Biodiversity net gain or habitats regulations assessment
One regime measures habitat units and asks for 10 per cent more. The other asks whether a designated site would be harmed, and refuses permission if doubt remains. They are not interchangeable.
Biodiversity net gain and habitats regulations assessment answer different questions under different statutes, and a scheme can pass one while failing the other. Net gain asks whether the development leaves measurably more biodiversity value than it found, scored on a statutory metric and secured for thirty years. An appropriate assessment asks whether the proposal will harm the integrity of a designated European site, and unless the answer is no beyond reasonable scientific doubt, permission cannot lawfully be granted.
One is an arithmetic obligation with a fixed target. The other is a legal bar with no target at all.
Two regimes, two legal bases
Mandatory biodiversity net gain comes from the Environment Act 2021, which inserted Schedule 7A into the Town and Country Planning Act 1990. It works through a biodiversity gain condition deemed to be attached to most planning permissions, requiring a gain of at least 10 per cent measured on the statutory metric, with habitat secured and managed for at least thirty years.
Appropriate assessment comes from the Conservation of Habitats and Species Regulations 2017. Where a plan or project is likely to affect a habitats site, the competent authority must assess the implications for that site's conservation objectives, and may only agree to the project having concluded that it will not adversely affect the site's integrity.
The difference in character matters more than the difference in source. A net gain shortfall is a problem to be solved by buying units. An adverse effect on integrity is not a problem to be bought off, except through a derogation that needs an absence of alternatives and imperative reasons of overriding public interest.
What the statutory metric measures
The metric converts habitat into units. Area habitats, hedgerows and watercourses are each scored separately, using distinctiveness, condition, strategic significance and, for proposed habitat, difficulty of creation and time to target condition with risk multipliers applied.
It is a proxy, deliberately. It does not measure species, it does not measure ecological function, and it does not care whether the habitat it scores sits next to a designated site or three miles from one, beyond the strategic significance multiplier. A wildflower meadow created on a car park generates units whether or not anything moves into it.
That abstraction is the metric's strength for administration and its weakness for ecology. Our note on habitat units as the currency of biodiversity net gain sets out how the scoring works in practice, and the wider statutory framework covers the duties that sit behind it.
What an appropriate assessment asks
The habitats test is specific to a site and its qualifying features. The questions are whether the proposal could affect the features for which the site was designated, through which pathways, and whether the conservation objectives for those features can still be met.
Pathways are the practical heart of it. Recreational disturbance from new residents, nutrient enrichment through the wastewater system, air quality effects from traffic on a heathland, hydrological change, lighting spill, and construction noise are all recognised routes by which a scheme several kilometres away can affect a designated site. Distance is not a defence.
Two features of the test catch developers out. Mitigation can be taken into account in the assessment, but the mitigation has to be certain, and vague commitments do not reduce the level of scrutiny. And where doubt cannot be removed, the authority must refuse, whatever the planning merits of the scheme.
Who decides, and when
Both regimes put the local planning authority in the driving seat, in different roles and at different moments.
| Biodiversity net gain | Habitats regulations assessment | |
|---|---|---|
| Legal source | Environment Act 2021, Schedule 7A TCPA 1990 | Conservation of Habitats and Species Regulations 2017 |
| What is measured | Habitat units on the statutory metric | Effects on a site's qualifying features and integrity |
| Target | At least 10 per cent gain | No adverse effect on integrity |
| Timing | Gain plan approved after permission, before work starts | Assessment completed before permission is granted |
| Key consultee | None statutory, though councils take ecological advice | Natural England |
| If it fails | Condition unmet, development cannot lawfully start | Permission cannot lawfully be granted |
The timing row is the one to internalise. An appropriate assessment that has not been done is a defect in the permission itself, and permissions have been quashed on that basis. A gain plan is a later stage, and the biodiversity gain plan as the document after permission explains how that approval runs.
Mitigation that can and cannot serve both
There is real overlap on the ground and very little overlap in law.
A wetland created to strip nutrients from wastewater may also score habitat units. A heathland management contribution made to address recreational disturbance may also improve habitat condition. In each case the ecological work is the same, and it is tempting to count it twice.
Two rules limit that. Net gain requires the gain to be additional to what would have happened anyway, so habitat delivered because another legal regime compels it is hard to claim as a metric gain, and councils increasingly ask the question directly. And mitigation relied on in an appropriate assessment has to be secured for the purpose of that assessment, so it cannot be traded, released or varied later to suit a metric calculation.
Where the site sits in a nutrient catchment, the interaction is particularly awkward and is covered in what nutrient neutrality stops and where.
Exemptions and thresholds
The two regimes have completely different entry points.
Net gain has statutory exemptions, including householder development, certain self-build and custom build schemes, development below a small de minimis threshold of on-site habitat, and development on a biodiversity gain site itself. Most other planning permissions carry the condition, with the small site regime applying since April 2024 and the major regime since February 2024.
The habitats regime has no exemptions of that kind. It applies wherever a likely effect on a habitats site can be identified, which means a single dwelling in the wrong catchment can be caught while a hundred houses in the right place are not. Screening, not scale, decides it.
What goes wrong when they are treated as one
Three failures recur.
The first is offering net gain as mitigation for an effect on a designated site. It does not work, because the metric measures habitat value generally while the habitats test asks about specific features of a specific site, and an inspector will say so.
The second is running the ecology programme as one workstream with one budget, which produces a preliminary appraisal scoped for the metric and silent on pathways to a site 4 kilometres away. Commission a Preliminary Ecological Appraisal that names both regimes in its brief.
The third is a timing failure: securing the habitats mitigation through the same agreement as the net gain habitat, with a single review clause, and then finding that the clause cannot be operated without reopening the appropriate assessment.
The Environment Act 2021 regime expects the statutory metric to be completed by a competent person, and ecological assessment for the habitats test needs a suitably qualified ecologist. A Biodiversity Net Gain Assessment drafted in advance gives that professional a starting point rather than a blank page.
This is general guidance, not legal advice. Check the position with your local planning authority, Natural England 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. For specialist technical reports, qualified professional input may be required.
Frequently asked questions
Can biodiversity net gain be used to offset harm to a protected site?
No. The statutory metric measures habitat value in general terms, while the habitats test asks whether specific qualifying features of a specific site can still meet their conservation objectives. Offering metric units as mitigation for an adverse effect on integrity will not satisfy the Conservation of Habitats and Species Regulations 2017, and inspectors say so plainly.
When does each assessment have to be completed?
The appropriate assessment must be completed before permission is granted, because a permission issued without it is defective and can be quashed. The biodiversity gain plan comes later, approved by the council after permission and before development starts, under the gain condition deemed to be attached to most permissions.
Does every application need a habitats regulations assessment?
No. It is triggered by screening rather than by scale. Where a likely effect on a habitats site can be identified through a pathway such as recreational disturbance, nutrient enrichment, air quality or hydrological change, the assessment follows, even for a single dwelling. Schemes with no identified pathway are screened out at the first stage.
Which developments are exempt from biodiversity net gain?
The statutory exemptions include householder development, certain self-build and custom build schemes, development below a small de minimis threshold of on-site habitat, and development on a biodiversity gain site itself. Major development has been within the regime since February 2024 and small sites since April 2024, so check the exemption against the current guidance.
Can the same habitat count for both regimes?
Rarely, and never by assumption. Net gain has to be additional, so habitat delivered because another legal regime compels it is hard to claim on the metric. Mitigation relied on in an appropriate assessment must also stay secured for that purpose, which means it cannot be varied later to suit a metric calculation.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.