Where does the requirement come from?
Mandatory biodiversity net gain sits in Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021. It applied to major development from 12 February 2024 and to small sites from 2 April 2024. The mechanism is a deemed pre-commencement condition: every permission it applies to is treated as carrying a condition that development may not begin until the authority has approved a biodiversity gain plan. There is no separate application to make, and the 10% is not negotiable downwards.
What changes on 6 August 2026?
Regulations laid before Parliament on 13 July 2026 (SI 2026/790) bring in the first batch of changes announced in Defra's April 2026 consultation response. From 6 August 2026:
- development on a site of 0.2 hectares or less is exempt, unless on-site priority habitat is negatively affected;
- a new exemption covers temporary development where land is reinstated within five years, subject to the same priority habitat carve-out;
- minor development that is not exempt may go straight to off-site units rather than working through the on-site hierarchy first;
- the existing self-build and custom build exemption is removed.
The changes apply to applications made from 6 August. Applications already submitted, and permissions already granted, remain under the current rules. Government analysis puts the effect of the 0.2 hectare exemption alone at roughly half of the residential permissions that would otherwise have required BNG.
How is the gain measured?
Baseline and post-development biodiversity are both calculated in biodiversity units using the statutory biodiversity metric, which scores habitat by distinctiveness, condition and strategic significance, and adds risk multipliers for the time created habitat takes to reach its target condition. Two consequences follow for site selection. Higher distinctiveness habitat is expensive to lose, because it can only be replaced like for like or better. And a parcel that reads as low value on an aerial photograph can score higher than expected once condition is assessed on the ground, so a metric run on assumed habitat types is indicative rather than settled.
What does the hierarchy require?
The order of preference is on-site gain first, then off-site units registered against the site, then statutory biodiversity credits bought from government as a last resort. Credits are deliberately priced above the market rate for off-site units, so a scheme relying on them is usually one whose layout has not been tested against the metric. Whichever route is used, the gain must be secured and maintained for at least 30 years through a planning obligation or a conservation covenant, which carries a management and monitoring cost sitting well outside the build programme.
Why test this at site-finding stage?
BNG changes developable area rather than simply adding a cost line. Land given over to on-site habitat creation cannot also carry plots, and habitat retained for its baseline value constrains layout. The practical question on a constrained site is how many units survive once the metric is satisfied, and that is answerable in outline from habitat mapping and aerial imagery long before a site is bought. Sites that fail this test tend to fail late, once the layout has already been drawn.
Common questions
Last updated 24 July 2026.