Is Biodiversity Net Gain Mandatory? What Developers Need to Know

Is Biodiversity Net Gain mandatory in England?

Yes: Biodiversity Net Gain is mandatory for most development within the English statutory planning framework, subject to exemptions and transitional arrangements. The minimum required gain is 10%, measured through the statutory biodiversity metric. Mandatory BNG commenced for major development on 12 February 2024 and for small development on 2 April 2024. Those dates concern the relevant application and transitional rules, not simply the day construction begins.

At Collington Winter Environmental, we start by checking the permission route, application history, development description and ecological baseline. This avoids two common mistakes: assuming that every project needs exactly the same BNG documents, and assuming that a small or previously developed site is automatically exempt.

Key takeaways for developers

• Most in-scope planning permissions carry a statutory general biodiversity gain condition.
• The biodiversity gain plan must be approved before development begins.
• Small development is not the same thing as exempt development.
• Exemptions have specific legal tests and should be evidenced.
• Mandatory BNG is additional to protected-species and designated-site requirements.
• Local policy and other planning obligations can remain relevant even where the statutory condition does not apply.

Where does the obligation come from?

The Environment Act 2021 introduced the BNG framework into the Town and Country Planning Act 1990 through Schedule 7A. For a planning permission within scope, the general biodiversity gain condition requires approval of a biodiversity gain plan before commencement. The statutory biodiversity gain objective compares pre-development biodiversity value with the compliant post-development position, including eligible off-site allocations or statutory credits where applicable.

The minimum is a 10% increase, not a requirement to landscape 10% of the application site. It is also not satisfied simply by replacing every removed tree. The biodiversity metric assesses habitats and applies distinctiveness, condition, strategic significance and relevant risk factors. Our guide to biodiversity metric calculations explains why the target depends on the baseline rather than on a standard planting allowance.

Local plans may contain additional biodiversity policies, including locally justified expectations above the national minimum. Their application needs to be considered in the relevant planning context. A developer should distinguish the statutory minimum from additional policy requirements rather than assuming the two are interchangeable.

Application dates and transitional arrangements

For major development, the mandatory regime generally applies to relevant applications made on or after 12 February 2024. For small development, the corresponding commencement date is 2 April 2024. Permissions linked to applications before those dates may fall outside the mandatory framework under transitional provisions.

Section 73 applications require particular care. A variation of an earlier permission does not automatically bring an otherwise transitional permission into mandatory BNG, and special provisions govern how the original permission is treated. Conversely, a fresh application should not be assumed to inherit the position of an older permission simply because it concerns the same land.

Outline permissions, reserved matters and phased schemes also need their application history checked. For phased development, the framework includes overall and phase biodiversity gain plan requirements. The correct answer depends on the legal route and facts, not just the label used on a project programme. Nationally significant infrastructure has a separate regime and timetable; the dates above should not be applied to it indiscriminately.

Which developments can be exempt?

The following explains the principal exemptions in the framework introduced in 2024. Exemption rules and commencement provisions should always be checked against the legislation and guidance applicable when the application is made, particularly where policy changes are proposed or introduced.

Householder applications

Householder applications are exempt from the mandatory general biodiversity gain condition. These commonly include qualifying domestic extensions and similar works within the defined householder application category. The exemption is tied to the legal application type, not merely to a project taking place near a house. A separate new dwelling is not automatically a householder proposal.

Qualifying self-build and custom-build development

The self-build and custom-build exemption has cumulative limits. Under the 2024 framework, development must comprise no more than nine dwellings, occupy a site no larger than 0.5 hectares, and consist exclusively of dwellings meeting the legal self-build or custom-build definition. A small speculative housing scheme is not exempt merely because purchasers might choose finishes.

The application should explain how the criteria are met and provide the evidence required by the authority. Ownership, procurement and delivery arrangements can matter. A scheme should not rely on an informal description of itself as self-build without considering the statutory definition.

The de minimis exemption

Under the 2024 exemption rules, development must not affect priority habitat and must affect less than 25 square metres of on-site area habitat with biodiversity value greater than zero, and less than five metres of relevant on-site linear habitat. The thresholds are strictly less than, not “up to and including”. Where both types are present, assess both limits rather than treating them as alternative allowances.

The relevant measure is habitat impact, not simply the building footprint. Access works, drainage, excavation and other construction effects can matter. A small red-line site is not automatically de minimis, while a proposal largely affecting sealed surfaces may need a different analysis. Accurate habitat mapping is a better starting point than assumptions based on site appearance.

