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What is Biodiversity Net Gain?

Overview​

Biodiversity Net Gain (BNG) is a legal requirement, in England, for most new development to leave biodiversity measurably better off than before — by at least 10% — and to secure that improvement for at least 30 years.

"Measurably" is the key word. BNG turns habitat into a currency: land is scored in biodiversity units using a standard government calculation (the statutory biodiversity metric), both before development (the baseline) and after (the post-development and any off-site gains). The scheme must show a net increase of 10% in units across that comparison.

Why it matters for BNG​

BNG reframes an old, vague planning expectation ("consider biodiversity") into a numeric, enforceable, tradable obligation. Three consequences flow from that:

  • Your baseline is money. Every unit of habitat you start with, and its condition and distinctiveness, changes the number you must beat. Getting the baseline right is not paperwork — it is the foundation of the whole calculation.
  • Data quality becomes commercially material. Whether a parcel is scored high or low distinctiveness depends on the habitat datasets behind it. That is why WildKnowledge cares so much about how datasets are made.
  • It has an order of operations. You cannot simply flatten a site and pay to offset. The biodiversity gain hierarchy forces on-site retention and enhancement first, off-site gains next, and government credits only as a last resort.

How it works — England​

BNG was introduced by the Environment Act 2021 (which inserted the biodiversity gain condition into the planning system via Schedule 7A to the Town and Country Planning Act 1990) and commenced in stages:

  • 12 November 2023 — mandatory for most major developments.
  • 2 April 2024 — extended to small sites.

The mechanics, in outline:

  1. Establish the baseline. Survey (or, for a desk-based assessment, model) the existing habitats and score them in units using the statutory metric.
  2. Design to the gain hierarchy. Avoid harm, then enhance and create habitat on-site, then deliver off-site, then — only if unavoidable — buy statutory credits.
  3. Reach ≥10% net gain in units against the baseline.
  4. Secure it for 30 years. On-site gains through planning conditions and a Habitat Management and Monitoring Plan (HMMP); off-site gains through a legal agreement and entry on the Biodiversity Gain Sites Register.

A statutory exemption list applies (e.g. some very small householder developments), and the Small Sites Metric is a distinct, simplified route for qualifying small sites.

Scope note

Some development types (e.g. Nationally Significant Infrastructure Projects) sit outside the initial mandatory regime and have been subject to separate timetables. Confirm current scope on GOV.UK before advising on a specific project type.

Nation differences​

BNG's mandatory 10% / 30-year metric regime is England-only. The devolved nations pursue biodiversity gain through policy and statutory duties, not an equivalent unit metric — see the fuller breakdown on the gain hierarchy page. In short: Wales (net benefit for biodiversity via Planning Policy Wales + Environment (Wales) Act 2016 s.6), Scotland (positive effects for biodiversity via NPF4), and Northern Ireland (biodiversity duty via the WANE Act (NI) 2011) share the ambition but not the machinery.

WildStack's take​

WildStack's take

BNG's real innovation is not the 10% — it is forcing biodiversity into a number that can be audited. That is a genuine improvement, but it has a failure mode: a number looks authoritative even when the evidence beneath it is thin. A baseline built on low-confidence modelled land cover produces a precise-looking unit figure that could be badly wrong.

Our position: the honest output of a desk-based BNG assessment is often not a final number but a clear split — the habitats we can confidently baseline from survey-grade data, versus the ones that need boots on the ground before the number means anything. BNG rewards precision; good practice refuses to fake it.

Official sources​

Last reviewed​

5 July 2026. Will need revision if the mandatory regime is extended to new development types (e.g. NSIPs), the metric version changes, the 10%/30-year figures are amended, or the exemptions list is updated.

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