Buying off site biodiversity units: the process
Units have to match the habitat you are losing, come from registered land, and be formally allocated to your development before the gain plan can be approved. Leaving that until after permission stops sites starting.
An off site biodiversity units purchase is the second tier of the biodiversity gain hierarchy, and most developers reach for it later than they should. The sequence matters: deliver what you can on site, buy registered off site units for the shortfall, and treat statutory credits as the last resort. Buying units is not a simple transaction. The land has to be on the biodiversity gain sites register, the units have to match the habitat types you are losing, and they have to be formally allocated to your development before the local planning authority can approve your biodiversity gain plan.
Leaving that allocation to the discharge of a condition after permission is the mistake that stops sites starting.
When off site units become necessary
Run the statutory metric on the proposed layout as soon as the layout stabilises, not at the end of the design process. The output tells you the on site position, and the shortfall against the required uplift is the number you have to buy.
Before buying anything, test whether the on site figure can be improved. Retaining a hedgerow rather than replacing it, keeping a strip of species-rich grassland, choosing a wildflower mix over amenity grass, or moving a road two metres to avoid a good habitat parcel will each move the score, and every one of them costs less than purchased units.
Two structural points push schemes off site regardless of design effort. Dense urban sites have almost no room for habitat creation, and sites whose baseline includes high distinctiveness habitat face trading rules that on site work cannot satisfy. Identify which of those applies to you, because it changes what you are shopping for.
The trading rules decide what you can buy
This is the part that catches buyers who assume a unit is a unit.
The metric applies trading rules requiring compensation to be like for like or better. Losses of high distinctiveness habitat must be replaced with the same habitat type. Losses of medium distinctiveness habitat must be replaced with the same broad habitat type or something better. Only at the lower distinctiveness bands can you compensate with a different habitat.
Three separate accounts run in parallel, and they do not cross-subsidise. Area habitats, hedgerow units and watercourse units each have to balance in their own right. A scheme that removes 200 metres of hedgerow cannot fix that by buying more area units, and a gain site full of created grassland will not help you at all if your loss is a watercourse.
So the search is for a specific habitat type in a specific place, which is a considerably narrower market than the phrase "buying units" suggests. Start it early.
The spatial risk multiplier and why location matters
The metric discounts units delivered further from the development, on the reasoning that biodiversity lost in one place is not fully replaced by a gain fifty miles away.
| Where the gain site sits | Effect on the units |
|---|---|
| Within the same local planning authority area or National Character Area as the development | No discount applied |
| In a neighbouring local planning authority area or National Character Area | Units discounted, commonly to three quarters of their face value |
| Further afield | Discounted more heavily, commonly to half |
Check the multipliers in the version of the metric you are using, because the tool is revised periodically. The practical effect is straightforward: distant units are lower priced per unit and you need more of them, so compare offers on the delivered unit cost after the multiplier rather than on the headline price.
Local authorities also increasingly express a preference for gains within their area, and some local plan policies say so. That preference is not always expressed as a hard requirement, and it is worth asking the case officer directly before signing a contract for units two counties away.
Finding a registered gain site
Land only counts once it is on the biodiversity gain sites register maintained by Natural England. Registration requires the landowner to demonstrate the habitat work proposed, to have the 30 year maintenance secured by a planning obligation or a conservation covenant, and to submit a habitat management and monitoring plan.
When you approach a seller, establish which of three states the land is in.
- Registered, with units available and unallocated. This is what you want.
- Application submitted but not yet determined. Workable, but the risk of refusal or delay sits somewhere, and the contract must say where.
- Land identified, nothing submitted. Treat this as an option on a future product rather than a purchase, and price the risk accordingly.
Ask for the register reference, the habitat types and condition targets, the number of units available by type, the allocations already made, and a copy of the legal agreement securing the 30 years. A seller reluctant to share the register entry is telling you something.
Local authority-run schemes, land trusts and commercial habitat banks all operate in this market, and their contractual terms vary widely. The underlying obligations they carry are the same 30 year commitments the rest of the regime imposes.
Allocating units to your development
Allocation is a formal step on the register, not a private matter between buyer and seller. The gain site operator records the allocation of specified units to your specific development, identified by its planning reference.
