works, including the HMMP and off-site habitat units.

Principal authoritative sources