Biodiversity gain sites and other routes

Development undertaken solely or mainly to fulfil the BNG condition for another development can fall within the biodiversity gain site exemption. This supports habitat delivery; it does not create a blanket exemption for any development on land described as a habitat bank. Qualifying high-speed railway development also has a specific exemption in the original framework.

Permitted development and retrospective permissions under section 73A have distinct treatment and should be checked separately. Exemption lists should not replace review of the actual permission route. Our plain-English BNG guide provides the broader ecological context for these legal distinctions.

What evidence belongs with the planning application?

If BNG applies, the application normally needs information identifying the pre-development biodiversity value, the relevant date and habitat evidence, alongside information about irreplaceable habitats and any applicable degradation history. The exact validation requirements should be checked with the planning authority. A clear draft delivery strategy helps demonstrate that the layout can achieve a compliant result.

If exemption is claimed, explain which exemption applies and why. Useful evidence may include a habitat plan, impact measurements, photographs or self-build supporting information, depending on the claim. An unsupported exemption statement can create avoidable validation or determination delays.

The biodiversity gain plan is normally approved after planning permission but before commencement. That does not make BNG an issue that can safely be postponed until after the design is fixed. A site unable to meet trading rules or accommodate credible habitats may need substantial redesign.

Off-site units, credits and starting work

Where on-site measures cannot deliver the required result, eligible off-site gains can contribute, subject to the biodiversity gain hierarchy. These must be appropriately secured, registered on Natural England's Biodiversity Gain Sites Register and allocated to the development. A reservation agreement alone is not the same as a completed allocation supporting an approved gain plan.

Government statutory biodiversity credits are the last resort, not the same product as privately sold biodiversity units. Their use must follow the relevant rules and evidence requirements. Before commencement, developers should check that the gain plan has actually been approved and that other pre-commencement conditions are also satisfied. Our guide to how to achieve biodiversity net gain connects these legal milestones to the practical project programme.

Exemption does not mean no ecology obligations

Protected species, designated sites, trees, hedgerows and local planning policy may still require surveys, mitigation or consent. A householder extension affecting a bat roost illustrates the distinction: exemption from statutory BNG would not disapply species protection. Likewise, a numerical net gain cannot justify harm that is unacceptable under another part of the planning or legal framework.

We therefore assess mandatory status separately from the broader ecological scope. Recording both decisions clearly helps applicants understand which documents are necessary, which requirements arise from law or policy, and which assumptions need to be revisited if the development changes.

Legal security, management and evidence

Biodiversity gains need more than a planting schedule. Significant on-site habitat enhancements and registered off-site gains must be secured for a minimum of 30 years. For off-site gains, the period runs from completion of the habitat enhancement works. Delivery arrangements should identify the land, the management obligations, the responsible parties, the monitoring programme and the remedies if habitat targets are missed. A section 106 planning obligation or conservation covenant can secure off-site delivery. Conservation covenants are agreements with designated responsible bodies; they are not simply private promises between a seller and buyer.

A Habitat Management and Monitoring Plan, often called an HMMP, translates the ecological proposal into practical tasks. It should describe the starting condition, intended habitat, establishment work, annual management, monitoring methods and measurable milestones. It should also explain how management will change if monitoring shows that the habitat is not developing as expected. Funding, access rights and responsibility after a land sale deserve attention before the documents are signed.

Registration is not the same as ecological approval

Natural England operates the Biodiversity Gain Sites Register for off-site gains. Registration and allocation link secured habitat gains to a development, helping prevent the same units being allocated twice. Register entry does not replace the developer's responsibility to demonstrate compliance, the local planning authority's approval of the biodiversity gain plan, or the land manager's duty to deliver the habitat. Ordinary on-site gains are not registered through this off-site register.

Read the metric alongside wider environmental duties

BNG does not replace protected-species legislation, designated-site protection, environmental assessment or other planning requirements. A scheme can show a numerical uplift and still be unacceptable because it damages an important ecological feature or fails another legal test. Irreplaceable habitats, such as ancient woodland, have special arrangements: losses are not compensated through ordinary metric trading, and bespoke compensation does not make an otherwise unacceptable loss acceptable.

Local Nature Recovery Strategies provide important spatial context. Where applicable, their mapped priorities inform strategic significance in accordance with metric guidance. They can also help identify better-connected habitat projects, but do not automatically make any planting scheme suitable. We consider ecological function, practical management and planning evidence together rather than treating the spreadsheet as the whole assessment.

Talk to our ecologists

Call 01204 939 608 or send us a message using the form on this page for an initial discussion about your site.