Understanding Biodiversity Net Gain Legislation

The 1949 National Parks and Access to the Countryside Act,¹ passed by the Attlee government, began decades of legislation to strengthen protections for the country's biodiversity. The most recent addition is the biodiversity net gain (BNG) law, which became mandatory in 2024² and requires most developments in England to provide a minimum 10% increase in biodiversity value.

BNG is reshaping how projects are planned, assessed, and delivered, but it's gone further than previous efforts. It has also addressed one of the historic weaknesses of voluntary biodiversity offsetting schemes: the lack of robust, long-term legal security. BNG is now a mandatory and enforceable requirement, with direct consequences for developers, landowners, and local planning authorities who don't meet it.

This guide gets into the meat of BNG legislation, how it has already changed in the years since its launch, who enforces it, and what developers must know to remain compliant.

What Is Biodiversity Net Gain Legislation?

Part of the Environment Act 2021,³ BNG legislation covers most developments in England, and seeks to make conservation part of the planning process, rather than an afterthought.

It requires wildlife habitats to be left in a "measurably better state"⁴ than before work began, and mandates developers ensure a minimum 10% increase in biodiversity value (calculated using a statutory biodiversity metric) that is protected and maintained for at least 30 years.

Secondary legislation, known as statutory instruments, underpins that high-level requirement. It covers which projects are exempt from BNG, how the planning process was amended to include BNG conditions, and the fee structure and penalties tied to the biodiversity gain site register.

If you're not sure where to start, we can help guide you.

How Biodiversity Net Gain Law Works in Practice

BNG law requires developers to calculate the biodiversity value of a site before work begins and afterward, using the statutory biodiversity metric.⁵

It scores on-site and off-site habitats by size, distinctiveness, condition and strategic significance, and reports the result in three types of biodiversity unit: area habitats, hedgerows and watercourses.

The calculated difference between the habitat's value before and after development gives a number to the impact on biodiversity when the work is finished, and becomes a key part of a developer's biodiversity gain plan. It is sent to the relevant local planning authority and details how the 10% gain will be achieved for the development to comply with BNG legislation.

The Biodiversity Gain Hierarchy

BNG legislation requires developers to follow a specific path when seeking the 10% gain, in line with wider environmental planning law: avoid harm first, minimise unavoidable harm, and only then compensate for any residual loss.

Under the law,⁶ the Biodiversity Gain Hierarchy prioritises on-site biodiversity gains for major development. That means major developments must create biodiversity on site wherever possible, enhancing or creating on-site habitats within the development boundary. For non-major development, applications made on or after 6 August 2026 treat on-site and registered off-site gains as equal options.⁷

If the required gain can't be met on site, then the developer can consider off-site BNG options. They can only be sourced from registered habitat banks, listed on the biodiversity gain site register.

Statutory biodiversity credits, bought directly from the government, are the last product in the hierarchy. They can only be used if developers cannot secure enough on-site solutions, or off-site BNG through the market.

BNG does not apply to irreplaceable habitats.⁸ Instead, developers must reach a bespoke compensation approach with the relevant local planning authority.

Additionality and What Counts as Genuine Biodiversity Gain

Any increase in biodiversity value created on a development site must be additional to count towards the 10% requirement.⁹ Existing conservation commitments or other regulatory obligations unrelated to a specific development will not count.

The additionality principle prevents developers from claiming credit for environmental improvements that would have happened anyway, or from counting an existing conservation measure twice, ensuring that BNG drives new investment in habitat creation and enhancement.

Developers relying on off-site biodiversity gains must ensure their chosen solution is new or enhanced beyond any pre-existing baseline commitment. The local planning authorities will determine if the proposed habitat enhancements meet the legal requirements and are additional.¹⁰

The Biodiversity Gain Plan and the Planning Process

Developers must submit a biodiversity gain plan to the local planning authority (LPA), and it must be approved before development starts.

It should include:¹¹

• a completed metric tool calculation
• pre-development and post-development plans
• a compensation plan if irreplaceable habitats are impacted
• biodiversity net gain register reference numbers if off-site units are being used
• proof of purchase if any statutory biodiversity credits are needed
• details of how significant on-site gains will be monitored and managed

Local planning authorities have eight weeks¹² to approve a biodiversity gain plan from the point of a valid submission, although this can be extended if the developer and LPA agree.

Because BNG is a condition of planning permission, work on developments can't start until the biodiversity gain plan is approved.

Outline Planning Permission

Outline planning permission is granted in two stages: agreement in principle, then approval of the detailed 'reserved matters' later. Because BNG applies when permission is granted, approving reserved matters later does not trigger a fresh BNG requirement: that happened at the outline stage.¹³

Phased Developments

Phased developments require:

• an overall biodiversity gain plan
• a phase biodiversity gain plan¹⁴

Phased developments raise additional complexity, so developers should get specific advice on how BNG applies to their sequencing.

Section 106 Agreements and Conservation Covenants

BNG takes a long-term view: any habitat creation or enhancement that counts towards the 10% gain must be legally secured for at least 30 years.

There are two ways to achieve this: a Section 106 agreement¹⁵, under the Town and Country Planning Act 1990, or a conservation covenant under the Environment Act 2021.¹⁶

Section 106 agreements are a long-established mechanism for securing planning obligations via local authorities, including habitat management commitments, and are widely used for on-site biodiversity gain.

Conservation covenants are newer and were introduced specifically by the Environment Act 2021. More flexible than section 106 agreements, conservation covenants are particularly well-suited for off-site biodiversity gain sites, such as habitat banks.

