Science-based expertise
The UK Department for Environment, Food & Rural Affairs (DEFRA) and Natural England developed the statutory biodiversity metric to provide a standardised way of measuring changes in the biodiversity value of habitats resulting from development. It compared the biodiversity units present before development with those expected after development, helping to determine whether the required Biodiversity Net Gain (BNG) will be achieved.
The metric supports transparent and consistent decision-making during the planning process. However, it does not replace ecological surveys, protected-species assessments or professional ecological judgement.
How does the statutory biodiversity metric work?
The statutory biodiversity metric assesses habitats rather than individual species. For area habitats, biodiversity units are calculated using factors including habitat area, distinctiveness, condition and strategic significance. Separate calculations are completed for hedgerows and watercourses, where relevant.
The baseline calculation establishes the biodiversity value of the site before development. The post-development calculation considers habitats that will be retained, enhanced or created. It also accounts for the time required for habitats to reach their intended condition, the difficulty of creating or enhancing them and, where applicable, spatial risk.
The percentage change is calculated as:
A qualifying development must normally achieve at least a 10% increase in biodiversity units. It must also comply with the metric's trading rules, which help ensure that habitat losses are replaced appropriately. The assessment should therefore report the results separately for area habitats, hedgerows and watercourses rather than combining them into one overall score.
The completed metric calculation forms part of the evidence supporting the Biodiversity Gain Plan, which must be approved by the local planning authority before development begins. Significant on-site gains and off-site gains must generally be managed and maintained for at least 30 years.