Big Changes for Small Sites: The New 0.2 ha BNG Exemption
This 0.2ha rules will come in to force sometime in the first half of 2026.
New Biodiversity Net Gain (BNG) Exemption for Small Sites
The UK government has announced a significant shift in Biodiversity Net Gain (BNG) requirements to support small-scale developers. As of December 2025, the housing minister confirmed that developments on sites smaller than 0.2 hectares will be exempt from the mandatory 10% BNG requirement.
What is Changing? Previously, even small sites were required to demonstrate a 10% increase in biodiversity, often using the Small Sites Metric (SSM). Under the new policy, sites with a total area under 0.2 hectares will no longer be required to meet this statutory 10% uplift. This threshold was selected as a compromise after earlier proposals suggested exempting sites up to 0.5 hectares.
Red Line or Built Area?
Measurement and Site Area The 0.2-hectare threshold is based on the entire site area as defined by the red line boundary of the development. It is not limited to the built area or the developed footprint. This means that if the total area within your red line boundary is 0.2 hectares or more, the exemption will not apply, even if your actual building footprint is very small.
Which Metric Do I Use?
Metric Calculators for Larger Sites For sites that exceed the 0.2-hectare threshold but are still classified as small sites (typically up to 9 dwellings or less than 1 hectare for residential), the Small Sites Metric (SSM) remains the primary tool. However, if a site is over 0.2 hectares and contains “priority habitats” (such as ancient woodland or specific grasslands), the more complex Statutory Biodiversity Metric must be used instead.
Why the Change? The primary goal is to reduce the administrative and financial burden on SME housebuilders. For micro-sites, the cost of ecological surveys and the requirement to secure 30-year management plans were often seen as disproportionate to the environmental impact. By removing these hurdles, the government aims to accelerate the delivery of infill housing and urban development.
Current Status and Parliament Timeline The government’s formal response to the BNG consultation and the full implementation timeline are expected in early 2026. Because this change requires secondary legislation to become law, it must be laid before Parliament as a Statutory Instrument. While a specific day has not yet been set, the legislative process is expected to move forward in the first half of 2026. Until this legislation is officially enacted and brought into force, the existing BNG rules—including the 10% mandate for small sites—remain the legal requirement.
Additional Consultations Alongside the 0.2-hectare rule, the government is consulting in early 2026 on a further exemption for residential brownfield sites up to 2.5 hectares. This is intended to incentivize development on previously used land with low ecological value.
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