Budgeting the statutory metric and the biodiversity gain plan

Two separate pieces of work sit behind the words biodiversity net gain, and they get billed separately. The first is the statutory biodiversity metric, completed from a habitat survey of your site before you apply. The second is the biodiversity gain plan, prepared after permission is granted and approved by the local planning authority before development can begin. Budget for both, plus a 30 year management and monitoring commitment, and plan the survey around the growing season rather than your programme.

For a self-builder putting up one to nine homes, the first question is not cost at all. It is whether you are exempt.

Who has to complete the metric

Mandatory net gain applies through Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021. It took effect on 12 February 2024 for major development and on 2 April 2024 for small sites. Most planning permissions now carry a deemed condition requiring a 10 per cent gain in biodiversity value, secured for at least 30 years.

The exemptions are where small schemes live or die. Householder applications are out. So is development below the de minimis threshold, which turns on a small area of on-site habitat and a short length of linear habitat rather than on the size of the building. And there is a self-build and custom housebuilding exemption for schemes of no more than nine dwellings on a site of no more than half a hectare, where the development consists exclusively of self-build or custom build housing.

Check your eligibility against the current government guidance rather than against a summary, because the drafting is tighter than most people expect and a single market unit in the scheme can remove the exemption. If you are exempt, you still need to say so on the application form and explain why.

Baseline survey timing and the growing season

The metric needs a habitat baseline, and the baseline needs a site survey. Grassland, scrub and ruderal vegetation can only be classified and condition-assessed when the plants are identifiable, which for botanical work in England means roughly April to September.

Miss that window on a site with any grassland interest and you have three options: wait, survey outside the optimum and accept a precautionary worst-case classification, or justify a desk-based assumption that the authority may reject. The first costs months. The second usually costs habitat units, because an assessor who cannot verify condition will not award good condition.

Book the survey as soon as you have site control. This is the single biggest programme risk on small sites, and it is entirely avoidable.

Habitat condition assessment and its evidence

The metric does not value habitat by area alone. Each parcel is classified to the UK Habitat Classification and then scored for condition against published criteria, so a half hectare of species-poor modified grassland and a half hectare of other neutral grassland in good condition give very different baselines.

That assessment is the part of the work a reviewer will probe. Expect the output to include a habitat plan with parcel boundaries, completed condition sheets, species lists, fixed-point photographs and a statement of survey limitations. A Preliminary Ecological Appraisal is often commissioned at the same time, because the surveyor is already on site and the two documents share the habitat map.

The statutory metric is expected to be completed by a competent person under the Environment Act 2021 regime, and the biodiversity gain plan is approved by the authority. A Biodiversity Net Gain Assessment drafted in advance helps you structure the evidence and brief that person, and it is a starting point for their work rather than a replacement for it.

The small sites metric and where it applies

The small sites metric is a simplified version with fewer inputs and a shorter condition assessment, which means less assessor time. It is available for smaller residential and non-residential schemes within the thresholds set out in the government guidance, broadly where the site area and dwelling numbers are low.

There are carve-outs. The small sites metric cannot be used where priority habitats are present on site, beyond limited exceptions, or where the development affects a protected site or European site. If your ecologist finds a priority habitat during the walkover, you move to the full metric mid-project, and the fee changes. Ask for the quote to state which metric is assumed and what happens if the assumption fails.

Producing the gain plan after permission

The biodiversity gain plan is a post-permission document. It sets out the pre-development biodiversity value, the post-development value of on-site habitat, any registered off-site gains, any statutory biodiversity credits purchased, and how the 10 per cent is met.

Three cost drivers sit here. The first is whether the on-site scheme can deliver the gain at all, because a tight plot with parking and drainage often cannot. The second is off-site units, bought from a habitat bank and registered on the biodiversity gain sites register. The third is statutory biodiversity credits, the government-run backstop, priced in published tiers that are deliberately set above market rates to discourage their use. Those prices are revised, so check the current schedule rather than any figure you were quoted last year.

Allow time as well as money. The gain plan has to be approved before development begins, and that approval is a separate determination with its own timescale. Builders who plan to start on site the week after permission are the ones who get caught.

Management, monitoring and legal costs

The 30 year obligation is the part self-builders underestimate most. Delivering on-site habitat means a habitat management and monitoring plan, secured either by a planning obligation under section 106 or by a conservation covenant with a responsible body.

That package involves three fees people forget: the ecologist writing the management plan, the solicitor drafting the agreement, and the authority's own legal and monitoring charges. Monitoring reports are usually required at set intervals across the 30 years, and someone has to be funded to produce them. Where the habitat sits in a private garden, expect the authority to ask hard questions about enforceability.

The honest position is that on very small plots an off-site unit purchase can cost less overall and is far simpler than a 30 year on-site commitment, even though it looks more expensive on the first invoice. Price both.

Programme risk if a season is missed

Put the sequence on a programme before you commit to a build start.

1. Confirm whether an exemption applies, in writing, on the basis of the current guidance.
2. Commission the habitat survey inside the growing season window.
3. Run the metric and test whether on-site gain is achievable alongside the layout.
4. Price off-site units and credits as alternatives, and keep the fallback live.
5. Submit the application with the metric output and a clear gain statement.
6. Prepare the gain plan immediately on grant, not when the contractor is ready.
7. Complete the section 106 agreement or covenant and allow for legal timescales.

Steps two and six are the ones that slip. The survey window is fixed by biology and the gain plan is fixed by statute, so neither bends to a contractor's start date.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified ecologist before relying on it, and see PlanScope pricing if you want to compare the cost of drafting documents in-house.

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