A Biodiversity Net Gain (BNG) assessment is a crucial concept for sustainable development practices. Under Schedule 7A of the Town and Country Planning Act 1990, introduced by the Environment Act 2021, qualifying developments in England must deliver at least 10% Biodiversity Net Gain (BNG).
The assessment begins with a habitat survey. The site is divided into habitat parcels, such as grassland, woodland, scrub or developed land. The survey records the type, size and condition of each habitat, along with its strategic importance in the local landscape. This creates the baseline: a measured picture of the site before development. A good baseline is important because every later calculation is built on it.
The next step is to assess the development plan. Habitats may be retained, enhanced, created or lost. Looking at these changes early can reveal opportunities to protect valuable areas, connect habitats or incorporate realistic biodiversity features into the layout.
The statutory biodiversity metric is the official calculation tool used for BNG. It converts habitat information into biodiversity units so that the existing site and the proposed development can be compared on a consistent basis. The metric has separate calculations for area habitats, hedgerows and watercourses. These unit types are not interchangeable. Where a development affects more than one of these habitat types, it must achieve the required net gain separately for each relevant type.
At its simplest, the assessment compares baseline units with post-development units.
The percentage change is calculated as:
((post-development units - baseline units) / baseline units) x 100.
The detailed calculation also considers factors such as habitat distinctiveness, condition and strategic significance. For habitats that will be created or enhanced, the metric also considers how long they will take to reach their intended condition and how difficult they are to establish successfully. Where habitat improvements are delivered offsite, their location in relation to the development and local nature priorities may also affect the calculation.
The statutory biodiversity metric provides a consistent method for calculating biodiversity value, but it does not replace ecological judgement. Habitats must be correctly identified, their condition assessed accurately, and any proposed enhancements based on realistic management measures. These factors influence both the calculation and whether the predicted gains can be achieved in practice.
A BNG assessment is a practical way to bring biodiversity into the design process. When it starts early, it can help a project understand its impacts, explore different options and plan habitat improvements that are measurable as well as achievable.
Related guidance: Current BNG regulations and exemptions | BNG units and statutory credits