Key changes confirmed to Biodiversity Net Gain

Biodiversity Net Gain (BNG) was introduced in February 2024, requiring development under the Town and Country Planning Act (TCPA) to deliver a minimum 10% net gain for biodiversity using the biodiversity metric.

The government was clear from the outset that the regime would evolve. Following reports of planning delays, high costs and a stalled off-site market, Defra undertook a consultation in 2025 on how exemptions, thresholds and metrics could be refined to ensure BNG is workable in practice without undermining its environmental objectives.

A second consultation at the same time investigated how BNG would work for Nationally Significant Infrastructure Projects (NSIP) which was expected to come into effect in May.

Defra published its response to both consultations on 15 April 2026 along with a new consultation on brownfield sites. The headline points are:

• Confirmation of an earlier announcement of a 0.2ha BNG exemptions from 31 July 2026.
• BNG for NSIP projects will become mandatory on 2 November 2026. There will be new ‘BNG boundary’ definition, and flexibility on where aggregated biodiversity gain is sited, with the exception of water impacts which must be in the catchment.

The CLA is continuing to analyse these updates in detail and what they mean for members. A brief overview is below.

Defra has confirmed that secondary legislation will be introduced in 2026 to improve how BNG operates in practice for minor, medium and brownfield development. This will be in from 31 July 2026 and reflects growing recognition that aspects of BNG policy are challenging for smaller development schemes.

A new 0.2 hectare area based exemption

Previously announced in December 2025, we now have more detailed on the UK Government’s intentions to introduce a new exemption for very small sites. This exemption will be introduced for developments where the site area is 0.2 hectares or below. These developments will still be subject to environmental protections via the National Planning Policy Framework (NPPF) and wider regulatory requirements but will no longer need to demonstrate a 10% net gain either on or off site. Planning applications will be refused where a significant harm to biodiversity cannot be avoided, mitigated or compensated against, even on an exempt site.

Removal of the self and custom build exemption

From 31 July 2026, it will no longer be possible to apply for exemption from BNG for self or custom build dwellings. It is anticipated that the new 0.2 hectare exemption will capture many of these developments.

Exemption for temporary planning permissions

New regulations are to be introduced to exempt temporary development which is for five years or less. This recognises that delivery of BNG on these sites is limited due to their temporary nature.

Amendment to the biodiversity gain hierarchy

Amendments will also be introduced for the biodiversity gain hierarchy. Regulations will amend how the hierarchy is used for minor development, where it is not exempt. The hierarchy remains the same for any other development.

In addition, it has been announced that the following will also become exempt from BNG later in 2026:

• Development with the primary objective to conserve or enhance biodiversity
• Development that enhances parks, playing fields and public gardens

It has also been revealed that the BNG metric will be updated to a digital web-based platform.

Alongside the suite of announcements, Defra has launched a consultation on targeted BNG exemption for brownfield residential development. The consultation, closing on 10 June, focusses on the detail of introducing a new exemption for brownfield sites of up to 2.5 hectares for residential development only.

The consultation acknowledges that further exemptions could substantially impact nature markets and the CLA will be reviewing this to ensure a thriving nature market whilst supporting members ambitions and development. There is a risk that the proposed exemption (in addition to the 0.2 hectare exemption introduced in July 2026) could result in a reduction of up to 48% of off-site market transactions.

Mandatory BNG for Nationally Significant Infrastructure Projects has been delayed but it now scheduled to be implemented from 2 November 2026. The original timetable anticipated earlier implementation, but the complexity, scale and variety of NSIP schemes has resulted in delays.

Defra is implementing the same BNG requirements for all NSIP sectors and types of development, but there are some changes from original plans. For example, only habitats negatively impacted will need to be included in the BNG baseline, with a new category of BNG boundary. This will include both permanently and temporarily affected habitats – there had been some calls to not include temporarily affected habitats.

There will also be flexibility on how the off-site regime will operate for these larger scale developments, allowing all BNG to be aggregated and delivered in any one of the Local Planning Areas affected by the project.

Over time BNG for NSIPs has the potential to increase demand for offsite biodiversity units, and for a wider range of unit types, as it can be more difficult to avoid habitats in larger schemes, although this will depend on how it is deployed and the balance of onsite.

If you have any questions regarding these announcements, please contact the CLA for more information.