Fractional BNG units: buying part of a unit
A fractional BNG unit is part of a biodiversity unit, for example 0.3 or 1.25 units. Most minor developments and many small commercial schemes need less than a whole unit and registered gain sites can allocate fractions, so buying exactly what the metric requires is normal and usually the fastest route to compliance.
Key facts
- The statutory metric produces decimal results; deficits are typically fractions.
- Fractional units are allocated on the biodiversity gain site register in the same way as whole units.
- From 6 August 2026 minor development can use off-site units on an equal footing with on-site gains.
- Area, hedgerow and watercourse fractions are separate: 0.3 area units cannot cover a 0.1 hedgerow deficit.
- Providers may apply a minimum price or minimum fraction; we confirm this before an introduction.
Last checked: 3 October 2026
Why do small schemes need fractions?
A scheme of five homes on 0.4 hectares of improved grassland might lose 1.2 units of habitat and need 1.32 units after the 10% uplift. If it recreates 1.0 unit on site in gardens and landscaping, the off-site deficit is 0.32 units. Buying a whole unit would mean paying three times what the metric needs.
How are fractional units priced?
Usually pro rata from the provider’s unit price, sometimes with a minimum charge to cover allocation and documentation. The prices page gives indicative ranges; the provider confirms the price.
Which unit types can be bought as fractions?
All three. Hedgerow and watercourse fractions are common for small schemes that remove a few metres of hedge or culvert a short ditch and they must be met with the same unit type. The multiplier applies to fractions in the same way: 0.32 units from an adjacent area means buying 0.43.
How do I buy a fraction?
- Get the deficit by unit type from your ecologist’s metric.
- Tell us the figures in the sourcing check.
- We find registered sites that will allocate the fraction you need and show the multiplier arithmetic.
- You agree price and terms with the provider and the fraction is allocated on the register.
A worked example
Six homes on 0.45 hectares of improved grassland: baseline 1.4 area units; post-development 0.9 on site; target 1.54; deficit 0.64 units. A local provider allocates 0.64 units at the 1.0 multiplier. Had the nearest supply been in an adjacent authority, the purchase would have been 0.86 units. At an indicative £25,000 per unit, that is £16,000 versus £21,500: the multiplier changed the cost by a third, not the fraction itself.
How providers handle fractions
Registered sites hold a total of units by habitat type. Each allocation, whole or fractional, reduces the unallocated balance on the register. Providers typically price pro rata and some set a minimum fraction or a minimum fee to cover allocation and documentation, which is worth checking before you compare quotes.
Why fractions keep small schemes viable
Before fractional allocation was common, small developers faced buying whole units they did not need. Allocating 0.3 or 0.6 units lets a provider serve several small schemes from one site and lets the developer pay for the metric result, not a round number. For a provider, fractions mean more allocations and more paperwork, which is why some set a minimum, but they also keep the register entry moving.
Key takeaways
- Fractions are normal; most minor developments need less than one unit
- Each unit type is fractioned separately
- The multiplier applies to fractions in the same way
- Check minimum fractions and fees before comparing quotes
Frequently asked questions
Is there a minimum fraction?
Some providers set one, often around 0.1 units. We flag it when we present options.
Can two small schemes share a unit?
Each scheme has its own allocation. A provider can allocate fractions of the same unit to different developments.
Do fractions take longer to allocate?
No. The process is the same as for whole units.
Can I round down my deficit?
No. The gain plan must show the full requirement is met. Round up to the metric result.
Can I round my fraction down to save money?
No. The gain plan must demonstrate the full deficit is met.
Is documentation the same for a fraction?
Yes. The allocation confirmation and register evidence are identical to a whole-unit purchase.
Sources
- Statutory biodiversity metric, gov.uk
- Biodiversity net gain guidance, gov.uk
- Biodiversity gain site register
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, the 0.2 hectare exemption, BNG unit types.
Need units for a live application?
Tell us your requirement and we'll start sourcing today.
Start the free sourcing check
One requirement, one point of contact, supply from across England.
Free for developers. Your details are only used to source your units.