Biodiversity net gain for small development sites in England

The biodiversity net gain rules for small developments changed on 6 August 2026. A developer relying on a 2024 checklist may now be budgeting for the wrong process, or overlooking an obligation that still applies.

The first step is to check the application date, the complete site area and the habitats affected. Being a small developer, proposing only a few homes or using a brownfield site does not by itself establish an exemption.

Check the new exemption carefully

For new planning applications made on or after 6 August 2026, development on a site no larger than 0.2 hectares can qualify for an exemption from the statutory biodiversity gain condition. This is 2,000 square metres, measured within the application’s red line boundary. The exemption does not apply where the development adversely affects on-site priority habitat. Both the area and habitat tests matter. See the government’s exemption guidance.

The new exemption is not a retrospective release for every existing small-site permission. The regulations exclude applications made before 6 August 2026 from this new area-based exemption. Check the original application and permission history, especially where a variation is proposed. See the 2026 amendment regulations.

In practical terms, commission a measured boundary check and obtain suitable ecological advice where habitat classification is uncertain. Do not treat the building footprint as the site area or assume that neglected land has little ecological value.

Do not rely on the former self-build exemption

The separate self-build and custom-build exemption no longer applies to new planning applications made on or after 6 August 2026. Transitional arrangements remain relevant to earlier applications and variations of qualifying exempt permissions. A new self-build proposal may qualify under another exemption, but its description alone no longer settles the question. Check the updated exemption guidance.

The de minimis exemption also remains relevant. Its habitat-impact thresholds are different from the new whole-site area test. If relying on that route, check the precise tests and the evidence expected by the local planning authority rather than substituting the total area of the site for the area of habitat affected.

Work out what a non-exempt scheme must provide

For developments within the statutory regime, the biodiversity gain objective is at least a 10% increase against the pre-development biodiversity value. The calculation uses habitat value, not simply the percentage of the site covered with planting. A Biodiversity Gain Plan must be approved before development commences where the statutory condition applies. Read the planning practice guidance.

Start the calculation while the layout can still change. The statutory metric considers factors including habitat type, area, condition and strategic significance, as well as delivery risks for proposed habitats. A planting proposal that looks generous on a plan may not provide the required units. See the statutory metric guidance.

Some eligible minor developments can use the Small Sites Metric. It is a simpler calculation route with its own eligibility criteria, not an exemption from delivering the required gain. Have the person preparing the assessment confirm that the tool is appropriate to the actual habitats and proposal.

Compare on-site and off-site options early

Since 6 August 2026, non-exempt small or minor developments can consider on-site and off-site provision together, without first having to pursue on-site provision ahead of off-site. Statutory government credits remain a last resort. The changes do not justify ignoring avoidable habitat loss. Read the current explanation of BNG delivery options.

For a compact site, compare the layouts and cost implications with ecological advice. An on-site area must be capable of achieving the intended habitat condition alongside access, gardens, drainage and future management. For off-site provision, obtain a quotation matched to the required unit types and confirm the registration and allocation arrangements. A headline price per unit may not describe what your particular scheme needs.

Budget for assessment, design coordination, legal arrangements where required, management and monitoring, as well as any unit purchase. Significant on-site gains and off-site gains normally need to be secured and maintained for at least 30 years. Agree responsibility and costs before promising a landscape arrangement that future owners will inherit. See the government’s explanation of habitat maintenance.

Test two examples against the rules

These are illustrative situations, not Horizon case studies.

A developer submits a four-home application in September 2026 on a 0.18-hectare site. If the full red line area is correctly measured and the development does not adversely affect on-site priority habitat, the new area-based exemption may apply. The same proposal on the same site, submitted in July 2026, cannot rely on that new exemption simply because its decision arrives later.

Another developer proposes six homes on 0.30 hectares. The unit count does not make the site eligible for the 0.2-hectare exemption. The team must check whether another exemption applies. If none does, it should plan for the statutory BNG process and check whether the Small Sites Metric and minor-development delivery flexibility are available.

In both cases, record the reasoning in the application material. A clearly evidenced position is easier to assess than a bare assertion that the site is exempt.

Keep ecology in the project even where BNG is exempt

An exemption from mandatory BNG does not remove protected-species legislation, habitat protections or relevant planning policies. Surveys, mitigation and suitable biodiversity measures may still be required. Local authority guidance explicitly distinguishes these obligations. See Wealden’s explanation of the August changes.

Before site clearance or committing to a layout, agree the ecological information needed and the scope of any specialist input. Then keep the habitat plan coordinated with the architectural, drainage and landscape proposals.

If you are preparing a small development, contact Horizon Architecture to discuss how the architectural layout and submission should be coordinated with the project’s ecological advice. Early agreement on the applicable route can make the next design stage clearer.

England guidance checked 6 November 2026. Rules, local policies and transitional arrangements should be rechecked for the particular application. This article is general information and does not replace ecological or planning advice.