Process · England

BNG assessment

A BNG assessment in England runs from a baseline habitat survey by a competent ecologist, through the statutory biodiversity metric calculation, design iteration to maximise on-site gains, sourcing of off-site units or last-resort statutory credits, to approval of a biodiversity gain plan before commencement and 30 years of habitat management. Timelines depend on season and site; professional ecological and legal advice is required for anything beyond a small site.

Process drawn from Planning Practice Guidance (updated 31 July 2026) and GOV.UK "Understanding biodiversity net gain" (updated 14 July 2026). Page reviewed 2026-09-17.

The assessment, step by step

1. Check whether BNG applies

   Confirm the application date, site area inside the red line, and whether an exemption (householder, permitted development, 0.2 ha, de minimis, temporary) applies. If exempt, declare it with reasons on the application form.

2. Commission a baseline habitat survey

   A competent ecologist maps every habitat parcel inside the red line boundary, classifies habitat type and assesses condition against the statutory metric criteria. Survey timing matters: many habitats can only be assessed in the growing season.

3. Run the statutory biodiversity metric for the baseline

   The ecologist enters area, type, condition and strategic significance to calculate pre-development biodiversity units for area habitats, hedgerows and watercourses. This baseline is a minimum national information requirement submitted with the planning application.

4. Design for on-site gains and iterate

   Follow the biodiversity gain hierarchy: avoid and minimise habitat loss through layout, retain the most valuable habitat, then create or enhance habitat on site. Re-run the metric with each design iteration, saving versions.

5. Secure off-site units or credits for any shortfall

   If the post-development total is below baseline × 1.10, source off-site units from a registered biodiversity gain site (recorded on the register) or, as a last resort, estimate and buy statutory credits.

6. Obtain planning permission with BNG information

   Submit the baseline metric, a statement of how the gain objective will be met, and details of significant on-site enhancements. The authority may impose conditions and section 106 obligations to secure the gains.

7. Submit the biodiversity gain plan before commencement

   After permission, submit the biodiversity gain plan with the final metric calculation, evidence of off-site allocations and any credit purchases. The local planning authority has 8 weeks to approve or refuse; development cannot lawfully start until approval.

8. Deliver and manage habitat for 30 years

   Create or enhance the habitat, then manage and monitor it for at least 30 years under the legal agreement and habitat management and monitoring plan. The authority can take enforcement action if commitments are not met.

Where the non-statutory estimator fits

The biodiversity net gain calculator on this hub is useful at step 1 and step 4: before you commission a survey, to understand how much value different parts of a site are likely to hold and whether the 0.2 ha exemption might apply; and during early design, to see how retaining a hedgerow or a strip of grassland changes the arithmetic. It cannot replace steps 2, 3 or 7. Those require the statutory biodiversity metric tool completed by a competent person and submitted to the local planning authority.

Who you will need