Biodiversity Net Gain (BNG)

How it works in practice

The site is surveyed against the government's Statutory Biodiversity Metric, which scores habitats by type, condition, area and connectivity. That gives you a baseline number of "biodiversity units".

After the development is built, the new on-site habitats are scored again. The post-development score must be at least 110% of the baseline. The 10% gap is the "net gain".

Hedgerows and watercourses have their own separate metrics — you can't swap one for the other.

What it means for small sites

From April 2024 BNG applies to "minor" sites too — 1–9 homes, or non-residential below 1,000 m². Small sites get a slightly streamlined version: surveys can sometimes be desk-based and the "Small Sites Metric" is simpler, but the 10% target is the same.

For a typical 4-home rural plot, BNG often adds £5–15k of cost — a survey, a metric calculation, some thoughtful on-site planting, and a 30-year monitoring plan. Sites with high baseline habitat value (mature hedgerows, wildflower meadow) are more expensive because you're starting from a higher number that has to be beaten.

Exemptions exist — self-build and custom-build plots are exempt from BNG when the plot is sold to a future occupier who will build their own home, as are house extensions and very small de minimis sites (under 25m² of habitat). The exemption is narrow — most ordinary sites are caught.

The flip-side: BNG as a landowner opportunity

If you own land that is too constrained to develop but could be enhanced ecologically, BNG creates a new income stream. Field margins, scrub-up corners, and low-grade pasture can be converted to wildflower meadow, woodland or wetland and sold as off-site biodiversity units into the national register.

Typical pricing today sits around £20,000–£35,000 per unit. A few acres of managed habitat creation can generate 20+ units. The catch: the land has to stay managed as habitat for 30 years, locked in by a Section 106 or Conservation Covenant.