BNG unit types explained: area, hedgerow and watercourse
Biodiversity net gain in England uses three unit types. Area habitat units measure habitats by area, hedgerow units measure hedgerows and lines of trees by length and watercourse units measure rivers, streams and ditches by length. Each is calculated separately in the metric and each deficit must be met with the same unit type.
Key facts
- Area habitat units: grassland, woodland, scrub, heathland, wetland, cropland, urban habitats, measured in hectares.
- Hedgerow units: hedgerows and lines of trees, measured in kilometres.
- Watercourse units: rivers, streams, ditches and canals, measured in kilometres.
- No substitution between the three types; all three need a 10% gain.
- Watercourse units are the scarcest and dearest; hedgerow units are limited in most regions.
Last checked: 3 October 2026
How are the three types calculated?
Each uses the same logic: size multiplied by distinctiveness, condition and strategic significance, before and after development, with risk multipliers for off-site delivery and time to maturity. Area habitat is the bulk of most metrics. Hedgerow and watercourse modules only appear where those features exist on site.
| Unit type | Measured by | Typical loss | Supply |
|---|---|---|---|
| Area habitat | Hectares | Grassland or arable under buildings and roads | Good for medium distinctiveness; limited for high |
| Hedgerow | Kilometres | Field boundary hedges removed for access or layout | Limited |
| Watercourse | Kilometres | Ditches culverted or realigned | Scarce |
Why are hedgerow and watercourse units harder to find?
Fewer gain sites create them, because they need linear features in the right condition and long-term management is more specialised. Developers who remove a hedge or culvert a ditch often discover late that the area units they planned to buy do not help. Identify linear losses at pre-application and, where possible, design around them.
What should I ask my ecologist?
- The deficit for each of the three types separately.
- The distinctiveness of each habitat lost, which sets the trading rule.
- Whether any loss is very high distinctiveness, which may be impossible to offset.
- How on-site measures reduce each deficit.
Then tell us the three figures in the sourcing check.
What each unit type looks like in the metric
The area module lists habitat parcels by UKHab type with area in hectares, distinctiveness, condition and strategic significance. The hedgerow module lists hedgerows and lines of trees by type and length in kilometres, with condition. The watercourse module lists rivers, streams, ditches and canals by length with a condition assessment based on the River Condition Assessment. Each module reports its own baseline, post-development value and percentage change and each must reach 10%.
Typical deficits by scheme type
| Scheme | Area | Hedgerow | Watercourse |
|---|---|---|---|
| Urban infill | Small fraction | Rare | Rare |
| Greenfield housing | Several units | Common where access removes a hedge | Occasional ditches |
| Logistics | Several units | Common | Common where ditches are culverted |
| Linear infrastructure | Many | Many crossings | Watercourse crossings |
Reducing linear deficits by design
Keeping a hedgerow in the layout, bridging rather than culverting a ditch or realigning a watercourse to equal or better condition removes the scarcest requirements before they arise. These decisions are cheapest at the masterplan stage, which is why we encourage a supply outlook at pre-application.
Key takeaways
- Three unit types, each calculated separately
- Hedgerow and watercourse are measured by length
- Each type present must reach a 10% gain
- Hedgerow and watercourse units are the hardest to find
Frequently asked questions
Can I buy fractions of hedgerow or watercourse units?
Yes. Fractions are common for all three types.
Are individual trees area habitat?
Individual trees are assessed in the area habitat module as a specific habitat type.
Does a pond count as watercourse?
Ponds and lakes are area habitats. Watercourse covers linear features such as rivers, streams and ditches.
Do I need all three to show 10%?
Each type present on site must show a 10% gain. If there is no hedgerow or watercourse on site, that module does not apply.
Is a line of trees a hedgerow?
Lines of trees are assessed in the hedgerow module.
Can a culverted ditch be reinstated to avoid the deficit?
Keeping or reinstating the ditch in equal or better condition avoids the watercourse deficit, which is usually the cheapest option.
Sources
Written by BNG Sourced by Green Acres BNG specialists and reviewed by Mike Dungait, Managing Director of Green Acres BNG, who has registered and manages two habitat banks. Indicative information, not legal or planning advice. Confirm with your ecologist and planning authority.
Related: BNG sourcing check, exemption checker, BNG unit prices, glossary, BNG trading rules, fractional BNG units.
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