Since 2024, most new developments in England must deliver a measurable gain for nature — not just avoid harm. Here's what Biodiversity Net Gain actually requires, and how to evidence it without derailing your programme.
What is Biodiversity Net Gain?
Biodiversity Net Gain (BNG) is an approach to development that leaves the natural environment in a measurably better state than before. It's no longer a voluntary best practice under the Environment Act 2021; BNG became a mandatory condition of planning permission in England.
The headline requirement is simple to state: every applicable development must deliver a minimum 10% net gain in biodiversity, measured and maintained for at least 30 years.
When did it become mandatory?
BNG became a statutory requirement for most developments in England on a rolling basis:
- 12 February 2024 — major developments under the Town and Country Planning Act 1990.
- 2 April 2024 — small sites (minor developments).
- Later phases — Nationally Significant Infrastructure Projects (NSIPs) follow on their own timeline.
If your planning application falls in scope, you cannot discharge planning without demonstrating the 10% gain.
How biodiversity is measured — the statutory metric
You can't manage what you can't measure, so BNG is quantified using the statutory biodiversity metric (Natural England's metric, currently version 4.0). It converts habitats into biodiversity units based on:
- Size — the area of the habitat (or length, for linear features).
- Quality (condition) — how good the habitat is, from poor to good.
- Distinctiveness — how ecologically valuable the habitat type is.
- Strategic significance — whether it's in a location identified as a priority for nature.
Crucially, three things are measured separately, and each must not fall below baseline:
- Area habitats (grassland, woodland, scrub, etc.)
- Hedgerows
- Watercourses (rivers and streams)
The maths that catches people out. The 10% gain is calculated against your pre-development baseline biodiversity units not against zero. Clear a good-quality habitat and you create a large deficit you then have to make up, plus 10%. Surveying and protecting what's already there is almost always cheaper than recreating it.
The biodiversity gain hierarchy
Like the carbon reduction hierarchy, BNG has an order of preference to deliver as high up the list as possible:
- On-site — achieve the gain within the red-line boundary of your development (retained and enhanced habitats, new planting, green roofs, SuDS).
- Off-site — if you can't hit 10% on-site, deliver or buy units on other land (your own or through the habitat market).
- Statutory biodiversity credits are a last resort: credits bought from the government. These are deliberately priced to be the least attractive option.
What developers must do — the practical steps
- Survey the baseline. Commission a suitably qualified ecologist to assess existing habitats and calculate baseline units using the statutory metric. Do this early; it shapes your whole layout.
- Design to retain and avoid. The cheapest units are the ones you never lose. Keep valuable habitats, hedgerows and trees where you can.
- Calculate the gain. Model your post-development habitats and confirm you reach at least 110% of baseline units for each of the areas, hedgerows and watercourses.
- Prepare a Biodiversity Gain Plan. Set out the baseline, the proposed habitats, how you meet the 10%, and how gains are secured and maintained for 30 years.
- Secure it legally. On-site and off-site gains are secured through planning conditions, planning obligations (Section 106) or conservation covenants, with long-term management and monitoring.
Common mistakes that cost time and money
- Surveying too late. Ecological surveys are seasonal. Missing a survey window can delay a scheme by months.
- Clearing habitat before baselining. Removing habitat before the baseline is agreed upon can mean it's still counted as if present (the "degradation" rule) or simply lands you with a bigger deficit.
- Ignoring the 30-year commitment. BNG isn't a one-off at planning; it's a three-decade management and monitoring obligation that must be resourced and funded.
- Treating it as separate from carbon. Low-carbon, nature-positive design decisions often overlap; the teams and data should talk to each other.
How BNG and carbon fit together
BNG and carbon reporting are separate regimes, but they pull in the same direction and increasingly land on the same desk. A contractor or developer who can evidence both a credible carbon record (PAS 2080, PPN 06/21) and a compliant Biodiversity Gain Plan is telling clients and planners a coherent sustainability story and doing it from one set of project data rather than two disconnected spreadsheets.