What Is Biodiversity Net Gain?
BNG requires developers to leave nature in a measurably better state than they found it. The baseline is calculated by surveying existing habitats using Biodiversity Metric 4.0, which scores habitats by type, condition, distinctiveness, and strategic significance. The post-development score must equal at least 110% of the baseline. Introduced by Environment Act 2021 Schedule 14, BNG has applied to major applications from November 2023 and minor applications from April 2024.
Exemptions
The de minimis exemption applies where development affects less than 25 square metres or 5 metres of linear habitat. Householder applications are generally exempt. Brownfield land does not carry automatic exemption — if notable habitats are present they are scored and must be exceeded.
- De minimis: <25 sqm or <5m linear habitat — BNG not required
- Householder applications: exempt
- Self-build and custom-build: exempt
- Class Q agricultural conversions: BNG does apply
On-Site Habitat Management Plans
On-site BNG is the preferred route. A habitat management and monitoring plan (HMMP) must be secured by a planning condition or section 106 agreement for 30 years. Annual monitoring reports are required for the first 5 years, then at years 10, 20, and 30.
Off-Site Habitat Banks — Income for Rural Landlords
Landowners can register eligible land as a biodiversity gain site with Natural England and sell biodiversity units to developers. Unit values vary by habitat type: lowland meadow (good condition) approximately £30,000–£60,000 per unit; wet woodland creation approximately £25,000–£50,000 per unit; hedgerow enhancement approximately £15,000–£30,000 per 100m unit. A 30-year legal agreement must be entered into, encumbering the land for the term.
Statutory Biodiversity Credits
Where on-site and off-site options are both unavailable, developers may purchase statutory biodiversity credits from Natural England at prices set by DEFRA — typically 2–4× private off-site unit prices. Credits fund strategic nature recovery projects nationally and are a last resort.
Development Appraisal Implications
BNG obligations must be costed into appraisals from the outset. A site with high-quality grassland or hedgerow baseline may have a significantly higher BNG liability — and lower residual land value — than an equal-area site with degraded habitats. Instruct an ecologist for a Phase 1 habitat survey (£500–£2,500) at pre-application stage and run a preliminary Biodiversity Metric 4.0 calculation before committing to a site.