Irreplaceable habitats and the biodiversity gain exception

Where a development would affect irreplaceable habitat biodiversity net gain works differently. The ten per cent objective in Schedule 7A to the Town and Country Planning Act 1990 does not apply to that habitat at all. Instead the biodiversity gain plan has to set out bespoke compensation agreed with the local planning authority, and the authority has to be satisfied that the loss was avoided or minimised first. The statutory metric cannot value these habitats, so there is nothing to trade.

That is a harder test than the percentage, not an easier one.

Which habitats are treated as irreplaceable

The Biodiversity Gain Requirements (Irreplaceable Habitat) Regulations 2024 define the category for the purposes of the biodiversity gain condition. The schedule to those regulations lists habitat types including ancient woodland, ancient and veteran trees, blanket bog, limestone pavement, coastal sand dunes and lowland fen. Read the schedule itself rather than a summary, because the list is specific and the habitat descriptions matter at the margins.

Identification is the first practical problem. Ancient woodland is mapped on the Ancient Woodland Inventory, which is a starting point and not a boundary survey. Inventory polygons are frequently wrong at the edges, and unmapped ancient woodland exists on plenty of sites. Veteran trees are not mapped at all in any systematic way, and a single veteran oak in a hedgerow can change a layout. An ecologist walking the site is the only reliable method, and that survey should happen before the masterplan is fixed rather than after.

Blanket bog and limestone pavement are less likely to be misidentified but more likely to be undervalued in an early feasibility, because their extent on a site is often larger than the visible core.

Why the percentage objective does not apply

The statutory biodiversity metric works by scoring habitat distinctiveness and condition, then comparing the pre-development and post-development scores. That method assumes the habitat can in principle be recreated somewhere. For irreplaceable habitat the assumption fails, so the metric excludes it and the percentage calculation runs on the rest of the site only.

This is a modification of the biodiversity gain condition rather than an exemption from it. A development affecting ancient woodland still needs a biodiversity gain plan, still needs to hit the ten per cent objective on the habitats that are covered, and additionally needs approved bespoke compensation for the irreplaceable element. Exemptions, such as those for householder development, self-build and the de minimis threshold, work differently and remove the condition altogether. Confusing the two is a common error in early advice.

What bespoke compensation means in practice

Bespoke compensation is negotiated, site-specific and not reducible to a number of units. What authorities and Natural England look for tends to fall into a few groups:

• Measures that reduce the residual loss further, such as redesigning to avoid part of the habitat, moving an access, or reducing a buffer incursion.
• Long term protection and management of retained habitat on the site, secured for decades rather than years, usually through a planning obligation.
• Restoration of degraded habitat of the same type elsewhere in the same landscape, where such habitat exists and can be brought into better condition.
• Measures addressing the specific mechanism of harm, for example deer fencing, hydrological restoration for a bog, or removal of a drainage ditch that has been dewatering a fen.

New planting is not compensation for ancient woodland. Everyone in the process knows that, so a strategy that offers a hectare of new broadleaf woodland in exchange for half a hectare of ancient woodland will be read as an admission that the harm was not avoided.

Illustrative example: a 90 unit scheme where the access road clips 0.12 hectares of an ancient woodland edge. The accepted package involves realigning the road to remove the direct loss, a 15 metre buffer of new native planting managed under a thirty year plan, permanent fencing to prevent garden encroachment, and a contribution to invasive species control across the remaining woodland. The metric then runs on the rest of the site as normal.

How the biodiversity gain plan handles it

The plan has to describe the irreplaceable habitat on the site, the steps taken to avoid and then minimise the adverse effect, and the bespoke compensation proposed. The authority cannot approve the plan unless it is satisfied on those points, and approval of the plan is what discharges the biodiversity gain condition before development starts.

Sequencing matters here more than on an ordinary site. Because the compensation is negotiated rather than calculated, it is not something that can be resolved quietly at the gain plan stage after permission. Bring it into the application, agree the principles before determination, and record them in the planning obligation. A Biodiversity Net Gain Assessment that identifies irreplaceable habitat and leaves the compensation to be settled later is storing up a problem at the gain plan approval stage.

The policy test that sits alongside it

The biodiversity gain condition is a statutory requirement. National planning policy is a separate and older constraint, and it is usually the harder one to satisfy. The National Planning Policy Framework maintains the position that development resulting in the loss or deterioration of irreplaceable habitats should be refused unless there are wholly exceptional reasons and a suitable compensation strategy exists. The Framework was reorganised into policy-coded chapters in the edition published on 17 August 2026, so check the reference against the edition you are citing before quoting it in a statement. Reading the restructured Framework alongside your local plan policy is worth an hour before drafting.

That bar is a high one, and the examples historically given are infrastructure of national importance and mineral extraction of national significance. A housing scheme rarely clears it. Which means the practical strategy on most sites is avoidance, not justification.

Evidence the authority will expect

Assemble four things. A habitat survey by a suitably qualified ecologist identifying and mapping the features, with the method stated. An arboricultural assessment of any ancient or veteran trees, including root protection areas, since the buffer is usually the binding constraint rather than the canopy. A plan showing buffers, construction exclusion zones and the route of every service. And a management plan for the retained habitat with a duration, a responsible body and a funding mechanism.

Buffer distances come from the published advice on ancient woodland and veteran trees, which recommends a minimum buffer around ancient woodland and a root protection area for veteran trees derived from stem diameter or canopy extent, whichever gives the larger area. Councils apply those figures closely, and a scheme that encroaches is usually asked to redesign rather than mitigate. A Preliminary Ecological Appraisal is the normal first step, and it should be commissioned early enough to influence the layout.

Avoidance as the working assumption

On a site with irreplaceable habitat, the realistic approach is to design the development so the habitat and its buffer are untouched, then treat everything else as the developable area. That is a land budget decision, not an ecology decision, and it belongs in the appraisal before the land is bought.

Where avoidance is not possible at all, start the conversation with the authority and Natural England early, and expect it to take months rather than weeks. As at September 2026 the regulations described here are in force, and small site and exemption rules differ, so check whether the small sites regime applies to your scheme before assuming the full framework does.

Biodiversity net gain work is legally required to be carried out by a competent person under the Environment Act 2021 regime, and the biodiversity gain plan is approved by the local planning authority. Any draft PlanScope produces is a starting point for that professional rather than a substitute, and for specialist technical reports, qualified professional input may be required. This is general guidance rather than 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.