Biodiversity Net Gain Assessment
Most planning permissions in England now come with a condition: the development must leave biodiversity at least 10% better than it found it, measured with the statutory metric, and the plan to achieve that must be approved before work starts.
A biodiversity net gain assessment measures the habitat on the site, runs the metric against your plans and shows how the gain will be met. For a small site expect £500 to £1,000 plus VAT for the baseline and metric.
- Baseline and metric
- £500 to £1,000plus VAT, small sites
- Required gain
- 10%Kept for at least 30 years
- Small sites exempt
- 0.2 ha or lessSince 6 August 2026
1Who needs it after the August 2026 changes
Mandatory net gain applies to applications made from 12 February 2024, and to smaller applications from 2 April 2024. On 6 August 2026 the rules changed (Defra), so check which applies to you:
| Type of application | Needs 10% net gain? |
|---|---|
| Householder applications (extensions, loft conversions) | No, exempt |
| Sites of 0.2 hectares or less | No, exempt since 6 August 2026, unless priority habitat is affected |
| Development affecting under 25 m² of habitat or 5 m of hedgerow | No, exempt (de minimis), unless priority habitat is affected |
| Temporary development for five years or less | No, exempt since 6 August 2026 |
| Permitted development | No |
| Self-build and custom build on sites over 0.2 hectares | Yes, for applications made on or after 6 August 2026 |
| Other minor and major development | Yes |
The full list of exemptions is on gov.uk. A brownfield housing exemption was consulted on in spring 2026 and has not been decided.
2What the assessment involves
An ecologist surveys every habitat on the site and scores its condition, which together set the baseline in biodiversity units. They then run the statutory metric on your layout, counting what is lost and what new planting and habitat will add, and work out whether the 10% gain can be met on the site.
Condition is scored most reliably when plants are growing, so the baseline survey is best done between April and September. Hedges, trees and watercourses are measured separately from area habitats.
- Habitat survey and condition assessment of the whole site
- The statutory metric calculation for the existing site and your proposals
- Advice on layout and planting to meet the gain on site
- A biodiversity gain plan for the council to approve before work starts
- A management and monitoring plan for significant on-site gains, kept for 30 years
Not sure if your scheme is exempt? Tell us the site size and the application type and an ecologist will confirm it.
Get a quote3If the gain cannot be met on site
The order of preference is on-site gain first, then off-site units bought from a habitat bank registered with Natural England, then statutory credits as a last resort. Since 6 August 2026, minor sites over 0.2 hectares can go straight to off-site units.
Statutory credits are deliberately expensive: from £42,000 a credit for the lowest tier, and two credits are needed for every unit short (gov.uk, updated 3 November 2026). A good layout that keeps hedges and adds planting is almost always cheaper.
Wales asks for a net benefit for biodiversity through planning policy, and Scotland asks major development to leave biodiversity demonstrably better under NPF4, without the English metric or a fixed percentage.
4What it costs
For a small site, the baseline survey and metric typically cost £500 to £1,000 plus VAT. The gain plan, management plan and any off-site purchase are extra and depend on the scheme. Where you also need a preliminary ecological appraisal, ask for the habitat survey to cover both. See the survey cost guide.
Questions we get asked
Does a self-build need biodiversity net gain?
For applications made on or after 6 August 2026, yes, unless the site is 0.2 hectares or less or another exemption applies. Before that date, self-build and custom build were exempt.
Can the baseline survey be done in winter?
It can, but condition is hardest to judge in winter and the council may question it. Most ecologists prefer April to September for the baseline.
What is a biodiversity gain plan?
The document approved by the council before work starts, showing the baseline, the gain and how it will be delivered and maintained for 30 years. It discharges the general biodiversity gain condition.
Do I need BNG in Wales or Scotland?
Not the English 10% rule. Both countries expect development to benefit biodiversity through planning policy instead.
Where our information comes from
Rules and timings come from the official and professional sources below, read on the dates shown on this page.
Get a quote for a biodiversity net gain assessment
Three short steps: where the site is, what you are planning, and how to reach you. About two minutes. If the council named a survey in a letter or validation checklist, choose that one.
Thank you. We have your enquiry.
- We will email or phone, normally within one working day, with the name of an ecologist who covers your area and the surveys you need.
- If you are happy for them to have your details, we pass them on. If not, nothing is shared.
- The ecologist confirms the scope, the season and the fee in writing. We send no marketing.