What is Biodiversity Net Gain?

BNG in plain English

Biodiversity Net Gain — usually shortened to BNG — is a legal requirement for most new developments in England. In simple terms, it means that any new building project must leave the natural environment in a measurably better state than before the development took place.

The improvement must be at least 10%. So if a housing development destroys habitat worth 100 biodiversity units, the finished development (plus any off-site habitat creation) must deliver at least 110 units in return.

This isn't just a guideline — it's a statutory planning condition. Developers cannot lawfully begin construction until their Local Planning Authority has approved a Biodiversity Gain Plan demonstrating how the 10% uplift will be achieved.

Why does BNG exist?

England is one of the most nature-depleted countries in the world. The 2023 State of Nature Report showed a 19% decline in the average abundance of wildlife species in the UK since the 1970s. Development has historically been a major driver of habitat loss, with no systematic requirement to compensate for the damage.

BNG was introduced under the Environment Act 2021 to reverse this dynamic. Rather than simply mitigating harm, developers must now deliver a net positive outcome for biodiversity. The policy creates a new tradeable commodity — biodiversity units — and a growing market of habitat banks, ecological consultancies, and compliance services.

Who does BNG apply to?

BNG applies to most new developments in England that require planning permission under the Town and Country Planning Act 1990. This includes:

• Residential developments (all sizes, from single homes to major housing estates)
• Commercial and industrial developments
• Mixed-use developments
• Infrastructure projects requiring planning permission
• From May 2026: Nationally Significant Infrastructure Projects (NSIPs)

Current exemptions

A small number of development types are currently exempt:

• Developments impacting less than 25 square metres of habitat (this threshold is increasing to 0.2 hectares in late 2026)
• Householder applications (extensions, loft conversions, etc.)
• Biodiversity gain site developments (the habitat creation itself)
• Permitted development rights (where no planning permission is required)
• Some self-build and custom housebuilding developments

Not sure if your development is exempt? Use our free BNG Exemption Checker to find out in under a minute.

How is biodiversity measured?

Biodiversity is measured in standardised biodiversity units using the DEFRA Statutory Biodiversity Metric 4.0. The metric assigns a numerical value to every piece of habitat based on several factors:

• Habitat type — what kind of habitat it is (grassland, woodland, wetland, etc.)
• Size — area in hectares, or length in kilometres for hedgerows and watercourses
• Distinctiveness — how ecologically rare or important the habitat is (Very High to Very Low)
• Condition — the current quality of the habitat (Good, Moderate, or Poor)
• Strategic significance — whether the habitat aligns with Local Nature Recovery Strategies

The metric produces a single number: the biodiversity unit value of the site. The developer must then show that their post-development habitat value exceeds the pre-development value by at least 10%.

The three ways to deliver BNG

Developers must follow a strict hierarchy — called the biodiversity gain hierarchy — when delivering their 10% uplift:

Developers can combine all three routes but must demonstrate they followed the hierarchy. Read the full guide to the mitigation hierarchy for worked examples and cost comparisons.

The 30-year management obligation

BNG habitats aren't just created and forgotten. Any habitat created or enhanced for BNG purposes — whether on-site or off-site — must be legally secured and managed for a minimum of 30 years.

This obligation is enforced through legal agreements:

• Conservation covenants — private agreements between a landowner and a designated responsible body
• Section 106 agreements — planning obligations between the developer and the Local Planning Authority

The landowner is legally responsible for creating or enhancing the habitat and managing it to achieve the target condition over the 30-year period. They must report progress to the responsible body or LPA.

What happens in practice?

Here's what the BNG process looks like for a typical development:

1. Pre-application: The developer engages an ecologist to survey the site and calculate its baseline biodiversity value using the statutory metric.
2. Design stage: The development is designed to avoid and minimise habitat loss where possible. On-site habitat creation is planned.
3. Planning application: The application includes biodiversity information: baseline value, proposed post-development value, and the BNG strategy.
4. Determination: The LPA assesses whether the 10% uplift is deliverable. Any S106 obligations for on-site or off-site gains are discussed.
5. Pre-commencement: A full Biodiversity Gain Plan is submitted and approved by the LPA. Off-site units are allocated from the register. Legal agreements are signed.
6. Construction: Development proceeds. On-site habitat measures are implemented per the gain plan.
7. Post-completion: Habitats are managed and monitored for at least 30 years, with regular reporting to the LPA or responsible body.

For detailed timelines, costs, and practical tips at each stage, see our stakeholder-specific guides for developers and landowners.

The BNG market in 2026

Two years into mandatory BNG, a functioning market for biodiversity units is taking shape:

• 197 registered gain sites are listed on the national register, up from just 46 a year earlier
• ~7,000 hectares of land is registered for BNG habitat creation
• ~28,000 habitat units are available nationally
• Only ~1,500 units have been sold so far — supply is healthy but demand is growing
• 202 of 309 LPAs still have no registered habitat banks, creating significant geographic gaps

Several major reforms are expected in 2026, including new exemptions, simplified compliance routes, and the extension of BNG to major infrastructure projects. Read our full guide to 2026 BNG reforms.

What should I do next?

Your next step depends on who you are:

• Developer? Read the Developer Compliance Guide for a step-by-step walkthrough, or check whether your site is exempt with the Exemption Checker.
• Landowner? Read the Landowner Guide to understand how to turn your land into BNG income.
• Lender? Read the Lender Guide for a BNG risk and due diligence framework.
• Planning authority? Read the LPA Guide for implementation best practice.
• Want to understand the metric? Continue to How the Biodiversity Metric Works.