--- title: "Biodiversity Net Gain (BNG) Explained: What Developers Must Do" description: "Mandatory Biodiversity Net Gain in England: the 10% rule, the statutory metric, and what developers must evidence for planning permission." metaTitle: "Biodiversity Net Gain (BNG) Explained" category: Compliance date: 2026-07-19 dateModified: 2026-08-24 readingTime: 7 min read lead: 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. cardDescription: The 10% rule, the statutory metric, the 30-year requirement, the on-site/off-site hierarchy, and exactly what developers must evidence to get planning permission. ctaHeading: Calculate your biodiversity units with VerdCore ctaBody: VerdCore includes a Statutory Biodiversity Metric calculator alongside your carbon tracking — assess habitat units for area habitats, hedgerows and watercourses, model your 10% gain, and keep the evidence in one place. footerNote: This guide is general information, not legal, planning or ecological advice. Refer to the statutory biodiversity metric and current GOV.UK / Natural England guidance for definitive requirements. ogTitle: Biodiversity Net Gain (BNG) Explained for Developers ogDescription: "Mandatory BNG in England: the 10% rule, the statutory metric, the 30-year rule, and what developers must submit." faq: - q: What is Biodiversity Net Gain? a: Biodiversity Net Gain is a mandatory condition of planning permission in England requiring most new developments to deliver a minimum 10% measurable improvement in biodiversity, maintained for at least 30 years. - q: When did BNG become mandatory? a: 12 February 2024 for major developments and 2 April 2024 for small sites, under the Environment Act 2021, with Nationally Significant Infrastructure Projects following their own later timeline. - q: How is the 10% net gain calculated? a: Against your pre-development baseline biodiversity units using the statutory biodiversity metric, not against zero, so clearing a good-quality habitat first creates a deficit that then has to be made up, plus 10%. - q: What is the biodiversity gain hierarchy? a: On-site gains first, then off-site gains if 10% cannot be achieved within the development boundary, with statutory biodiversity credits bought from government as a deliberately unattractive last resort. --- ## 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**](https://www.legislation.gov.uk/ukpga/2021/30/contents); BNG became a **mandatory condition of planning permission** in England, per [gov.uk's biodiversity net gain guidance](https://www.gov.uk/guidance/biodiversity-net-gain). 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**. Like the increasing planning-authority requirement for a [Whole-Life Carbon Assessment](/resources/whole-life-carbon-assessments.html), BNG has moved from a nice-to-have to a hard condition of permission on larger schemes. ## 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: | Measured separately | Examples | | --- | --- | | **Area habitats** | Grassland, woodland, scrub, and similar | | **Hedgerows** | Field boundaries, tree lines | | **Watercourses** | Rivers and streams | Each category needs its own 10% gain — a surplus of area habitat units can't cover a hedgerow shortfall. > **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: 1. **On-site** — achieve the gain within the red-line boundary of your development (retained and enhanced habitats, new planting, green roofs, SuDS). 2. **Off-site** — if you can't hit 10% on-site, deliver or buy units on other land (your own or through the habitat market). 3. **Statutory biodiversity credits** are a **last resort**: credits bought from the government. These are deliberately priced to be the least attractive option.
Biodiversity gain hierarchy
1
On-site
Retained and enhanced habitats, new planting, green roofs, SuDS — within the development's red-line boundary.
2
Off-site
Deliver or buy units on other land, your own or through the habitat market, for whatever 10% you can't hit on-site.
3
Statutory credits
Bought from government — deliberately priced as the least attractive option, a genuine last resort.
Deliver as high up the list as possible — on-site units are cheaper and more defensible than buying your way out later.
## What developers must do — the practical steps 1. **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. 2. **Design to retain and avoid.** The cheapest units are the ones you never lose. Keep valuable habitats, hedgerows and trees where you can. 3. **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. 4. **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. 5. **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 — the same discipline a live [Environmental Management Plan](/resources/environmental-management-plans-explained.html) is built for, rather than a document filed once and forgotten. * **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](/resources/pas-2080-for-contractors.html), [PPN 06/21](/resources/ppn-06-21-explained.html)) **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 — which is exactly what VerdCore's [BNG reporting software](/bng-reporting-software.html) is built for.