Budgeting biodiversity net gain on a small site

Mandatory biodiversity net gain reached small sites on 2 April 2024, and the cost of the evidence base on a four or five plot scheme routinely surprises people who budgeted for a survey and nothing else. The biodiversity net gain cost on a small site has five parts: the habitat survey, the metric, any off-site units or statutory credits, the biodiversity gain plan, and thirty years of management and monitoring secured by a legal agreement. The first two are the small numbers. The last three are where a scheme becomes unviable.

There is one exemption worth checking before spending anything, because it removes the whole regime for a category of small developer.

What the small sites regime requires

The duty comes from the Environment Act 2021, which inserted a biodiversity gain condition into planning permissions in England. Development must deliver at least a ten per cent gain in biodiversity value, measured with the statutory metric, and the gain must be maintained for at least thirty years.

Small development, broadly one to nine dwellings on a site under one hectare or a site under half a hectare where the number is unknown, can use the small sites version of the metric. It asks for less habitat detail and takes less time to complete. Crossing to ten dwellings moves you onto the full metric, as covered in what changes across the major development threshold.

Several exemptions apply. Householder applications are out. So is development below the de minimis threshold, which covers schemes affecting less than 25 square metres of on-site habitat and less than five metres of linear habitat such as hedgerow. And self-build and custom housebuilding is exempt where the scheme is no more than nine dwellings on a site of no more than half a hectare and each dwelling is built or commissioned by the occupier. That last one matters to a lot of readers of this article, and it is checked far too late.

Habitat survey and its seasonal window

The baseline survey maps every habitat parcel on the site to the UK Habitat Classification and assesses its condition against published criteria. On a small site that is usually half a day on site and a day of desk work.

The constraint is the season. Condition assessment for grassland and other vegetated habitats depends on identifying the species present, which realistically means April to September, and a survey done in January will either be impossible or will have to make cautious assumptions that cost you units. Botanical work in July on a site you bought in November is a nine month gap in the programme.

Two related costs often ride along. Where protected species may be present, a Preliminary Ecological Appraisal covers both purposes in one visit and costs less than commissioning them separately. Where bats, newts or reptiles are indicated, the Protected Species Survey work that follows has its own narrower seasons, and those surveys must be carried out by a suitably licensed ecologist. That is a legal requirement rather than a preference, and an AI-assisted draft is a starting point for that ecologist, not a substitute for the survey.

Completing the statutory metric

The metric itself is a spreadsheet published by the government and free to download. The cost is the competence to fill it in correctly.

Getting it wrong is expensive in a specific way. Overstate the baseline and you need more gain than you can deliver. Understate it and a validating officer rejects the calculation. Mis-assign a condition category and the trading rules change, because habitats of medium or higher distinctiveness have to be replaced with the same habitat type rather than with anything convenient.

Hedgerows and watercourses are counted in separate modules with their own ten per cent requirement, so a scheme that gains area units while losing 30 metres of hedgerow has not complied. That single point accounts for a good share of small site failures.

The baseline is normally taken at the date of the application. Where habitat has been degraded since 30 January 2020, the baseline reverts to the earlier state, so clearing a scrubby corner before applying gains you nothing and can cost you a great deal.

Off-site units and statutory credits as a cost line

Most small sites cannot deliver ten per cent on site once the houses, drives and gardens are laid out. Gardens score poorly. That leaves three routes, in the order the legislation prefers them: on-site gain, off-site gain, and statutory biodiversity credits bought from the government.

Off-site units are bought from a habitat bank or delivered on other land you control. Prices are set by the market and vary by habitat type and by the national character area, so get quotes rather than assuming. The land must be registered on the biodiversity gain sites register, and the registration itself carries a fee.

Statutory credits are the last resort and are deliberately priced above the market to keep them that way. The tariff is published by government and is revised, so check the current figures rather than a number from an article. On a small scheme, needing credits usually means the layout should be revisited first.

The biodiversity gain plan and its approval

The gain plan is submitted after permission and approved by the council before development starts. It sets out the pre-development biodiversity value, the steps taken to minimise loss, the post-development value, and the off-site or credit arrangements.

Two practical costs attach. The council charges nothing for the approval itself in most cases, but the time it takes is a programme risk, and a refused gain plan stops the site. And the thirty year commitment has to be secured legally, through a planning obligation under section 106 or through a conservation covenant with a responsible body. Legal drafting of a section 106 for a five unit scheme is a real number, and the council's own legal costs are usually payable too.

Thirty years of management and monitoring

This is the line most first-time budgets miss entirely.

A habitat management and monitoring plan has to say who does what, when, for thirty years, and how it is reported to the council. Typical items are annual meadow cuts with arisings removed, hedgerow management on a rotation, tree and shrub replacement, invasive species control, and monitoring surveys at defined intervals with reports submitted to the authority.

On a small site the money usually comes from a management company funded by service charge, from a commuted sum paid to the council, or from the off-site provider where the units were bought. Whichever route is chosen, it has to be costed and evidenced at the planning stage, and an officer who sees no funding mechanism will ask for one.

Where the budget usually goes wrong

1. Designing the layout first and surveying second, so the baseline is fixed before anyone knows what it will cost to beat it.
2. Clearing vegetation before the application, which triggers the degradation rule and raises the baseline you must exceed.
3. Missing the hedgerow module, so the area units balance and the scheme still fails.
4. Ignoring the self-build exemption and paying for a full evidence base that was never required.
5. Budgeting for the survey but not for the legal agreement, the registration fee and the thirty year management.
6. Booking the survey in October.

The order that works is: check exemptions, commission the baseline survey in season, design the layout around the habitat you must retain, run the metric, then price the shortfall. Sequenced that way, the same discipline applies as in budgeting a contaminated land investigation, where the site work has to lead the design rather than follow it.

Biodiversity net gain work under the Environment Act 2021 is expected from a competent person, and a Biodiversity Net Gain Assessment drafted by PlanScope is a structured starting point for that person rather than a replacement for them.

This is general guidance, not legal advice. The regime and the credit tariff change, so check the current government guidance and your local planning authority's position before relying on it.

See PlanScope pricing if you are working out where the drafting fits against the survey budget. Reports should be reviewed before submission, and we do not guarantee planning approval.