The landscape of mandatory Biodiversity Net Gain (BNG) has just shifted significantly.
In a major announcement on December 16, 2025, as part of broader planning reforms aimed at boosting housebuilding, the UK government confirmed critical changes to BNG regulations. These changes are designed to reduce the regulatory burden on small developers and SME builders.
The Headline Change: The New 0.2 Hectare Exemption
The most significant aspect of the Written Ministerial Statement is the confirmation of a new, blanket exemption threshold. The Housing Secretary has confirmed that the government will exempt smaller developments of up to 0.2 hectares from mandatory Biodiversity Net Gain.
Up until now, many small sites — unless qualifying for specific "de minimis" or self-build exemptions — faced the same 10% net gain requirement as larger developments, often requiring complex ecological surveys and the use of the Small Sites Metric (SSM). This new 0.2ha rule is intended to streamline the planning process for the smallest housing projects, removing a significant cost and time barrier for SME developers working on minor sites.
Beyond 0.2ha: Proposed Brownfield Exemptions
The reforms do not stop at the smallest sites. The announcement also revealed that Defra will "consult rapidly" on further targeted exemptions. The government is proposing an additional BNG exemption for residential brownfield development, testing a site size threshold of up to 2.5 hectares for this exemption.
Simplified Requirements for Medium Sites
For sites that do not fall under the new 0.2ha exemption, the government has also signalled its intention to introduce a suite of "simplified requirements" to improve BNG implementation on remaining small and medium sites. This likely points to future reforms of the Small Sites Metric and potentially reduced monitoring periods.
Clarification: The New Rule vs. The Self-Build Exemption
The current Self-Build and Custom Housebuilding exemption (in force since early 2024) remains separate — it applies to sites under 0.5 hectares with no more than 9 dwellings, exclusively for self-build. The new announcement is a broader, area-based exemption for any development under 0.2 hectares.
When Do the New BNG Rules Come into Force?
While the 0.2ha exemption is now confirmed government policy, it is not yet law. To implement this exemption, the government must pass secondary legislation. Full implementation details and consultation responses were due to be published in early 2026.
As of the date of this announcement, existing BNG rules still strictly apply to planning applications. Developers should watch closely for the legislative updates that will officially enact this change.