Budgeting biodiversity net gain: units, credits and monitoring
Biodiversity net gain is four commitments, not one: the metric work, the units themselves, the legal mechanism and thirty years of management. Here is how to budget each line.
Biodiversity net gain cost is not a single line in a development appraisal. It is four separate commitments: the ecological work that produces the baseline and the metric, the on-site or off-site delivery of the units themselves, the legal mechanism that ties them down, and thirty years of management and monitoring. Schemes that budget only for the first two are the ones that discover a problem at the biodiversity gain plan stage, when the permission is already granted and the design is fixed.
What actually drives the bill
The mandatory gain requirement comes from Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021. Most planning permissions now carry a deemed condition that development may not begin until the local planning authority has approved a biodiversity gain plan showing at least a 10 per cent increase against the pre-development baseline, measured with the statutory metric, and secured for 30 years.
Four variables move the number more than anything else. The first is the quality of what is already on site, because a baseline of species-rich grassland or mature hedgerow is expensive to replace and a baseline of hardstanding is not. The second is how much of the site the layout leaves available for habitat. The third is habitat distinctiveness, since the metric's trading rules require like for like or better, so losing a medium distinctiveness habitat cannot be made good with amenity grassland. The fourth is time, because delayed creation and risk multipliers reduce the units a new habitat delivers.
That last point is the one developers find counter-intuitive. Creating woodland does not credit you with mature woodland. The metric discounts for the years it takes to reach the target condition and for the risk of failure, which is why a plan that relies on slow-forming habitats needs more area than the headline percentage suggests.
On-site delivery against off-site units
On-site habitat is usually the cheapest route in cash terms, because you are paying for landscape works you would partly have done anyway. It is not free. It removes developable area, it commits the management company to a specification for three decades, and it brings a monitoring reporting duty that someone has to fund.
Off-site units are bought from a habitat bank or a landowner and registered on the biodiversity gain sites register. Price varies by habitat type, by the national character area and by how scarce that habitat is in your region, which is why a broad market figure is unhelpful for a specific appraisal. Get two or three written quotes for the unit types your metric shortfall actually needs, and get them before you fix the layout, because a shortfall in a scarce habitat type can cost several times what a common one does.
Factor in the register application fee charged by Natural England and the cost of the legal agreement that binds the off-site land, which is normally a conservation covenant or a section 106 agreement. Both sit outside the unit price.
Statutory credits as the last resort
Statutory biodiversity credits are sold by the government where on-site and off-site delivery cannot be achieved. Their prices are published and deliberately set above market rates so that credits remain the final option rather than a convenient one, and the schedule is reviewed, so check the current published prices on gov.uk rather than a figure quoted in an older appraisal.
Treat credits as a risk line in the appraisal, not a plan. Most councils will want evidence that the hierarchy was worked through before they accept a credits-based gain plan.
Thirty years of management and monitoring
The 30-year obligation is where budgets most often fall short. It covers the habitat management works themselves, the condition monitoring at the intervals set in the habitat management and monitoring plan, the reports submitted to the authority, and the administration of whatever body holds the obligation.
Price it as an annual figure across the period and then capitalise it, rather than treating it as a small contingency. Ask three questions early. Who holds the land in year 25? Who pays if a monitoring visit finds the habitat has not reached target condition and remedial works are needed? And is the council asking for a monitoring fee of its own under the section 106 agreement, which many now do on a published scale?
Legal agreements, registration and metric work
Legal costs cover the section 106 or conservation covenant, the council's legal fees, and the registration of an off-site gain site. These are quotable at the outset and should be, because they are fixed costs that do not scale with the size of the gain.
The metric and the ecological survey work behind it are a separate line. A Preliminary Ecological Appraisal establishes what is there, and the habitat condition assessment that feeds the metric has to be done in the growing season for most habitats, which is a programme constraint as much as a cost. The Environment Act 2021 regime expects the statutory metric to be completed by a competent person, so this is work a licensed or suitably qualified ecologist signs, and a Biodiversity Net Gain Assessment drafted in advance is a starting point for that person rather than a substitute for them.
Where design changes reduce the bill
The cheapest unit is the one you do not have to buy. Four design moves reduce the requirement more than any amount of negotiation later.
- Survey and run the metric before the layout is drawn, so that the highest value habitat sits inside the retained area rather than under the access road.
- Keep hedgerows intact and route services away from them, because hedgerow units are traded separately from area units and are often the harder shortfall to fill.
- Design habitat into land you cannot build on anyway, such as attenuation basin margins and buffer strips, provided the management regime is actually compatible.
- Avoid creating habitat types that take decades to reach target condition where a faster-forming type of the same distinctiveness would do.
Illustrative example: a 40-unit scheme on improved grassland with two boundary hedgerows. Running the metric at concept stage showed the proposed spine road crossing the better hedge, which created a hedgerow unit shortfall that no on-site planting could fill within the trading rules. Moving the access 30 metres removed the shortfall entirely and cost nothing but a fortnight of design time.
Building the whole obligation into the appraisal
Set out the commitment as a schedule with six lines: survey and metric, on-site habitat creation works, off-site units, legal and registration, 30-year management and monitoring, and council monitoring fees. Attach an assumption to each, and mark which ones are quoted and which are estimated.
Exemptions are worth checking before any of this. The regime does not apply to development below the de minimis threshold, to householder applications, or to self-build and custom build schemes meeting the stated conditions, and the exemption has to be claimed on the application form rather than assumed. If your scheme sits near a threshold, the cost of the exemption analysis is trivial against the cost of the obligation.
For the drafting side of the evidence base, PlanScope's pricing is one free report, £50 per report or £150 per month for unlimited reports on the Professional tier, which is a fixed and known figure against the variable ecological costs above. That helps with the document, not with the units.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
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.
Frequently asked questions
Why are statutory biodiversity credits so expensive?
The government sets credit prices deliberately above market rates so that on-site and off-site delivery remain the preferred routes. The published schedule is reviewed periodically, so check the current prices on gov.uk rather than relying on a figure in an older appraisal. Most councils also expect evidence that the hierarchy was worked through first.
Who pays for the 30 years of monitoring?
Whoever the legal agreement says pays, which is why the point needs settling before the section 106 is engrossed. Budget the management works, the condition monitoring visits, the reports to the authority and the administration as an annual cost capitalised over the period. Many councils also charge their own monitoring fee on a published scale.
Can I reduce the requirement by changing the layout?
Often, yes. Running the metric before the layout is fixed lets you keep the highest distinctiveness habitat outside the developable area and avoid severing hedgerows, which are traded separately from area habitats. A shortfall created by an access road crossing a hedge can sometimes be removed entirely by moving the access.
Which schemes are exempt?
Development below the de minimis threshold, householder applications, and self-build and custom build schemes meeting the stated conditions sit outside the regime. The exemption must be claimed on the application form with supporting information rather than assumed. Where a scheme sits close to a threshold, get the analysis done before committing to a route.
Who has to complete the statutory metric?
The Environment Act 2021 regime expects the metric to be completed by a competent person, in practice a suitably qualified ecologist who can defend the habitat condition assessments. Condition assessment for most habitats has to be done in the growing season, so the survey window is a programme constraint as well as a cost line.
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.