Biodiversity net gain or ecological impact assessment
The statutory metric and ecological impact assessment answer different questions and are secured by different mechanisms. What each covers, what the metric is silent on, and how to run one survey programme for both.
Biodiversity net gain or ecological impact assessment is not a choice for most major schemes. They answer different questions, they are secured by different mechanisms, and a site with protected species or a nearby designated site will need both. The confusion costs money when a developer commissions one, assumes it covers the ground, and finds out at consultation that the county ecologist wanted the other.
Net gain measures habitat as a tradeable quantity and asks whether the scheme leaves more of it. Ecological impact assessment asks what the scheme does to the features that matter and whether the effects are acceptable. A site can deliver a comfortable metric result and still be refused because it removed a bat roost.
Two different questions
The biodiversity gain condition in Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021, is deemed to be attached to most planning permissions in England. It requires a biodiversity gain plan showing at least a 10 per cent increase in biodiversity value, calculated with the statutory metric and maintained for 30 years. That regime applied to major development from 12 February 2024 and to small sites from 2 April 2024.
Ecological impact assessment is not a statutory regime at all. It is a professional method, set out in the CIEEM guidelines, that identifies important ecological features, predicts effects on them, applies the mitigation hierarchy and reports residual effects. Its legal anchors are elsewhere: the Conservation of Habitats and Species Regulations 2017, the Wildlife and Countryside Act 1981 and the duty in section 40 of the Natural Environment and Rural Communities Act 2006.
So one is arithmetic with a statutory backstop. The other is judgement with statutory consequences.
Units against effects on receptors
The statutory metric converts habitat into units using distinctiveness, condition, strategic significance and area, then applies multipliers for spatial risk, delivery difficulty and time to target condition. The output is a number of habitat units, hedgerow units and watercourse units, compared before and after. The arithmetic is unforgiving and the habitat units that form the currency of net gain behave in ways that surprise people used to area-based thinking.
Ecological impact assessment works on receptors rather than units. It assigns importance to each feature at a geographic scale, from international down to site level, then describes effects by magnitude, extent, duration, reversibility and timing. The conclusion is an expert view on whether an effect is material, not a figure.
Both start from the same survey work. A Preliminary Ecological Appraisal with a habitat survey mapped to the UK Habitat Classification gives the metric its baseline and gives the assessment its feature list. Commission that once, properly, and both products follow from it.
Where the metric is silent
The metric counts habitat. It does not count a great deal else, and this is the gap that catches developers out.
| Issue | Covered by the statutory metric | Covered by ecological impact assessment |
|---|---|---|
| Area and condition of grassland, woodland, hedgerow | Yes | Yes, as context |
| Bat roosts, badger setts, great crested newt populations | No | Yes |
| Effects on a designated site through recreation or nutrients | No | Yes |
| Lighting, noise and severance effects on foraging routes | No | Yes |
| Invasive non-native species on site | No | Yes |
| Functional connectivity to habitat beyond the red line | Only through strategic significance | Yes |
That table is the argument in one place. A metric-only submission on a site with mature trees and a watercourse will draw an objection from the local records centre before the consultation period closes.
Protected species sit outside the metric entirely
Nothing in the net gain regime touches the species licensing system. A Protected Species Survey and any mitigation licence require a suitably licensed ecologist under the Wildlife and Countryside Act 1981 and the Habitats Regulations 2017, and no biodiversity gain plan substitutes for that.
Survey seasons drive the programme far more than the metric does. Bat emergence surveys run from May to September, with the optimal window May to August. Great crested newt environmental DNA sampling runs from 15 April to 30 June. Reptile surveys run April to October, best in April, May and September. Miss those windows and a scheme waits a year, whatever the habitat units say.
Plan the ecology programme backwards from the survey seasons, not forwards from the planning submission date.
How each is secured in a decision
The biodiversity gain plan is approved by the local planning authority after permission is granted and before development begins. On-site gains are usually secured by condition. Off-site gains are secured through a section 106 agreement or a conservation covenant and registered on the biodiversity gain sites register, with the 30 year management obligation attached. Statutory biodiversity credits are the last resort in the hierarchy and are priced to be unattractive.
Ecological mitigation is secured the ordinary way: conditions requiring a construction environmental management plan for biodiversity, a lighting strategy designed to protect commuting routes, timing restrictions on vegetation clearance during the breeding bird season, and a licence obtained before works start.
The difference matters for programme. A biodiversity gain plan that is not approved stops development lawfully starting. An unmet ecology condition usually stops one operation rather than the whole site.
Cost, competence and programme
The Environment Act regime expects the statutory metric to be completed by a competent person, and the biodiversity gain plan to be approved by the authority. A Biodiversity Net Gain Assessment prepared without that competence will not survive scrutiny, and an authority can refuse to approve the plan.
For ecological impact assessment the position is similar in substance. Councils expect a suitably qualified ecologist, normally a CIEEM member working within their own competence, and professional review is strongly advised.
Budget for one survey programme feeding two outputs. The waste comes from commissioning a metric assessment in November from a desk study, then repeating the habitat condition survey the following summer when the ecologist points out that condition cannot be assessed from aerial imagery in winter.
Submitting both without duplication
Run them as one report family with a shared baseline chapter. The habitat survey, the desk study records, the designated sites search and the site description appear once. The metric calculation and the biodiversity gain plan form one output. The impact assessment, mitigation hierarchy and residual effects form the other.
Cross-reference rather than repeat. Where the assessment concludes that a hedgerow must be retained for bat commuting, the metric should show that hedgerow retained, and the landscape drawing should show it too. Inconsistency between the three is the most common reason an ecologist's work is queried at committee.
For a smaller scheme deciding between outline and full permission, note that an outline application still engages the gain condition, and the metric has to be run on the parameters rather than on a detailed layout.
This is general guidance, not 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. For biodiversity net gain and protected species work, a licensed or competent professional is legally required, and a draft is a starting point for that professional rather than a substitute.
Browse all 66 report types at PlanScope to scope the ecology package alongside the rest of the evidence base.
Frequently asked questions
Does a biodiversity net gain assessment replace an ecology report?
No. The statutory metric measures habitat area, distinctiveness and condition. It says nothing about bat roosts, badger setts, great crested newts, lighting effects on foraging routes or recreational pressure on designated sites. Those are assessed through ecological impact assessment and protected species survey work, which carry their own licensing requirements.
Is ecological impact assessment a legal requirement?
Not as a named statutory document. The obligations behind it are legal, including the Conservation of Habitats and Species Regulations 2017, the Wildlife and Countryside Act 1981 and the duty in section 40 of the NERC Act 2006. Councils apply the CIEEM guidelines as the expected method and will ask for the assessment through validation or consultation.
When did mandatory biodiversity net gain start?
The biodiversity gain condition applied to major development from 12 February 2024 and to small sites from 2 April 2024, under Schedule 7A to the Town and Country Planning Act 1990 as inserted by the Environment Act 2021. Nationally significant infrastructure projects followed on a separate timetable. Exemptions include householder applications and certain small self-build schemes.
Who is allowed to complete the statutory biodiversity metric?
The Environment Act regime expects a competent person to complete the metric, and the local planning authority must approve the resulting biodiversity gain plan. In practice that means an ecologist able to assess habitat condition to the published criteria. A metric completed from aerial imagery or from a winter walkover is unlikely to be accepted.
Can I run the metric and the assessment from one survey?
Yes, and you should. A preliminary ecological appraisal with habitats mapped to the UK Habitat Classification supplies the metric baseline and the feature list for the assessment. Commissioning them separately usually means paying for two habitat surveys and reconciling two versions of the same site description.
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.