Walkthrough: a twelve unit scheme and its gain plan

A biodiversity gain plan example is more useful than a summary of the rules, because the difficulty in delivering the biodiversity gain condition is almost never the 10 per cent. It is the sequence: survey in the right season, metric before layout is fixed, off-site units secured before the plan is submitted, and a management obligation that someone will still be honouring in 2056.

What follows is an illustrative walkthrough of a twelve unit scheme on a 0.9 hectare edge-of-village site in England. Twelve dwellings makes it major development, so it falls under the regime that applied from 12 February 2024 rather than the small sites arrangements that followed on 2 April 2024.

The site, the baseline survey and who did it

The site was a single field of improved grassland in poor condition, with a defunct species-poor hedgerow along the road frontage, a hedgerow with trees on the eastern boundary, and a block of bramble scrub in one corner. No watercourse.

The survey was carried out in June by an ecologist working to the UK Habitat Classification and the condition assessment sheets that come with the statutory metric. That timing matters. Botanical condition assessment needs the growing season, and a February survey would have produced a defensible habitat map with an indefensible condition score.

Biodiversity net gain is a regime where a competent person is expected to complete the statutory metric, so this is not work that can be done from aerial photography by a non-specialist. A Biodiversity Net Gain Assessment drafted as a starting point still needs that professional to run and sign off the calculation, and the local planning authority approves the resulting gain plan.

The same visit picked up the protected species questions, which is the usual arrangement, with the ecology baseline reported through a Preliminary Ecological Appraisal alongside the metric work.

Running the metric and the starting units

The statutory metric keeps three separate accounts: area habitats measured in habitat units, hedgerows in hedgerow units, and watercourses in watercourse units. Each account has to show at least a 10 per cent gain in its own right. Surplus area units cannot be used to cover a hedgerow shortfall, which is the trading rule that catches out schemes where a hedge is removed for an access.

For this illustrative site the baseline came out at roughly 3.6 habitat units across the grassland and scrub, and about 1.9 hedgerow units across the two boundaries. Those are illustrative figures used to show the method, not a result you can transfer to another site.

The metric scores each parcel on distinctiveness, condition and strategic significance, then applies risk multipliers to anything created or improved: difficulty of creation, and a temporal factor reflecting how long the habitat takes to reach its target condition. Created species-rich grassland is discounted heavily because it takes decades to get there and often fails. That discount is why on-site delivery usually underperforms the drawing. The underlying mechanics of these units are set out in the note on habitat units as the currency of biodiversity net gain.

Run the metric before the layout is fixed. On this scheme the first layout put the attenuation basin through the eastern hedgerow, which cost more hedgerow units than the whole rest of the site could replace.

On-site delivery and where it fell short

The revised layout retained both hedgerows, reinforced the frontage hedge with native planting to lift it from poor to moderate condition, created a wildflower meadow strip along the eastern boundary, and put the attenuation feature in the low corner as a seasonally wet grassland.

The hedgerow account passed on site, because reinforcing an existing defunct hedge is one of the few interventions the metric rewards quickly. The area account did not, and it was short by roughly 0.86 units after the houses, gardens, roads and parking had taken their share of the field.

That outcome is normal. A residential density of thirteen or fourteen dwellings per hectare leaves very little space for habitat that scores, and private gardens carry a low value in the metric whatever is planted in them.

Finding off-site units

The hierarchy runs on-site first, then off-site, then statutory biodiversity credits as a last resort. Credits are deliberately priced above the market so that habitat is created on the ground rather than paid away, and the price schedule is published by government and revised, so check the current one.

The scheme bought units from a habitat bank in the same national character area, about eleven miles from the site. Distance costs money in the metric through a spatial risk multiplier, which reduces the value of units delivered outside the local planning authority area or the relevant strategic area. Buying 0.86 units of shortfall therefore meant buying rather more than 0.86 units of habitat.

Three practical points came out of that search. Units must be registered on the biodiversity gain sites register and allocated to the specific development, so an informal agreement with a neighbouring landowner is not enough. Allocation happens at a defined point in the process, and the gain plan cannot be approved without it. And trading rules apply off site as well: medium or higher distinctiveness habitat lost on site has to be replaced with the same broad habitat type or better, wherever the replacement sits.

The habitat management and monitoring plan

Substantial on-site habitat works and all off-site gains have to be maintained for at least 30 years, secured through a planning obligation or a conservation covenant.

The management plan for this scheme covered the meadow strip, the reinforced hedgerow and the wet grassland. It set out the establishment regime for the first five years, the long-term cutting and arisings removal schedule, the hedgerow cutting cycle and rotation, who holds responsibility, and the monitoring points. Monitoring reports were required at years 1, 2, 3, 5 and then at five-yearly intervals, submitted to the authority.

Two things made it workable. A management company funded through an estate service charge, rather than an expectation that individual homeowners would cut a meadow correctly. And a remedial trigger written into the plan, so that a habitat failing to meet its condition target by year 5 triggers a specified intervention rather than an argument.

The gain plan after permission

Permission came with the biodiversity gain condition, which is deemed to be attached to most planning permissions. Development cannot lawfully start until the local planning authority has approved a Biodiversity Gain Plan.

The plan set out the pre-development biodiversity value with the date it was calculated, the steps taken to avoid and minimise harm in line with the mitigation hierarchy, the post-development value on site, the registered off-site units allocated to the scheme, and the completed metric as an appendix. No statutory credits were used.

Treat approval of the gain plan as a condition to be discharged on the critical path, not an administrative formality. The discipline is the same as for any other condition that has to be cleared before work starts, set out in the walkthrough on discharging pre-commencement conditions.

Programme, cost and the legal agreement

The programme, not the cost, is what surprises developers. On this scheme the ecologist's survey window pushed the application by two months because the site was picked up in early spring. The off-site search took another six weeks. The section 106 deed securing the 30 year management ran alongside the gain plan and had to complete before it.

Budget for four separate things: the baseline survey and metric, the off-site units, the legal agreement, and 30 years of management and monitoring. The last of these is a long liability and it needs a funding mechanism that outlives the developer.

One honest uncertainty. The market in off-site units is still maturing, availability varies a lot by region, and prices are not published in any consistent way, so early engagement with brokers and habitat banks is worth more than any assumption drawn from another scheme.

To see which ecology and supporting documents a site is likely to need, enter your postcode on PlanScope and work through the guided questions.

This illustrative example is general guidance, not legal advice, so 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. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.