Biodiversity net gain: Government confirms major relaxation for smaller and brownfield developments

Mandatory BNG was introduced as a major reform of the planning system, requiring most developments in England to deliver at least a 10% uplift in biodiversity value. While there has been positive progress – including good practice guidance, trained practitioners and the growth of a national off-site biodiversity unit market – early implementation also exposed some challenges.

In particular, evidence from local planning authorities, developers and consultants showed that:

• BNG, whilst key for protecting the environment, can be disproportionately costly and complex for smaller sites.
• Administrative burdens are often highest where biodiversity gains are lowest.
• Brownfield sites raise particular technical and viability issues.
• Negotiations over on-site delivery can delay planning outcomes.

The 2025 consultation, therefore, focused on targeted reforms aimed at simplifying BNG, without undermining its significant role in nature recovery.

The most significant reform is the introduction of a new area-based exemption for development with a site area of 0.2 hectares or less.

• Developments on sites of 0.2 hectares or below will no longer be required to deliver BNG.
• The exemption will not apply where priority habitats are present on site, regardless of site size.
• Government expects around 50% of residential permissions that would previously have required BNG to become exempt.

This marks a fundamental shift. Previously, BNG exemptions were complex and use-based. For the first time, an area threshold will apply across development types, removing the smallest schemes entirely from the BNG system.

For small developers, SME housebuilders and landowners promoting infill and minor residential schemes, this change removes the cost of metric calculations, the need for legal agreements securing habitat delivery and potential viability risks associated with small on-site biodiversity features. It should also ease pressure on local authority ecology resources, allowing them to focus on larger schemes where more meaningful biodiversity gains can be achieved.

Alongside the new area-based exemption, the government will remove the existing self and custom build exemption.

In practice, this is likely to have a limited negative effect. Many self/custom build schemes will now fall within the new 0.2 hectare threshold and be exempt on that basis. Where they do not, the government considers that they should contribute to biodiversity outcomes in the same way as comparable development.

The government has confirmed that regulations will introduce new targeted exemptions for specific categories of development, including:

• Development whose primary objective is to conserve or enhance biodiversity.
• Temporary planning permissions granted for a maximum of 5 years.
• Development enhancing parks, playing fields and public gardens.

These exemptions respond to concerns that certain forms of development were being unintentionally penalised by BNG, despite delivering clear public or environmental benefit.

For landowners and promoters of meanwhile uses, park improvements or biodiversity-led schemes, this provides welcome clarity and reduces regulatory friction.

For minor development only (where not otherwise exempt), the biodiversity gain hierarchy will be amended so that:

• Off-site biodiversity gains are placed on the same footing as on-site habitat creation and enhancement.
• Statutory biodiversity credits will remain a last resort.

This reform is designed to remove uncertainty and delay, particularly where:

• On-site delivery would result in token or impractical habitat features.
• Local planning authorities have been reluctant to accept off-site solutions.
• Negotiations over landscaping details have delayed decisions.

Developers will still be able to source off-site units from anywhere in England (subject to the spatial risk multiplier which may affect the number of units required), supporting growth of the national biodiversity market and providing greater choice and price competition.

For many minor schemes, especially in constrained urban areas, off-site delivery may now be the most efficient and commercially sensible route.

The government has acknowledged that BNG on brownfield land can be more complex and disproportionately costly, particularly where open mosaic habitat (OMH) is present. As a result, Defra is undertaking a further consultation on a targeted exemption for residential brownfield development. This will:

• consider appropriate definitions of brownfield land; and
• test potential exemption thresholds, up to 2.5 hectares.

Depending on the outcome, further secondary legislation later in 2026 may introduce:

• A new targeted brownfield residential exemption.
• Changes to the de minimis threshold for BNG.
• Further refinements to ensure proportionality.

This is an area developers and landowners should watch closely, particularly where brownfield constraints have been a barrier to site promotion.

Subject to parliamentary scheduling, Defra intends to bring forward secondary legislation before the summer recess 2026, with changes expected to come into force before 31 July 2026.

These regulations will:

• Introduce the 0.2 hectare area-based exemption.
• Remove the self and custom build exemption.
• Exempt temporary permissions of up to 5 years.
• Amend the biodiversity gain hierarchy for minor development.

Further legislation later in 2026 is expected following the brownfield consultation.

These reforms present both opportunities and risks if not anticipated early. It is recommended that those seeking to maximise the value of development sites:

• Review pipeline sites to identify schemes likely to fall within the new 0.2 hectare exemption.
• Revisit viability appraisals where BNG costs were previously material.
• Engage early with planning strategies for brownfield land ahead of further consultation outcomes.
• Monitor local authority approaches to off-site delivery under the revised hierarchy.
• Ensure priority habitat constraints are properly assessed, as these will override exemptions.

For sites currently in the planning system, careful timing and advice may be needed to determine whether applications can benefit from the new regime.