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The Biodiversity Gain Hierarchy

Overview​

The biodiversity gain hierarchy is the ordered set of choices a development must work through when it delivers its statutory Biodiversity Net Gain (BNG). It is not a menu. It is a sequence, and you are expected to exhaust each step before you are allowed to reach for the next:

  1. Avoid harm to existing habitat — especially medium, high, and very high distinctiveness habitat — through site design.
  2. Enhance the habitats you are keeping on-site.
  3. Create new habitat on-site.
  4. Deliver off-site biodiversity units (on your own land elsewhere, or bought from a habitat bank / the market) for whatever gain you cannot achieve on-site.
  5. Buy statutory biodiversity credits from the government — explicitly the last resort.

Every development in scope must reach at least a 10% net gain in biodiversity units against its pre-development baseline, and hold that gain for at least 30 years. The hierarchy governs how you are permitted to get there.

Why it matters for BNG​

The 10% figure gets the headlines, but the hierarchy is what actually shapes a scheme. Two developments can both hit "+10%" and be judged completely differently, because BNG is designed to make the cheap, easy route — paying to offset habitat you flattened — the last route, not the first.

The practical consequences:

  • Distinctiveness drives everything. Losing a high-distinctiveness habitat and replacing it with an off-site meadow is not a like-for-like trade. The metric's trading rules and the hierarchy together push you to retain what is hard to recreate. This is where dataset quality bites: whether a parcel is scored as high or low distinctiveness depends on the habitat data you feed the metric.
  • On-site beats off-site beats credits — on cost and on acceptability. Statutory credits are priced deliberately high to discourage their use, and a planning authority can reasonably ask why you did not do more on-site first.
  • You must show your working. Reaching for off-site units or credits requires demonstrating that the earlier steps were genuinely maximised, not skipped.

How it works — England​

BNG became mandatory for most major developments in November 2023, and for small sites in April 2024, under the framework introduced by the Environment Act 2021, which inserted the biodiversity gain condition into the planning system (Schedule 7A to the Town and Country Planning Act 1990). Gain is measured using the statutory biodiversity metric published by Natural England and Defra.

The hierarchy operates at two moments:

  • At design stage, through the mitigation-hierarchy logic baked into the metric's trading rules — you cannot freely swap a valuable habitat for a common one and still score well.
  • At delivery stage, through the order of preference above: on-site enhancement and creation first, then off-site units, then credits.

Off-site gains and statutory credits are tracked so that the same unit cannot be sold twice: off-site gains are secured through legal agreements and recorded on the Biodiversity Gain Sites Register, and credits are sold by the government as the backstop.

Statutory citation — confirm before relying on it

The steps of the hierarchy are well established in practice and in Defra guidance, but the exact statutory/regulatory anchor is worth citing precisely. The gain condition sits in Schedule 7A to the Town and Country Planning Act 1990 (inserted by the Environment Act 2021); the ordered hierarchy is expressed through the Town and Country Planning (Development Management Procedure) (England) Order 2015 as amended and the statutory metric. Verify the precise article references against legislation.gov.uk before quoting them in an advisory context.

Nation differences​

BNG's mandatory 10%/30-year regime is an England-only regime. The other three nations pursue biodiversity gain through policy and duties rather than an equivalent statutory metric — so the hierarchy as described above does not transplant cleanly.

  • Wales — No statutory BNG metric or 10% requirement. Development is expected to deliver a net benefit for biodiversity through the "step-wise approach" in Planning Policy Wales, backed by the section 6 biodiversity duty of the Environment (Wales) Act 2016. Same direction of travel, different mechanism — no unit currency.
  • Scotland — No mandatory BNG. National Planning Framework 4 (NPF4) requires development to secure positive effects for biodiversity, applying a mitigation-hierarchy logic, but without a required percentage or a statutory metric.
  • Northern Ireland — No BNG regime. Biodiversity is handled through the biodiversity duty in the Wildlife and Natural Environment Act (Northern Ireland) 2011 and planning policy, not a gain metric.

The takeaway for a cross-border practitioner: the idea of avoid-first, compensate-last is broadly shared, but only England turns it into a numeric, tradable, register-tracked obligation.

  • Priority Habitat Inventory (PHI) — a primary input to distinctiveness scoring at step 1. Whether the metric "sees" a valuable habitat you are meant to avoid often depends on whether PHI mapped it.

WildStack's take​

WildStack's take

The single most common confusion we see is people treating the mitigation hierarchy ("avoid, mitigate, compensate" — a general ecological-impact principle) and the biodiversity gain hierarchy (the statutory delivery order above) as the same thing. They rhyme, but they are not interchangeable: the mitigation hierarchy is about reducing ecological harm; the gain hierarchy is about how you are allowed to deliver the required net gain units. A scheme can follow the mitigation hierarchy impeccably and still be pushed up the gain hierarchy toward off-site units — because they answer different questions.

Our second observation: the hierarchy is only as honest as the baseline it starts from. "Avoid" presumes you know what is there to avoid. On a desk-based assessment, that knowledge is only as good as the habitat datasets — and a habitat that PHI or Living England never mapped is a habitat the metric will happily let you lose for free. The hierarchy does not protect what the data cannot see. That is exactly why we are opinionated about which datasets carry which confidence.

Official sources​

Last reviewed​

5 July 2026. This page will need revision if: the mandatory BNG regime is extended (e.g. to Nationally Significant Infrastructure Projects), the statutory metric version changes in a way that alters trading rules, or any of Wales, Scotland, or Northern Ireland introduces a statutory gain metric of its own. The exact DMPO article references in the "How it works" section should be confirmed against legislation.gov.uk at next review.

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