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 qualifying developments must deliver at least a 10% increase in biodiversity value. This is calculated using the statutory biodiversity metric and secured for at least 30 years. The gain may be provided 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 considers factors including the type, size, condition and distinctiveness of each habitat, as well as its location and the time and difficulty involved in creating or enhancing it.

The completed development must provide at least 10% more biodiversity units than the site’s pre-development baseline. A local planning policy may require more than 10% where a higher requirement is properly justified.

BNG does not replace the need to avoid or reduce ecological harm. A development should first avoid harm where possible, then mitigate unavoidable effects and compensate for remaining losses. The creation of additional biodiversity units comes after these steps.

When did mandatory biodiversity net gain take effect?

Mandatory BNG applies to most planning permissions granted in England following applications made on or after 12 February 2024.

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

Transitional arrangements mean that some later permissions remain outside the mandatory regime. For example, a Section 73 permission will generally not attract mandatory BNG where the original application was made before 12 February 2024.

Different arrangements are being developed for nationally significant infrastructure projects.

Which developments are currently exempt?

As at 29 July 2026, the principal exemptions include:

householder development, such as extensions to an existing home;
development granted through permitted development rights;
development affecting less than 25 square metres of habitat and less than five metres of linear habitat, provided no priority habitat is affected;
qualifying self-build and custom-build schemes of no more than nine homes on sites no larger than 0.5 hectares;
urgent Crown development;
certain development undertaken to fulfil BNG obligations for another development; and
development connected with the high-speed railway network.

BNG does not apply to retrospective planning permissions granted under Section 73A. It has also not yet been commenced for certain other routes to permission, including local development orders, neighbourhood development orders and successful enforcement appeals.

The exemptions are changing from 6 August 2026. The date on which the planning application is made will therefore be important.

What is changing on 6 August 2026?

New regulations have been made and will take effect on 6 August 2026. Broadly, the changes will apply to new planning applications made on or after that date. Applications already submitted will continue to be dealt with under the existing rules.

A new exemption for sites of 0.2 hectares or less

Development will be exempt from mandatory BNG where the site within the red line boundary is no more than 0.2 hectares.

The exemption will apply across different forms of development rather than only to residential schemes. It will not apply where the development would negatively affect an onsite priority habitat.

This is an area-based exemption. It is separate from the existing de minimis exemption, which is based on the amount of habitat affected rather than the overall size of the application site.

The Government estimates that the new exemption will remove around half of the residential planning permissions previously required to provide mandatory BNG.

An exempt development must still comply with national and local policies protecting biodiversity. Exemption from mandatory BNG is not permission to cause otherwise unacceptable ecological harm.

A new exemption for temporary development

Development intended to be temporary will be exempt where the land is to be reinstated within five years or less.

The exemption will not apply where an onsite priority habitat would be negatively affected. The development must also fall within the detailed conditions set by the regulations.

The self build and custom build exemption will be removed

The existing exemption for qualifying self-build and custom-build development will be removed for applications made from 6 August 2026.

These developments will instead be considered under the same exemptions as other development. A self-build proposal on a site of no more than 0.2 hectares may therefore qualify for the new area-based exemption, provided it does not adversely affect onsite priority habitat.

A self-build scheme on a larger site will not be exempt merely because it is self-build or custom-build housing.

Minor development will have greater flexibility to use off site units

The biodiversity gain hierarchy will be modified for minor development that remains subject to BNG.

At present, the hierarchy generally prioritises enhancing or creating habitat onsite before registered off-site gains are used. From 6 August 2026, off-site gains will be placed at the same level of preference as onsite habitat enhancement and creation for minor development.

This should allow a developer to use registered off-site biodiversity units without first having to demonstrate that onsite delivery is impracticable. Avoiding and mitigating harm will remain important, while statutory biodiversity credits will continue to be a last resort.

For this purpose, minor development means development that does not meet the statutory definition of major development.

What other changes are proposed?

Further reforms have been announced but are not yet in force.

The Government intends to introduce:

an exemption for development whose primary purpose is to conserve or enhance biodiversity;
a targeted exemption for qualifying improvements to parks, public gardens and community or local authority playing fields; and
changes to the statutory biodiversity metric and its supporting guidance.

A separate consultation has considered a possible targeted exemption for certain residential development on brownfield land. It included potential site-size thresholds of up to 2.5 hectares. That consultation closed on 10 June 2026 and the Government has not yet confirmed whether an exemption will be introduced.

Depending on its response, the Government may also change the existing de minimis exemption.

Proposed metric changes include reforms to the spatial risk multiplier, clearer treatment of open mosaic habitat on previously developed land, a possible new urban habitat category and greater flexibility where open mosaic habitat cannot be replaced on a like-for-like basis. Changes to watercourse assessments are also being explored.

