A scheme that failed on biodiversity net gain

A biodiversity net gain refusal rarely happens at the planning application stage. It happens afterwards, when the biodiversity gain plan goes in for approval and the authority says no, and the site cannot lawfully start. This walkthrough follows an illustrative composite of a seven dwelling scheme on a paddock, built from the errors that recur rather than from any single real case. No figures here are quoted from a real decision.

The permission had been granted. The developer had a start date, a contractor and a funding drawdown tied to commencement. Then the gain plan came back rejected because the hedgerow units went backwards, and eleven weeks disappeared.

The site, the scheme and the baseline survey

The site was a 0.6 hectare paddock on the edge of a village, grazed intermittently, with a species rich hedgerow on the road frontage, a line of ash along one boundary and a small seasonal pond in the corner. Seven detached houses, a shared private drive, and an access taken through the frontage hedge.

Because the site was over 0.5 hectares and the scheme was not self-build, the statutory regime applied in full, and the exemptions did not help. The baseline survey was carried out in late January by an ecologist appointed late in the design process, after the layout had been drawn.

That sequence is the root cause of most of what followed. A baseline survey taken after the layout is fixed can only measure what the layout is about to destroy.

Where the first metric output went wrong

Three errors sat in the first metric return, and they are all common.

The grassland was classified as modified grassland in moderate condition. A February visit cannot assess grassland condition properly, because the sward is not identifiable, and the ecologist had flagged this as a limitation in the report. The authority's ecologist read the limitation, disagreed with the condition score, and required a spring resurvey. A higher baseline condition means more units to replace.

The hedgerow was included in the area habitat calculation rather than reported as hedgerow units. The statutory metric treats area habitats, hedgerows and watercourses as separate currencies, and the 10 per cent gain has to be achieved separately for each. Netting a hedgerow loss off an area habitat surplus does not work.

And the pond was recorded as absent because it was dry on the day of the visit. A seasonal pond is a habitat, and on a site with great crested newt potential it carries implications well beyond the metric.

The habitat the layout quietly destroyed

The access through the frontage hedge removed about 12 metres of species rich hedgerow with trees. In metric terms that is high distinctiveness habitat, and the trading rules require loss of high distinctiveness habitat to be compensated with the same habitat type in the same or better condition. You cannot replace a species rich hedgerow with a new mixed native hedge and call it equivalent, because a newly planted hedge starts in poor condition and takes decades to reach good condition.

The temporal risk multiplier in the metric applies a discount for the time a created habitat takes to reach its target condition. A hedgerow assessed at 30 years to target condition loses a substantial proportion of its theoretical units to that discount.

So the first gain plan showed a comfortable surplus on area habitats and a deficit on hedgerow units. One currency passed, the other failed, and the plan was rejected.

The ash line brought a second problem. Trees with ash dieback still count in the baseline at their current condition, and felling them for safety reasons during construction does not remove the obligation to replace their value. That needed a Preliminary Ecological Appraisal and a tree assessment read together, which had not happened.

On-site options tested and their limits

Four options were modelled before anything was bought.

1. Re-route the access to an existing field gate at the eastern end of the frontage, avoiding the species rich section entirely. This recovered most of the hedgerow baseline.
2. Plant new native hedgerow along the internal plot boundaries and the rear of the site, adding roughly 85 metres of new linear habitat.
3. Convert the remaining paddock margins from modified grassland to other neutral grassland under a management regime, which raises the post-development condition score.
4. Retain and enhance the pond with a shallow margin and native marginal planting.

Options one and two did most of the work. Option three was constrained by what the eventual residents could realistically be required to maintain, which is a recurring tension on small schemes: the metric rewards habitat that needs active management, and private gardens are not managed habitat.

The option that was rejected was hedgerow translocation. It is sometimes possible and it is rarely creditable at the original condition, because survival is uncertain and the metric does not reward optimism.

A Landscape Strategy prepared alongside the revised metric is what turned these options into a layout the authority could approve, rather than a list of intentions.

Buying off-site units and the cost

After the on-site redesign, the hedgerow currency was still short by a fraction of a unit. The hierarchy in the regime is clear: on-site first, then off-site units, then statutory biodiversity credits as a last resort.

Off-site units have to come from a site registered on the biodiversity gain site register, with the units allocated to the development, and the spatial risk multiplier applies a discount where the off-site land sits outside the local planning authority area or the relevant national character area. A unit bought locally is worth more in the metric than the same unit bought at a distance.

Statutory credits are deliberately priced above the market for off-site units, and the schedule is published by Defra, so the credit route is the expensive backstop by design rather than an alternative to look at first. Check the current schedule if you need the figures.

The fraction of a hedgerow unit was bought from a registered site in the same national character area. Small residual deficits are the normal case on small schemes, and the brokers who deal in them will not usually sell fractions below a floor, so there is a minimum transaction size to allow for.

The revised gain plan and its approval

The revised submission comprised a spring habitat resurvey with a corrected condition assessment, a revised metric with area, hedgerow and watercourse currencies reported separately, an amended layout showing the relocated access, a habitat management and monitoring plan covering 30 years, and the allocation evidence for the off-site units.

The 30 year commitment was secured through a planning obligation, with monitoring reports due at set intervals and a named responsible party. On a seven unit scheme that obligation outlives the developer's involvement entirely, which is why the authority wanted the management body identified rather than left to a future management company.

The plan was approved. Total delay from first rejection to approval was close to three months, and the layout change cost two plots a metre of frontage.

Biodiversity net gain work requires a competent person under the Environment Act 2021 regime, and the biodiversity gain plan has to be approved by the local planning authority. An AI-assisted Biodiversity Net Gain Assessment is a starting point for that professional rather than a substitute for them, and the metric itself has to be completed by someone competent to do it. If you need a reviewer, the professional directory is the route.

Lessons for the next application

Commission the ecology before the layout, not after it. Survey in season, which for grassland condition means spring or summer rather than a winter visit with a caveat attached. Report the three currencies separately from the start and check each against the 10 per cent requirement individually. And treat any access through a hedgerow as a design decision with a unit cost attached, because that is exactly what it is.

The pattern of commissioning technical work too late shows up in other scenarios too, including the solar array on agricultural land where the land classification decided the outcome.

This is general guidance, not legal advice, and the walkthrough is illustrative rather than a report of a real decision. For specialist technical reports, qualified professional input may be required. Check the position with your local planning authority or a qualified professional before relying on it.

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