Mandatory Biodiversity Net Gain: Exemptions and Solutions

Biodiversity net gain (BNG) became mandatory in England in February 2024,¹ under the Environment Act 2021.² To secure planning permission and be able to break ground, most new developments must deliver a minimum 10% increase in biodiversity value.

By March 2026, the BNG Federation was reporting more than 200 habitat banks and over 29,000 biodiversity units, created across 45 habitat types.³ Understanding which projects are exempt from BNG and which are not is crucial for developers of all sizes.

This guide examines where exemptions apply, where they do not, and how we can help if your development is not exempt.

What Biodiversity Net Gain Means

The purpose of BNG is to leave building sites and development locations in a measurably better state - at least 10% higher⁴ - than before work began.

BNG rules apply only to developments in England. For most developments they operate through planning permission and are enforced by local planning authorities (LPAs). Applications that are not exempt must include the pre-development biodiversity value and the completed metric. The biodiversity gain plan, which sets out how the 10% will be achieved, must be approved before work starts.⁵

For major development, on-site enhancements, such as habitat creation, must be prioritised. If that is not possible, developers can use off-site biodiversity units from companies like us or, as a last resort, buy statutory biodiversity credits, which Natural England sells on behalf of Defra.⁶ For non-major development, applications made on or after 6 August 2026 treat on-site and registered off-site gains as equal options.⁷

We can help identify the best way forward for your development project.

BNG Exemptions: Which Projects Qualify?

Most permanent developments that need planning permission are subject to mandatory BNG. But there is also a list of exemptions; categories of development that are excluded from the mandatory requirement.

Sites of 0.2 Hectares or Less

Planning applications made on or after 6 August 2026 are exempt if the site is 0.2 hectares (2,000 square metres) or less. The area is measured within the application's red line boundary. The exemption does not apply if the development impacts priority habitat on site.⁸

De Minimis

A development is exempt under the de minimis rule if it impacts less than 25 square metres of on-site habitat (a square of 5 metres by 5 metres), or less than 5 metres of on-site linear habitat, such as a hedgerow. Impact means a decrease in biodiversity value. The exemption does not apply if the development impacts priority habitat on site.⁹

Evidence may be needed to support a de minimis claim.¹⁰ There is no separate exemption for development that has no impact on habitat. The de minimis limits are the closest provision.¹¹

Householder Applications

Householder applications are exempt. A householder application is for development to an existing dwellinghouse, or within its curtilage for a purpose incidental to enjoying the dwelling, such as a home extension, conservatory or loft conversion.¹² It does not include a change of use or a change in the number of dwellings. An application to convert a house into flats is therefore not a householder application, and is not exempt on that basis.¹³

Temporary Development

From 6 August 2026, a development is exempt if the whole development is temporary and planning permission is granted for five years or less. Defra's announcement describes it as an exemption for land that is to be reinstated within five years or less.¹⁴ The exemption does not apply if the development impacts priority habitat on site.¹⁵ Successive permissions for the same temporary development cannot add up to more than five years.¹⁶

Self-Build and Custom Build

The self-build and custom-build exemption no longer applies to planning applications made on or after 6 August 2026. It still applies to applications made before that date, and to a variation applied for on or after that date where the original permission was exempt.¹⁷ A small self-build site may instead qualify under the 0.2 hectare exemption.

Applications Made Before BNG Began

BNG applies to planning permissions granted on applications made on or after 12 February 2024.¹⁸ Earlier applications are exempt. Minor developments were given longer: those with applications made before 2 April 2024 are also exempt.¹⁹

Variations to Earlier Permissions

A section 73 application varies an existing permission. If the original permission was not in scope of BNG, for example because it was applied for before 12 February 2024, BNG will not apply to the variation. If the original permission was subject to BNG, the biodiversity gain condition cannot be varied or removed through a section 73 application. A new biodiversity gain plan is needed only if the variation changes the post-development value in the approved plan.²⁰

The 0.2 hectare and temporary exemptions do not apply to a section 73 permission where the original application or grant came before 6 August 2026.²¹

Development That Creates Habitat for Another Development

Development is exempt if it creates or enhances habitat solely or mainly to meet the BNG requirement of another development.²²

High-Speed Rail

Development that is part of, or ancillary to, the high-speed rail network is exempt. That includes connections between the places listed in section 1(2) of the High Speed Rail (Preparation) Act 2013.²³

Urgent Crown Development and Development Orders

The Environment Act 2021 also exempts urgent Crown development, and development granted planning permission by a development order. Permitted development rights are an example of a development order.²⁴

Exemptions Still to Come

Defra has said it will introduce two further exemptions. As of November 2026, work on them is underway, neither is in force and no date has been confirmed. One covers development whose primary objective is to conserve or enhance biodiversity. The other is a targeted exemption for development that enhances parks, playing fields and public gardens.²⁵

Exemptions and Local Policy

Decision makers should not give weight to local policies that require biodiversity gains from development that is exempt under the statutory framework. Other local biodiversity policies may still apply.²⁶

Developments That Are Not Exempt from BNG

A development on a site of 0.2 hectares or less, a de minimis development or a temporary development that impacts priority habitat on site is not exempt from BNG.²⁷

Mandatory BNG also applies to:

• development that impacts 25 square metres or more of on-site habitat, or 5 metres or more of on-site linear habitat, where no other exemption applies²⁸
• new self-build and custom-build applications made on or after 6 August 2026, unless another exemption applies²⁹
• development that affects irreplaceable habitat, such as ancient woodland. Permission will only be granted in exceptional circumstances. The 10% gain applies to the rest of the site, and the loss of the irreplaceable habitat needs separate compensation agreed with the LPA³⁰
• nationally significant infrastructure projects with applications submitted on or after 2 November 2026³¹

If you are in any doubt, get a qualified professional to conduct a preliminary ecological appraisal to identify if mandatory BNG applies to your development. They will enable you to determine which exemptions could be available and what information the LPA is likely to need.

How BNG Exemptions Are Assessed in Practice

LPAs determine whether a development is subject to BNG or not. An applicant who believes the development is exempt must include a statement giving the reasons and identifying the exemption or transitional provision. A location plan showing the red line boundary is also required.³²

Applications that are not exempt must include the pre-development biodiversity value and the completed metric. The biodiversity gain plan follows after permission is granted, and the LPA has eight weeks to decide on it. The plan includes the completed metric calculation, plans showing on-site habitat before and after development, and a compensation plan if irreplaceable habitat is affected.³³

Biodiversity Gain Sites

Developers whose projects do not or cannot deliver the full 10% gain on site must deliver the rest off site, from their own land or by buying units from registered biodiversity gain sites. Statutory biodiversity credits are the last resort.³⁴

Each biodiversity gain site is assessed using the statutory biodiversity metric, assigned a defined number of biodiversity units, and listed on the public biodiversity gain sites register.³⁵

We advise landowners on creating biodiversity gain sites.

Sourcing the Right Biodiversity Solution for You

If you are unsure whether an exemption applies, check with your ecologist or your LPA before you apply. We can recommend trusted ecologists.

If you need support preparing a biodiversity gain plan or sourcing off-site biodiversity units, we can help. To buy units, you choose a habitat bank and the units you need, then pay a £150 deposit that holds your allocation for 30 days. Units are secured by a section 106 agreement or a conservation covenant, with management and monitoring plans in place.

If you are a landowner considering creating a habitat bank, our expert team can advise you on the options.

Email us at sales@greenshank-environmental.com or fill in this contact form and we'll get back to you as soon as possible.

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