They allow a landowner to make a private, voluntary, but nonetheless legally binding, agreement with a designated responsible body (a local authority, public body, charity or other organisation with conservation aims) to conserve a habitat, either for a defined term or for generations.

The Habitat Management and Monitoring Plan

Whether a developer uses a Section 106 agreement or conservation covenant, it must include a habitat management and monitoring plan (HMMP) compiled with an ecologist or competent professional.

This critical part of the BNG legal framework bolsters the section 106 agreement or conservation covenant by offering more detail about:¹⁷

• how off-site gains or significant on-site enhancements will be managed (including any legal restrictions and requirements)
• when and how habitats will be monitored
• when and how monitoring results will be reported
• when and how management proposals will be reviewed
• any changes to the way the habitat is managed to achieve specific goals

For off-site gains, the HMMP should also include details of the responsible body or local authority.

Developers of nationally significant infrastructure projects (NSIPs)¹⁸ must submit their draft HMMP with the outline biodiversity gain plan, including details of the reviewer (usually the local planning authority). If there are any changes to the on-site enhancements, the HMMP must be updated and submitted.

The Biodiversity Gain Site Register

Not every development can achieve the 10% gain through on-site habitat creation, but biodiversity net gain legislation has a solution for that too. Off-site BNG, via projects on the biodiversity gain site register.¹⁹

Maintained by Natural England on behalf of the government,²⁰ it lists hundreds of registered sites across England that have been formally approved to supply biodiversity units to help developers achieve their BNG requirements.

The biodiversity gain site register lists information on the habitat type, its condition, the number of biodiversity units available, and the legal mechanism (either a section 106 agreement or conservation covenant) that secures it.²¹

Trading rules built into the statutory biodiversity metric demand any BNG enhancements, whether on-site or off-site, to be of the same standard or higher than habitat lost during development. The biodiversity gain site register allows developers to find habitat solutions offering exactly what they need.

Statutory Biodiversity Credits

In situations where off-site biodiversity gains from the register still can't fully meet a development's 10% requirement, statutory biodiversity credits can be bought directly from the government.

These credits are deliberately more costly than typical off-site biodiversity units, reflecting their position at the bottom of the biodiversity gain hierarchy.

The intention is that developers should exhaust all on-site and off-site options before relying on statutory biodiversity credits to meet the requirement.

Compliance, Monitoring and Enforcement

Whoever monitors and enforces compliance with the approved biodiversity gain plans depends on the type of agreement underpinning it.

Any on or off-site gains covered by a section 106 agreement are overseen and enforced by the local planning authority. If a conservation covenant is in place, it falls to the responsible body.²²

Both can take action if the landowner does not meet their obligations over the 30-year period.

Exemptions from Biodiversity Net Gain Legislation

Biodiversity net gain requirements only apply to planning applications made after BNG became mandatory in 2024. Projects whose applications were made before then are not subject to BNG, although transitional provisions may apply to certain variations of existing permissions.

Current exemptions are:²³

• Householder development as defined within Article 2(1) of the Town and Country Planning (Development Management Procedure) (England) Order 2015.
• Development granted planning permission by a development order under section 59, including permitted development rights.
• Development subject to the de minimis exemption.
• Development related to the high speed railway transport network.
• Urgent Crown development granted permission under section 293A of the Town and Country Planning Act 1990.
• Development of a biodiversity gain site.

Exemption applies only to applications made on or after 6 August 2026 for:

• Temporary development that is permitted for a period of five years or less
• Development no larger than 0.2 hectares which does not impact an on-site priority habitat.

Biodiversity Net Gain Legislation and the National Planning Policy Framework

BNG legislation sits alongside the National Planning Policy Framework, which requires local plans to support and enhance biodiversity wherever possible.²⁴ Local planning authorities are also expected to have regard to Local Nature Recovery Strategies when assessing biodiversity gain plans.

They identify priority areas for habitat creation and restoration at a local level. The statutory biodiversity metric rewards habitat creation in these areas with a higher strategic significance score,²⁵ encouraging biodiversity gain to be deployed where it will have the most benefit.

Section 73 Applications and Biodiversity Net Gain

A common question developers raise with their local planning authority concerns Section 73 applications - requests to vary or amend an existing planning permission - and if these trigger a fresh biodiversity net gain requirement.

If the original permission was granted, or applied for, before BNG became mandatory in 2024, a Section 73 permission is outside BNG. If the original permission was subject to BNG, the biodiversity gain condition carries over and cannot be varied or removed.²⁶

A new biodiversity gain plan is needed only if the variation changes the post-development biodiversity value in the approved plan. Otherwise, the earlier plan stands. From 6 August 2026, the new small-site and temporary exemptions do not apply to a Section 73 permission whose original permission was applied for or granted before that date.²⁷

Developers considering a Section 73 application to an existing permission should seek specific advice on whether it is likely to trigger mandatory BNG requirements, given their specific circumstances.

Helping You Navigate a Changing Landscape

Biodiversity net gain legislation has already evolved from the 2024 launch, with changes to exemptions and more amendments on the way. Keeping up with this changing framework can feel like a full-time job. We can help.

Whether you're a developer or land manager seeking to understand how BNG legislation applies to your project, if you need support preparing a biodiversity gain plan or sourcing off-site biodiversity units, we can help.

Our team has practical experience of BNG legislation, conservation covenants and the biodiversity gain site register. We can guide you through every stage, from initial site assessment, through metric calculation and formal submission, to the long-term ongoing management of your habitat commitments and legal obligations.

Email us at sales@greenshank-environmental.com or fill in this contact form and we'll be in touch as soon as possible.

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