Biodiversity Net Gain

Biodiversity net gain (BNG) is a legal requirement introduced by the Environment Act 2021, mandating that most new development in England must deliver a measurable increase in biodiversity value of at least 10% compared to the site’s pre-development condition. The requirement became mandatory for major developments in February 2024 and for small sites in April 2024.

BNG is measured using the Defra Biodiversity Metric (currently version 4.0), a standardised calculation tool that quantifies the biodiversity value of habitats based on:

• Habitat type and condition: Each habitat type (grassland, woodland, hedgerow, etc.) is assigned a distinctiveness score, and its current condition is assessed against defined criteria
• Area or length: The spatial extent of each habitat
• Strategic significance: Whether the habitat is within an area identified in the Local Nature Recovery Strategy

The metric produces a score in “biodiversity units.” The post-development habitat plan must deliver at least 10% more units than the pre-development baseline.

BNG can be achieved through:

1. On-site habitat creation: Incorporating new habitats within the development layout, such as native planting, wildflower meadows, ponds, and green roofs
2. Off-site habitat creation: Creating or enhancing habitats on land outside the development site, registered on the national BNG register
3. Statutory credits: Purchasing credits from the government as a last resort (priced to incentivise on-site and local off-site delivery)

A Biodiversity Gain Plan must be submitted with the planning application (or as a pre-commencement condition), demonstrating how the 10% net gain will be achieved and maintained for at least 30 years.

While BNG and nutrient neutrality are separate requirements, they often interact on the same sites. Aegaea’s environmental team coordinates both assessments to identify synergies — for example, wetland creation that delivers both nutrient mitigation and biodiversity units.