Glossary

BNG glossary: 37 terms, explained plainly

The vocabulary of biodiversity net gain in England, from allocation to watercourse unit, defined in a sentence or two each. Every guide on this site links here on first use.

Last checked: 3 October 2026

A B C D E F H I L M N O P R S T W

Adjacent area: A local planning authority or National Character Area that borders the one the development is in. Units from an adjacent area carry a spatial risk multiplier of 0.75.
Allocation: The formal recording on the biodiversity gain site register that specific units from a gain site are assigned to a specific development. Allocation is the evidence a planning authority needs.
Area habitat unit: A biodiversity unit generated by an area-based habitat such as grassland, woodland, scrub or wetland. One of the three unit types, which cannot substitute for each other.
Baseline: The biodiversity value of a site before development or habitat creation, calculated with the statutory biodiversity metric.
Biodiversity gain hierarchy: The order of preference for delivering BNG: avoid harm, then on-site gains, then off-site gains, then statutory credits as a last resort. For minor development, off-site gains sit on an equal footing with on-site gains from 6 August 2026.
Biodiversity gain plan: The document submitted after planning permission that sets out how the 10% net gain will be delivered. The planning authority must approve it before development begins.
Biodiversity gain site register: The public register maintained by Natural England of off-site habitat sites and the units allocated from them.
Biodiversity net gain (BNG): The requirement in England that development leaves biodiversity in a measurably better state, with a minimum 10% net gain secured for at least 30 years.
Biodiversity unit: The measure of biodiversity value produced by the statutory metric, combining habitat area or length, distinctiveness, condition and strategic significance.
Condition: An assessment of how well a habitat is functioning, from poor to good, which multiplies its unit value in the metric.
Conservation covenant: A legal agreement between a landowner and a responsible body that secures land management for conservation, used as an alternative to a Section 106 agreement to secure a gain site.
Distinctiveness: A ranking of habitat types from very low to very high reflecting their ecological value. High and very high distinctiveness habitat must be replaced like for like or better.
Exemption: A category of development that mandatory BNG does not apply to, including sites of 0.2 hectares or less with no priority habitat affected, temporary permissions of up to five years and householder development.
Forward allocation: An agreement to reserve units from a gain site before planning permission is granted, usually through an option.
Fractional unit: Part of a biodiversity unit, for example 0.3 units, commonly bought by minor and small developments.
Habitat bank: Land where habitat is created or enhanced in advance of development demand so that units can be sold to developers. Green Acres BNG runs two.
Habitat management and monitoring plan (HMMP): The plan that sets out how a gain site will be managed and monitored for at least 30 years.
Hedgerow unit: A biodiversity unit generated by hedgerows and lines of trees, measured by length. One of the three unit types.
Introduction fee: The fee a BNG provider pays BNG Sourced by Green Acres BNG where our involvement results in an introduction. The service is free for developers.
Local Nature Recovery Strategy (LNRS): One of 48 strategies across England that map priorities for nature recovery. The government plans to move the spatial risk multiplier to LNRS areas.
Local planning authority (LPA): The council or authority that determines planning applications and approves biodiversity gain plans.
Major development: Development above the minor thresholds, for example 10 or more dwellings or a site of 1 hectare or more, subject to mandatory BNG since 12 February 2024.
Minor development: Smaller development, including residential schemes of 9 or fewer dwellings on a site of 1 hectare or less, subject to mandatory BNG since 2 April 2024.
National Character Area (NCA): One of 159 landscape areas across England used, alongside LPA boundaries, to define what counts as local for the spatial risk multiplier.
Nationally significant infrastructure project (NSIP): Large energy, transport, water and waste projects consented through development consent orders. Mandatory BNG applies to applications submitted on or after 2 November 2026.
Off-site gain: Biodiversity units delivered on land outside the development site, bought from a gain site and allocated to the development.
On-site gain: Biodiversity units delivered within the development site.
Priority habitat: Habitats of principal importance listed under Section 41 of the NERC Act 2006. Negatively affecting on-site priority habitat removes the 0.2 hectare exemption.
Responsible body: An organisation designated to hold conservation covenants.
Section 106 agreement: A legal agreement under the Town and Country Planning Act 1990 used to secure a gain site for at least 30 years.
Spatial risk multiplier: The factor applied to off-site units depending on their distance from the development: 1.0 local, 0.75 adjacent, 0.5 elsewhere in England.
Statutory biodiversity credits: Credits sold by the government as a last resort where off-site units cannot be found, deliberately priced above the market.
Statutory biodiversity metric: The official calculation tool for measuring biodiversity value before and after development.
Strategic significance: A metric multiplier reflecting whether a habitat is in a location identified as a priority in local strategies.
Temporal risk multiplier: A metric discount applied where habitat takes years to reach its target condition.
Trading rules: The rules that determine which habitats can replace which: like for like or better for high and very high distinctiveness habitat and no substitution between area, hedgerow and watercourse units.
Watercourse unit: A biodiversity unit generated by rivers, streams and ditches, measured by length. The scarcest of the three unit types.

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