Sequence it properly. The biodiversity gain plan cannot be approved without the allocation being in place, development cannot lawfully begin until the plan is approved by the authority, and the allocation cannot be made until the gain site is registered. Any one of those steps slipping stops the programme.
Ask three questions about the mechanics before exchanging contracts. Who submits the allocation and by when? What happens if the planning reference changes, for example through a section 73 variation or a reserved matters approval under a different number? And can the allocation be transferred if the site is sold before work starts?
Contract terms worth reading twice
The unit purchase agreement is doing more work than a typical land contract, because you are buying a 30 year performance promise.
The terms that repay attention are the conditionality on registration, the trigger for payment and whether any of it is refundable, what happens if the habitat fails to reach its target condition, who carries the cost of remediation, whether the seller holds a bond or insurance, the position on seller insolvency, and whether you have any step-in rights or recourse if the allocation is not made.
Also check the obvious commercial points: whether the price is per unit or per hectare, whether VAT applies, whether the units are index-linked, and whether the seller can reallocate units to another buyer if your scheme is delayed.
One trap recurs. A contract that transfers the units on payment but leaves the 30 year obligation entirely with the seller is fine while the seller is solvent and worth very little when it is not. Ask what happens to the land in that event, and whether the conservation covenant or planning obligation survives independently of the seller's continued existence.
Timing against the planning programme
Work backwards from the start of works, not forwards from planning permission.
Start the metric at design stage, and run the off site search in parallel with the application rather than after determination. Agree heads of terms before the decision so the cost is known when the section 106 is negotiated. Complete the purchase and secure the allocation in the window between permission and the start on site, allowing for the time the register process takes.
Evidence all of it in the biodiversity gain plan: the pre-development and post-development metric calculations, the register reference for the gain site, the allocated units by habitat type, the multipliers applied and the resulting percentage uplift. Attach the allocation confirmation rather than describing it.
The whole exercise rests on an accurate baseline, and that comes from an ecologist on site in the right season. A Preliminary Ecological Appraisal commissioned early gives the habitat mapping the metric needs, and shapes what the Biodiversity Net Gain Assessment can properly claim. On site opportunities that reduce the purchase are usually found through the Landscape Strategy rather than the ecology report.
PlanScope drafts supporting documents structured to UK planning policy and flags where site facts are still missing. Enter a postcode on PlanScope to see which reports a site is likely to need. The statutory metric and the biodiversity gain plan must be completed by a competent person under the Environment Act 2021 regime, so any draft is a starting point for that professional rather than a substitute. Reports are AI-assisted drafts that should be reviewed before submission, and we do not guarantee planning approval. This is general guidance, not legal advice: take your own advice on any unit purchase agreement.
Frequently asked questions
Can I buy any biodiversity units to cover my shortfall?
No. Trading rules require like for like or better, so high distinctiveness losses need the same habitat type and medium distinctiveness losses need the same broad habitat type or better. Area habitats, hedgerow units and watercourse units also balance separately and cannot substitute for each other.
What is the spatial risk multiplier?
A discount applied by the statutory metric to units delivered away from the development. Units in the same local planning authority area or National Character Area are undiscounted, neighbouring areas are commonly reduced to three quarters, and more distant sites to half. Compare offers on delivered cost after the multiplier.
When does the allocation of units have to happen?
Before the local planning authority approves your biodiversity gain plan, which in turn must happen before development can lawfully begin. The gain site must already be registered for an allocation to be made, so all three steps have to be sequenced into the programme rather than left to a condition after permission.
What should I check before contracting for units?
Whether the land is already registered, who bears registration risk, the payment trigger and refundability, what happens if the habitat fails to reach target condition, who pays for remediation, any bond or insurance, the position on seller insolvency, and whether you have step-in rights if allocation is not made.
Can I reduce the number of units I need to buy?
Often, yes. Retaining a hedgerow rather than replacing it, keeping species-rich grassland, choosing a wildflower mix over amenity grass, or shifting a road to avoid a good habitat parcel all move the on site score, and each of them costs less than buying units. Run the metric while the layout can still change.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.