Biodiversity net gain (BNG) means leaving wildlife habitat in a measurably better state than it was before a development. In England it is the law: most new development must deliver at least a 10% gain in biodiversity value, measured in biodiversity units with the statutory biodiversity metric and maintained for at least 30 years (GOV.UK).
This page gives the short definition. For the full explanation, read our complete guide to biodiversity net gain.
BNG in one sentence
BNG = 10% more biodiversity units after development than before, delivered on-site first, then off-site, then through statutory credits, and secured for 30 years. It is mandatory in England for most planning applications made since February 2024, with new exemptions for small sites from 6 August 2026.
The definition, in five parts
- Biodiversity is measured through habitat, not species. Each habitat on a site is scored for its area, distinctiveness, condition and strategic significance.
- Net means the total after development is compared with the total before, including all habitat inside the red line boundary.
- Gain means at least 10% more biodiversity units than the baseline. Local plans can ask for more.
- Delivery follows a hierarchy: avoid harm, improve habitat on-site, use registered off-site land, and buy statutory credits only as a last resort.
- Commitment means the new or improved habitat is legally secured and managed for at least 30 years.
Where BNG comes from
BNG is required by Schedule 7A of the Town and Country Planning Act 1990, inserted by Schedule 14 of the Environment Act 2021. It became mandatory on 12 February 2024 for major development and on 2 April 2024 for small sites. In 2026 the rules were amended: from 6 August 2026, new applications on sites of 0.2 hectares or less are exempt unless they affect priority habitat, and the self-build exemption was removed (SI 2026/790). From 2 November 2026, BNG also applies to nationally significant infrastructure projects (GOV.UK).
Go deeper
- Biodiversity net gain explained: the full guide, including the off-site market, credits and the evidence so far.
- BNG regulations 2026: the legislation, every exemption and how gains are secured.
- How the BNG metric works: the formula, the scores and a worked example.
- The statutory biodiversity metric: strengths and criticisms: what the research says.
Frequently asked questions
What does biodiversity net gain mean?
Biodiversity net gain means leaving the natural environment in a measurably better state than before a development. In England it is a legal requirement for most new development: a gain of at least 10% in biodiversity units, measured with the statutory biodiversity metric and maintained for at least 30 years.
Is biodiversity net gain a legal requirement?
Yes, in England. It became mandatory for major development on 12 February 2024 and for small sites on 2 April 2024, under Schedule 7A of the Town and Country Planning Act 1990, inserted by the Environment Act 2021. It applies to nationally significant infrastructure projects from 2 November 2026.
What is a biodiversity unit?
A biodiversity unit is the standard measure used for BNG. The statutory biodiversity metric calculates units from a habitat's area, distinctiveness, condition and strategic significance. A development must have at least 10% more units after development than before.
Who is exempt from biodiversity net gain?
Householder applications, permitted development, de minimis development and, for applications made from 6 August 2026, sites of 0.2 hectares or less and temporary development of 5 years or less, unless onsite priority habitat is affected. Self-build homes are no longer exempt for new applications.
Sources: Environment Act 2021, Schedule 14; GOV.UK, Understanding BNG; GOV.UK, exempt developments; SI 2026/790; GOV.UK, BNG for NSIPs.
