BNG exemption for brownfield sites

Mandatory BNG: a brief reminder

Subject to exemptions and transitional arrangements, mandatory BNG requires most development in England to deliver at least a 10% increase in biodiversity value against the pre-development value of on-site habitat. This can be achieved through on-site improvements, registered off-site gains or, as a last resort, statutory biodiversity credits. Where the statutory biodiversity gain condition applies, a biodiversity gain plan must be approved before development starts.

What is the current small-site exemption?

For planning applications made on or after 6 August 2026, development on sites with a red-line area of 0.2 hectares or less is exempt from mandatory BNG, provided it does not decrease the biodiversity value of onsite priority habitat. The exemption is not restricted to brownfield land or residential development and does not apply retrospectively to applications made before that date.

What has the Budget announced?

The Government has announced its intention to exempt residential brownfield developments on sites of up to 1.5 hectares in England from the mandatory BNG requirement. This is a targeted proposal for brownfield housing, not a general exemption for all development below that threshold.

The announcement follows the Government’s consultation earlier this year on a targeted exemption for residential brownfield development. The Budget confirms the intended site-size threshold, but the full consultation response and impact assessment are still awaited, as is the implementation timetable.

Potentially very good news, but the devil is in the detail

In our view, this could be very good news for developers of qualifying schemes. Removing mandatory BNG requirements could reduce the cost of securing biodiversity gains and the associated approval and long-term management requirements, helping to improve viability and simplify delivery.

However, the practical benefit will depend on the final rules. In particular, developers will need clarity on:

• Which sites qualify: how residential brownfield development will be defined, including the treatment of sites containing both previously developed and undeveloped land, and mixed-use schemes
• What safeguards apply: whether particular habitats will be excluded from the exemption or subject to additional requirements
• When the exemption starts: no commencement date has yet been confirmed
• How existing schemes are treated: whether there will be transitional arrangements, and what these mean for pending applications, existing permissions, and schemes already subject to BNG commitments

What should developers do now?

For now, the announcement should be treated as a potentially valuable change, not an exemption that can already be relied on. Continue to comply with the current rules and retain appropriate BNG allowances in project appraisals and programmes until eligibility and timings are clear.

Existing planning conditions, section 106 obligations, and offsite biodiversity unit contracts will also need separate consideration: the announcement does not automatically release those commitments. Nor would an exemption from mandatory BNG remove other applicable environmental protections or planning policy requirements.

We’ll be watching for the detailed response and implementing legislation. In the meantime, contact our Planning team to discuss what the proposed changes could mean for your development.