The Government expects further metric changes later in 2026, subject to the necessary parliamentary process. Transitional arrangements are promised for projects already using the existing metric.

These measures should be treated as proposals until the relevant legislation, metric and guidance have been finalised.

How can a development deliver the required gain?

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

Onsite habitat: habitat may be retained, enhanced or created within the development site. Early coordination between the design team and an ecologist can help integrate meaningful habitat into the scheme rather than treating BNG as a later addition. Significant onsite habitat enhancements must usually be secured and managed for at least 30 years. This may be achieved through a planning condition, a Section 106 agreement or a conservation covenant.

Registered off site biodiversity gains: where the requirement cannot be met entirely onsite, the developer may create habitat on other land or purchase biodiversity units from a third-party provider. The off-site habitat must be recorded on the biodiversity gain sites register and allocated to the particular development. It must also be legally secured and managed for at least 30 years. The metric generally rewards off-site habitat located close to the development. Units obtained farther away may be affected by the spatial risk multiplier, increasing the number required.

Statutory biodiversity credits: statutory credits may be purchased from the Government where the required units cannot be obtained onsite or through the off-site market. Credits are intended to be a last resort and are priced to avoid undercutting the private market. They should not be treated as a routine alternative to developing a workable BNG strategy.

What must accompany a planning application?

Where mandatory BNG applies, the application must include minimum information establishing the site’s pre-development biodiversity position. This normally includes:

confirmation that the biodiversity gain condition applies;
the pre-development biodiversity value;
a completed statutory biodiversity metric calculation;
plans identifying the existing onsite habitats;
details of any irreplaceable habitat; and
information about any activities that have reduced the biodiversity value of the site before the application.

A local planning authority may request further information where this is reasonably necessary to understand whether the development can meet its BNG obligations.

Where an applicant considers the development to be exempt, the application must state why the biodiversity gain condition does not apply and provide sufficient supporting evidence.

What is a biodiversity gain plan?

The biodiversity gain condition is automatically applied to most qualifying planning permissions.

Before development begins, the applicant must submit a Biodiversity Gain Plan to the planning authority. The plan explains how the development will achieve the required gain and confirms the final metric calculation, onsite measures, registered off-site allocations and any statutory credits.

The planning authority must approve the plan before the development commences. Beginning work without that approval may amount to a breach of planning control.

For phased development, an overall Biodiversity Gain Plan is followed by a plan for each phase before that phase begins.

Does biodiversity net gain apply to irreplaceable habitat?

Irreplaceable habitats are treated differently because their biodiversity value cannot meaningfully be recreated within a reasonable period. Examples include ancient woodland, ancient and veteran trees and certain other exceptionally valuable habitats prescribed by regulations.

The usual 10% calculation does not apply to the affected irreplaceable habitat. Harm must be avoided as far as possible, and any permitted impact requires a bespoke compensation strategy agreed with the planning authority.

BNG should not be used to justify development that would otherwise cause unacceptable harm to an irreplaceable habitat.

Why should BNG be considered at the start of a project?

The site’s existing habitat can influence the developable area, layout, viability and programme.

Considering BNG only after a scheme has been designed may reveal that important habitat cannot be retained, sufficient onsite gain cannot be achieved or expensive off-site units are required. The availability and type of off-site units can also affect the strategy.

Early ecological surveys, a reliable baseline and coordination between the ecologist, planner, landscape architect and design team can reduce these risks. The red line boundary should also be considered carefully, particularly because the new 0.2-hectare exemption will depend on the total application-site area rather than only the development footprint.

Artificially drawing a boundary to avoid BNG may create planning, access, implementation or enforcement difficulties. The boundary must properly encompass the land required to carry out the development.

Key points

Most qualifying development in England must provide at least 10% biodiversity net gain.
The gain is calculated using the statutory biodiversity metric and generally maintained for at least 30 years.
BNG can be delivered onsite, through registered off-site units or, as a last resort, through statutory credits.
New applications on sites of no more than 0.2 hectares will generally be exempt from 6 August 2026, unless priority habitat is negatively affected.
The self-build and custom-build exemption will be removed on the same date.
Further exemptions and metric reforms have been announced but are not yet in force.
BNG should be considered during site selection and design rather than left until after planning permission has been granted.

When should professional advice be sought?

Planning and ecological advice should be obtained early where a site contains valuable habitat, the development may fall close to an exemption threshold, off-site units may be required or the proposal involves phased development. Advice may also be important when defining the application boundary, agreeing the habitat baseline and determining how long-term delivery will be secured.

Updated 29 July 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