Biodiversity Net Gain 2026: Architect's Guide + Free BNG Calculator

Check the current exemptions before calculating. For relevant applications made on or after 6 August 2026, development on a site of 0.2 hectares or less is exempt only if it does not impact onsite priority habitat. The former self/custom-build exemption is limited by the transition date; do not keep applying the old nine-dwelling rule to new applications. Section 73 and other transitional cases need a separate check. Exemption from statutory BNG does not remove protected-species or other planning duties.

The statutory biodiversity metric is distinct from the older Biodiversity Metric 4.0. Habitat type, condition and other inputs affect the result; delivery risks and trading rules matter too. Hedgerow, watercourse and area habitat units cannot simply be added together.

Do not assign all car parks one unit per hectare or all green roofs a fixed unit score. The earlier examples on this page used unsupported fixed weights and have been removed. Habitat classification and condition need suitable evidence; mapped land use alone is not a statutory habitat baseline.

Enter baseline and proposed units from the same applicable metric category. For a positive baseline, the arithmetic uplift is (proposed − baseline) ÷ baseline × 100. An illustrative baseline of 10 units and proposed total of 11 units gives 10% uplift. This example demonstrates arithmetic only.

A zero baseline needs separate handling; a percentage increase from zero is undefined. A result of 10% or more does not validate inputs, exemption eligibility, habitat trading rules, ecological feasibility, offsite registration or the biodiversity gain plan. Keep those checks outside this simple comparison.

Ask the ecologist to identify retained habitats before drawing replacement landscaping. Test the location and long-term management of proposed planting, roofs, drainage features and habitat connections against the architecture, servicing and access strategy. A feature can have several design benefits without automatically attracting a particular number of biodiversity units.

Maintain an option table with habitat assumptions, metric version, area or length, proposed condition, delivery period, management responsibility and outstanding evidence. Mark unverified design assumptions clearly. Update the metric when the layout changes, rather than treating an early comparison as a permanent result.

The biodiversity gain plan normally needs approval before development commences under the biodiversity gain condition; that is different from saying the complete plan must be approved before planning permission can be granted. Significant onsite enhancements and offsite gains have long-term securing requirements, generally at least 30 years. Not every planted feature has the same legal securing route.

Read the government explanation of delivery before assuming all sites must exhaust onsite delivery first: since 6 August 2026, qualifying non-exempt minor development can consider onsite and offsite gains together. Purchased offsite biodiversity units and government statutory credits are different mechanisms. Identify the route and evidence required, rather than labelling both as habitat-bank credits.