What is mandatory 10% BNG
Mandatory under the Environment Act 2021, in force from Feb 2024 for major schemes and Apr 2024 for small sites. Each major planning permission carries a pre-commencement condition that the approved biodiversity gain plan must deliver at least a 10% net gain measured against the statutory biodiversity metric [Statutory Biodiversity Credits guidance].
Three delivery routes
- On-site: habitat creation or enhancement within the red-line site boundary.
- Off-site: purchased biodiversity units from a registered habitat-bank or landowner.
- Statutory credits: last-resort government-sold credits at a tiered, deliberately-expensive rate.
2026 reform package (in force 6 Aug 2026)
The 2026 amendments (SI 2026/790) are confirmed and in force for applications submitted on or after 6 Aug 2026: a 0.2ha small-site exemption, removal of the self and custom-build exemption, and a relaxed gain hierarchy for non-major development. Statutory credits remain a deliberate last resort, materially more expensive than market off-site units. See /bng/exemptions-2026.
30-year management plan in the deed
The S106 deed (or a unilateral undertaking) carries the 30-year habitat management and monitoring plan as a binding obligation. The plan typically includes a per-year monitoring fee to fund LPA officer time tracking habitat condition.