Biodiversity net gain, for small sites

What it is

Biodiversity net gain became a condition of most planning permissions in England for major development in February 2024 and for small sites in April 2024. The requirement is a 10% improvement in biodiversity value, measured with the statutory biodiversity metric against a baseline survey of the site as it is now, and secured for at least thirty years.

The gain is delivered in a hierarchy: on site first, then off site through units purchased from a habitat bank, and only as a last resort through statutory biodiversity credits bought from the government, which are deliberately priced to be unattractive.

What changed on 6 August 2026

Following the 2025 consultation on implementation for minor, medium and brownfield development, the government amended the regime. The two changes that matter most to small sites:

• A site area threshold. Development where the site area within the red line boundary is 0.2 hectares or less is exempt. This replaced the previous approach based on unit and floorspace thresholds, and it takes a large number of small infill and conversion schemes out of the regime entirely.
• The self-build and custom build exemption was withdrawn for applications made on or after 6 August 2026. Applications submitted before that date keep it.

Because the position depends on your application date, transitional arrangements matter. An application submitted in July 2026 and a materially identical one submitted in September 2026 can be in different regimes.

The other exemptions

• Householder development. Extensions, conservatories, loft conversions and the like at an existing house are exempt.
• The de minimis exemption. Development affecting less than 25 square metres of on-site habitat, or less than five metres of linear habitat such as hedgerow. It does not apply where priority habitat is affected.
• Applications made before 2 April 2024 for small sites, and before 12 February 2024 for major development.
• Various other categories including certain permitted development, urgent Crown development and some highways work.

What it means in practice on a small site

1. A baseline habitat survey by a competent ecologist, following UK Habitat Classification, before you submit. Habitat condition assessment is seasonal, which is the constraint that most affects programme.
2. A metric calculation showing the pre-development baseline and the post-development value, demonstrating at least 10% gain.
3. A biodiversity gain plan, submitted after permission is granted and approved by the Council before development may begin. This is the step people forget: BNG is secured by a pre-commencement condition, so an unapproved gain plan stops the job.
4. Securing the gain for 30 years, through a section 106 agreement or a conservation covenant where habitat is created or enhanced.

Where small schemes get caught

• Removing a hedgerow. Hedgerows are scored separately and are expensive to replace in metric terms. Retaining one is almost always cheaper than losing it.
• Building over the whole plot, leaving nowhere to deliver on-site gain and forcing an off-site purchase.
• Assuming a garden lawn is low value. Mown amenity grassland scores low, which helps; species-rich grassland does not, which does not.
• Leaving the survey until after refusal, then finding the only survey window has passed.
• Treating the gain plan as paperwork and discovering it is a pre-commencement condition.

How we help

We are planning consultants, not ecologists, and we do not pretend otherwise. What we do is tell you at the outset whether BNG applies to your site at all, which for a good number of small schemes it now does not; commission and co-ordinate the ecologist if it does; make sure the metric and the gain plan arrive at the right points in the process; and negotiate the condition wording so the gain plan does not become an unnecessary pre-commencement obstacle.