Biodiversity net gain costs in a development appraisal

A BNG cost appraisal has to carry six separate lines, and the one most often missed is the largest. Ecology fees, on-site habitat capital cost, thirty years of management and monitoring, off-site unit purchase, statutory credits where nothing else works, and the development value of the land you take out of the developable area. That last line is where a tight urban site becomes unviable and a large greenfield site barely notices.

Which costs the appraisal has to carry

Mandatory biodiversity net gain took effect on 12 February 2024 for major development and 2 April 2024 for small sites, through the biodiversity gain condition inserted into the Town and Country Planning Act 1990 by the Environment Act 2021. The requirement is a ten per cent gain, measured with the statutory metric, secured and managed for at least thirty years, and set out in a biodiversity gain plan the local planning authority has to approve.

Each of those words has a cost attached:

• Baseline habitat survey and classification, metric completion, and the biodiversity gain plan itself
• Habitat creation and enhancement works on site, at a habitat specification rather than an amenity landscape specification
• A habitat management and monitoring plan, then thirty years of delivering it, including monitoring surveys and reporting to the authority
• Off-site biodiversity units purchased and registered where on-site delivery falls short
• Statutory biodiversity credits where off-site units are not available
• The residual development value of land removed from the developable area

Set against those, a few exemptions remove the requirement entirely: householder applications, self-build and custom build of up to nine dwellings on a site under 0.5 hectares, and the de minimis case where the development affects less than 25 m² of on-site habitat and less than 5 metres of on-site linear habitat. Test the exemptions before you price anything, because they are the least costly answer available.

On-site delivery against off-site units

On-site delivery looks lower cost per unit and often is not, because the metric rewards distinctiveness and condition rather than area. Creating a modest area of species-rich grassland in good condition generates more units than a large area of amenity grassland, but it needs soil stripping or nutrient reduction, a proper seed mix, and a management regime that a housing estate service charge will not naturally fund.

Off-site units are bought from a habitat bank or a landowner whose site is entered on the biodiversity gain sites register. The price per unit varies by habitat type, because trading rules require like-for-like or better, and a scheme losing hedgerow or watercourse units cannot make them up with grassland. Two further costs ride alongside the unit price: the registration fee payable for entering an allocation on the register, and the legal cost of the planning obligation or conservation covenant that secures the off-site land for thirty years.

Distance costs money. The metric applies a spatial risk multiplier that reduces the value of units delivered further from the development, so units in the same local planning authority area or national character area go further than units bought two counties away. Price the multiplier, not just the headline unit rate.

Statutory credits as the deliberate last resort

Statutory biodiversity credits are sold by the government and are priced, deliberately, above what the market charges for equivalent units. They exist so that a scheme cannot be blocked by the absence of available habitat, not as a convenient route. The prices are published in tiers by habitat type and are reviewed, so take the current figures from the published schedule rather than from an appraisal template.

Treat credits as a risk allowance rather than a base case. An appraisal that assumes credits from the outset will show a cost the authority reads as avoidable, and an appraisal that assumes market units with no fallback carries an unpriced risk. The defensible approach is to price market units as the base case and carry the credit cost as a sensitivity.

Thirty years of management and monitoring

This line is routinely understated. Thirty years of managing species-rich grassland, a wetland feature or a new woodland edge is not thirty years of mowing, and the monitoring obligation adds surveys at intervals set in the management plan plus a report to the authority each time.

Build the figure from the management plan, not from a rate per hectare. Annual operations, periodic operations such as coppicing or scrub control on a rotation, replacement planting for failures, monitoring survey fees, reporting fees, and any monitoring fee the authority charges for checking compliance. Then decide how it is funded, because that decision changes the appraisal.

Three funding routes appear in practice. A capitalised sum paid to a third party or to the authority at the start. An annual charge on a management company or estate service charge, which works on larger schemes and is resisted on small ones. Or retention by the landowner under a covenant, which suits a farm holding and rarely suits a developer who intends to sell and leave.

State which route you have assumed, and index the cost. A flat annual figure held constant for thirty years is not an assumption anyone will accept.

Land taken out of development, valued properly

If the habitat solution takes a quarter of a hectare out of a 1.5 hectare residential site, the cost to the appraisal is not the cost of sowing the seed. It is the residual value of the plots that no longer fit.

And value it at the site's residual land value per hectare, or by removing the units and re-running the appraisal. Valuing it at agricultural value, which happens more often than it should, understates the cost by an order of magnitude and produces an appraisal the authority's consultant will dismantle.

There is a legitimate efficiency here, and it needs care. Land can serve more than one function, so an attenuation basin, a green corridor or an area of open space can also carry habitat. What it cannot do is score well in the metric while being used intensively. A play space mown weekly is amenity grassland in poor condition, and claiming it as species-rich grassland in good condition will be corrected. Design the dual use honestly and the saving is real.

Evidence the authority will accept

Quotes, not estimates. A written quotation from a habitat bank for the units required, dated, with the habitat types and the spatial multiplier applied. Landscape contractor rates for the creation works. The management and monitoring plan itself as the source of the ongoing cost, with unit rates attached. The published credit price schedule where credits are in the appraisal. The metric output as the authoritative statement of how many units are needed.

The metric and the gain plan need a competent person. The Environment Act 2021 regime expects the statutory metric to be completed by someone competent to do it, and the biodiversity gain plan is approved by the authority on the strength of that work, so a Biodiversity Net Gain Assessment drafted with AI assistance is a starting point for a qualified ecologist rather than a substitute for one. The habitat baseline usually comes out of a Preliminary Ecological Appraisal done by the same team.

Where the numbers get challenged

Six recurring arguments, in rough order of how often they succeed:

1. The baseline habitat condition was scored low, inflating the gain achieved by modest works
2. Land take was priced at agricultural or amenity value rather than residual development value
3. Management costs were taken from a rate card rather than from the management plan, and were not indexed
4. The same land was counted for open space, drainage and habitat without meeting the metric's condition criteria
5. Off-site units were priced without the spatial risk multiplier
6. On-site delivery was assumed on land outside the applicant's control, with no mechanism to secure it

The fourth and sixth are the ones that unravel an appraisal completely rather than just adjusting it. Where delivery depends on land or works outside your control, the mechanism matters as much as the money, and the choice between Grampian conditions and planning obligations is the right place to start. Where the whole appraisal is contested, the approach taken to viability at appeal and confidential appraisals applies to the biodiversity line like any other.

Put the biodiversity cost into the Viability Assessment as a named line with its own evidence, rather than folding it into a contingency. Authorities have got much better at spotting that.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and take credit prices and registration fees from the current published schedules. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.