On-site biodiversity units against buying off-site

Whether to deliver biodiversity net gain on site or off site is a land economics question dressed up as an ecology one. On-site units cost you developable area and a 30-year management obligation. Off-site units cost cash but arrive discounted by the statutory metric's spatial risk multiplier, so you buy more of them than the shortfall suggests. The comparison that matters is the value of the land you would give up against the price of the units plus the management liability you avoid.

Do that arithmetic before the layout is fixed, because after planning consent the options narrow sharply.

How the statutory metric values on-site delivery

Mandatory biodiversity net gain under Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021, requires a minimum ten per cent gain measured in biodiversity units using the statutory metric, secured through a biodiversity gain plan approved by the local planning authority.

Units are not a simple function of area. The metric multiplies habitat area by distinctiveness, condition and strategic significance, then applies risk multipliers for how difficult the habitat is to create, how long it takes to reach target condition and where it is. That means two hectares of species-poor amenity grassland created on site may generate fewer units than a smaller area of a more distinctive habitat, and a habitat that takes 25 years to reach good condition is discounted heavily for the delay.

Two consequences follow for layout. Enhancing an existing habitat is usually more efficient than creating a new one from a hard surface, because you start from a baseline that already has distinctiveness. And retaining anything of moderate or higher distinctiveness on site avoids a loss you would otherwise have to replace at a penalty, since the trading rules require like for like or better.

A Biodiversity Net Gain Assessment is the document that holds the calculation, and under the Environment Act regime the metric has to be completed by a competent person, with the gain plan approved by the authority. PlanScope drafts are a starting point for that professional rather than a substitute for their judgement.

Spatial risk and the multipliers that apply

The metric discounts units delivered further from the development. Units created on the development site itself attract no spatial discount. Units delivered off site within the same local planning authority area or national character area attract a smaller discount, and units delivered further away attract a larger one.

Read the multiplier values from the current published metric rather than from any secondary source, because they have been revised and the arithmetic is sensitive to them. What matters strategically is the direction: a distant off-site unit delivers less than a nearby one, so a shortfall of ten units may need substantially more than ten units purchased at distance.

That reverses a common assumption. Off-site delivery is not simply a cash substitute at par. It is a cash substitute at a discount, and the discount grows with distance.

Thirty-year management and its cost

On-site delivery carries a 30-year obligation, and this is the item developers price wrongly.

The habitat has to be managed and monitored for 30 years from completion, secured through a planning condition, a section 106 obligation or a conservation covenant with a responsible body. The cost goes well beyond cutting grass. It covers a written habitat management and monitoring plan, annual or periodic management operations, condition monitoring surveys at intervals across the 30 years, reporting to the authority, remedial works where condition targets are missed, and the administration of whoever holds the duty.

Who holds it matters commercially. On a housing scheme the usual answer is a management company funded by an estate charge, which transfers the cost to purchasers but leaves a reputational and legal tail. On a commercial scheme the landowner retains it. Either way, a 30-year liability discounted to present value is a real number that belongs in the appraisal alongside the other long-tail items, in the same way that utility connection costs belong there rather than in a contingency.

Monitoring reporting is where schemes quietly fail. A condition requiring a report in years 1, 2, 5, 10, 15, 20, 25 and 30 outlives the development company that agreed to it.

What an off-site unit purchase includes

Buying off site means buying registered units from a habitat bank or a landowner whose site is entered on the biodiversity gain sites register. What you are paying for should be set out clearly.

A properly structured purchase includes the units allocated to your development and recorded on the register, the legal agreement securing the habitat for 30 years, the management and monitoring obligation sitting with the seller, the seller's own reporting duty, and evidence that the units were available and unallocated when you bought them. The gain plan then names the off-site site and the allocated units.

Three checks are worth making before contract. Confirm the units are registered and unallocated rather than promised. Confirm the habitat type satisfies the trading rules against what you are losing, since you cannot replace a distinctive habitat with a low-distinctiveness one. And confirm the spatial relationship, because a site two counties away changes the number of units you need.

The advantage is clean. The liability transfers, the land stays developable, and the 30-year administration is someone else's problem.

Statutory credits as the last resort

Government sells statutory biodiversity credits as a backstop where on-site and off-site delivery cannot be arranged. The price schedule is published and is deliberately set above expected market rates, so that credits are the least attractive route rather than a convenient default. Check the current schedule rather than working from an older figure, and note that credit prices are tiered by habitat type.

Treat credits as evidence of a failed strategy rather than a plan. An authority reading a gain plan that relies on statutory credits for most of the requirement will ask why on-site delivery was not pursued, and the answer needs to be better than commercial preference.

Land take against unit cost in the appraisal

Here is the comparison to build, expressed per unit rather than per hectare.

An illustrative example. A scheme of 48 houses on 2.1 ha shows a shortfall of 4.2 biodiversity units after on-site enhancement of retained hedgerow and a wet swale. Delivering the remainder on site would need roughly 0.35 ha of species-rich grassland, which at that plot density is about three houses. If three plots carry a residual land value materially above the cost of buying the equivalent discounted units nearby, off-site purchase wins even after the higher unit count. If the land is low value or the units are scarce locally, it reverses. Every figure here is illustrative and the answer is site specific.

Building the comparison into the layout decision

Run the metric early, in outline, on two or three layout options rather than once on the final one.

1. Get the baseline habitat survey done in the growing season, before layout is fixed.
2. Calculate the baseline units and the loss implied by each layout option.
3. Test on-site enhancement of retained features first, since that is the most efficient source of units.
4. Price the residual shortfall both ways, including the discounted present value of 30 years of management.
5. Check off-site availability and spatial relationship in your area before assuming purchase is straightforward.
6. Fix the layout, then finalise the gain plan.

A Landscape Strategy that was drawn with the metric in mind will usually deliver more units for the same area than one drawn for appearance and then measured afterwards. That single sequencing decision is worth more than most of the trading around it.

One honest uncertainty. Off-site unit availability and price vary a great deal by area and by habitat type, and a scheme that assumes purchase is available at a national average can find nothing suitable within the same national character area. Check the market before you commit the layout.

See PlanScope pricing for the report side of the work: one free report, £50 per report, or Professional for unlimited reports.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.