Budgeting an off-site biodiversity credit purchase

Off-site biodiversity units are the second choice in the statutory hierarchy and statutory credits are the last, which means the price you pay rises the further you move from your own red line. The biodiversity credit cost small site question has no single answer, because off-site unit prices are set by the market and vary with habitat type and location, while statutory credit prices are published by Defra in a tiered schedule and are deliberately pitched above expected market rates. What you can budget accurately is the structure of the cost, and that is worth doing before the planning application rather than after.

When off-site units become necessary

Mandatory biodiversity net gain under the Environment Act 2021 has applied to major development since 12 February 2024 and to small sites since 2 April 2024. The requirement is a 10 per cent gain measured by the statutory metric, secured for at least 30 years.

The hierarchy is on-site first, then off-site, then statutory credits as a last resort. Small sites reach the off-site stage quickly and for an obvious reason: a quarter-acre plot that loses a hedgerow and some species-poor grassland has very little room left to create anything of higher distinctiveness once the house, the drive and the garden are laid out. Gardens score poorly in the metric, which surprises self-builders every time.

Two particular triggers push a small site off-site. Hedgerow units are measured separately from area units, so losing a length of hedge creates a hedgerow shortfall that cannot be made up with wildflower grassland. And the trading rules require like-for-like or better replacement of medium and higher distinctiveness habitat, so losing anything beyond poor grassland narrows your options sharply.

How the metric generates the shortfall

The metric does the arithmetic and the arithmetic is where the money is decided, so it pays to understand which numbers you can influence.

Each habitat parcel is scored for distinctiveness, condition, and strategic significance, then multiplied by area. The post-development score applies further multipliers for the time it takes a created habitat to reach target condition and for the difficulty of creating it. Off-site units then attract a spatial risk multiplier: full value where the off-site land is in the same local planning authority area or national character area, a reduction for adjacent areas, and a larger reduction beyond that.

That spatial multiplier is the one most people miss when budgeting. Buying units a long way away means buying more of them to deliver the same gain, so a lower headline price per unit can cost more in total.

The metric must be completed by a competent person. That is an expectation of the Environment Act 2021 regime rather than a presentational nicety, and a Biodiversity Net Gain Assessment prepared as a draft still needs that competent person to take responsibility for the figures. The habitat baseline behind it comes from a survey, usually a Preliminary Ecological Appraisal with habitat mapping, and a baseline recorded in the wrong season will cost you units.

Off-site units against statutory credits

The two routes behave quite differently and should be priced separately.

Off-site units are bought from a landowner or a habitat bank, with the gain site registered on the national biodiversity gain site register and the units allocated to your development. Price is negotiated, varies by habitat type, and reflects the real cost of creating and managing that habitat for 30 years plus the provider's margin and risk.

Statutory credits are bought from the government as a backstop. Defra publishes the price schedule by habitat tier and reviews it, and the prices are set high on purpose so that the market route remains more attractive. Check the current schedule rather than a figure quoted in an older article. Credits also carry a procedural catch: they are a last resort, and the local planning authority will expect to see evidence that on-site and off-site options were properly explored before it approves a gain plan relying on them.

The 30 year management cost behind a unit

A unit is not a commodity, it is a 30-year obligation that somebody has priced. Understanding what sits inside the price tells you whether a quotation is credible.

The provider has to establish the habitat, manage it to reach and hold target condition, monitor it and report, and accept the risk that condition is not achieved. On grassland that means a cutting and grazing regime, scrub control, and a monitoring survey at defined intervals. On woodland creation it means establishment maintenance, beating up failures, and a much longer time multiplier. The cost is front-loaded and the income is not, which is why habitat banks sell units at a premium over the bare management cost.

A quotation that looks unusually low usually means one of three things: the habitat is low distinctiveness and the units are priced low for a reason, the management commitment is thin, or the legal security is weak. Ask to see the management plan and the monitoring schedule before you commit.

Legal costs of securing off-site land

Securing is a separate cost line from buying, and on a small site it can be a surprising share of the total.

Off-site gain has to be secured by a planning obligation under section 106 or by a conservation covenant with a designated responsible body, and registered on the biodiversity gain site register. If you are buying allocated units from an established habitat bank, the legal work is largely done and you are paying for an allocation. If you are arranging your own off-site land, perhaps a field belonging to a family member, you are paying for the drafting, the council's legal fees, the registration fee and any monitoring fee the authority charges.

Budget for the council's legal costs explicitly, because they are recovered from you and they are not trivial for a bespoke agreement. Where a small scheme is marginal, the legal cost of a bespoke arrangement often exceeds the saving over buying allocated units.

Timing of purchase against the gain plan

The sequence creates a cash flow problem that catches small developers.

Permission is granted subject to the biodiversity gain condition. The biodiversity gain plan then has to be submitted and approved by the local planning authority before development may begin. The plan has to show how the gain will be achieved, which means the off-site units need to be identified and secured, or the credits purchased, before you can start on site.

So the money goes out before the first brick, and before any sales income. That is a pre-start obligation of exactly the kind that derails programmes, and it belongs on the same list as every other condition you cannot discharge quickly.

Two practical responses. Get the metric done at design stage, not after permission, so you know the shortfall and can negotiate while you have time. And get a written quotation with a validity period from a provider before you submit the application, so the figure in your appraisal is a real one.

What drives price differences between providers

Four things explain most of the variation in quotations for the same number of units.

Habitat type and distinctiveness, because creating species-rich grassland or wet woodland costs far more than enhancing poor grassland, and the trading rules may force you into the expensive category. Location, through the spatial risk multiplier, which means local units are worth more to you even at a higher price per unit. Security and credibility, since a habitat bank with a registered gain site, a signed covenant and a monitoring regime is selling something different from a landowner with an intention. And stage of delivery, because units from habitat already created and in condition carry less risk, and less risk costs more.

Ask every provider the same five questions: which habitat type, which local planning authority and national character area, what spatial multiplier applies to my site, is the gain site already registered, and what happens if target condition is not met.

A worked illustrative budget for a small site

Illustrative example, with the arithmetic shown and the unit price left as the quotation you obtain, because publishing a price would be a guess.

A self-builder has permission for two dwellings on a 0.3 hectare plot. The baseline includes modern species-poor grassland and 40 metres of species-poor hedgerow with gaps. The post-development design retains 25 metres of hedge, creates garden and a small area of ornamental planting, and achieves part of the gain on site.

The metric reports a residual shortfall of 0.42 area units and 0.08 hedgerow units. An off-site provider in the adjacent local planning authority is identified, so the spatial risk multiplier reduces the value of the units bought and the purchase has to be grossed up accordingly. The budget then has these lines: the ecological survey and metric work, the off-site area units at the quoted price grossed up for the multiplier, the off-site hedgerow units separately, the allocation and registration costs, the council's legal costs for the section 106 or the covenant, any authority monitoring fee, and a contingency for a second iteration of the metric if the layout changes.

Build that table in your appraisal with the provider's quotation dropped in, and revisit it whenever the layout moves a hedge. The shortfall is sensitive to small design changes, and a scheme that keeps 10 more metres of hedgerow can reduce the hedgerow purchase to nothing.

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. See PlanScope pricing if you are comparing the cost of drafting your own supporting documents. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.