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Biodiversity net gain baselines: what has to be right before the metric is run
The biodiversity net gain rules in force in November 2026, where approved gain plans go wrong, and how to build a baseline that survives a second look.
- Biodiversity net gain of at least 10 per cent has applied to most planning applications in England since 2024, and to new nationally significant infrastructure applications from 2 November 2026.
- A 2026 audit of 263 approved gain plans found defects in 104 of them. Most were evidential: baseline condition, transcription and missing metric files.
- A baseline that is mapped, dated and repeatable is the cheapest protection a developer or habitat bank can buy.
A biodiversity net gain baseline is the count of habitat units on a site before development. The statutory biodiversity metric turns habitat into that number, and every later promise rests on it. This guide covers the rules as they stand in November 2026 and the places where baselines most often fail.
What are the biodiversity net gain rules in November 2026?
| Date | What applies |
|---|---|
| 12 February 2024 | At least 10 per cent net gain becomes mandatory for major planning applications in England |
| 2 April 2024 | Small developments are brought in |
| 6 August 2026 | Amending regulations take effect, including an exemption for development with a red line boundary of 0.2 hectares or less, unless priority habitat on the site is harmed (Defra) |
| 2 November 2026 | Net gain becomes mandatory for new nationally significant infrastructure project applications in England (Defra) |
Gains must be secured for at least 30 years. Off-site gains are recorded on the biodiversity gain site register, which Defra said held over 350 sites in late October 2026.
What are the three ways to deliver net gain?
On site
Habitat created or improved inside the red line. It is the first choice in the hierarchy, and it leaves the developer holding a 30 year management duty on land that people will live or work on.
Off site
Units bought from a registered gain site, often a habitat bank. The gain is recorded on the public register and secured by a planning obligation or a conservation covenant.
Statutory credits
The last resort. Two credits are needed for each unit, and prices start at £42,000 a credit (GOV.UK). They are priced so that on-site and off-site units are almost always cheaper.
Where do biodiversity net gain baselines go wrong?
The Bristol Tree Forum read 263 approved gain plans from 31 planning authorities and found 143 defects across 104 plans. Fewer than half of the plans were published with their metric file.
The audit is the work of volunteers and covers a sample. Treat it as a strong signal and not a national rate. The pattern is the useful part: the weak points are evidential.
- Baseline condition. Scores that cannot be traced to a dated survey.
- Transcription and metric errors. Figures that change between the survey, the metric and the plan.
- Missing metric files. A plan published without its metric cannot be checked.
- Register mismatches. Off-site gains claimed in a plan with no matching entry on the register.
Have a metric you need to rely on? We can reproduce it independently, parcel by parcel.
Ask about a re-testWhat does a baseline that holds up look like?
- Condition scores with no survey date
- Parcel areas that do not sum to the red line
- A gain plan published without its metric
- Figures retyped between documents
- Off-site units with no register entry
- Every score tied to a dated field record
- Every parcel mapped, areas summing to the site
- The metric file published with the plan
- One data set feeding survey, metric and plan
- Register reference quoted for every off-site unit
Map every parcel
Each habitat parcel has a boundary and an area. The parcels sum to the site, and the site matches the red line.
Tie every score to a survey
A reviewer should be able to go from the number to the dated field record in one step.
Publish the metric with the plan
A plan without its metric cannot be checked by anyone.
Have it read independently
A second read from satellite and LiDAR catches errors while they are still cheap to fix.
In our own sealed test, the Vireo Neural Map reproduced the statutory metric on 48 of 49 parcels. That is what an independent re-test offers: a second answer, reached separately, before the figure carries a 30 year obligation.
What changes for infrastructure projects?
From 2 November 2026, new nationally significant infrastructure applications in England must deliver at least 10 per cent net gain. Defra guidance published on 27 August 2026 says the requirement applies to the habitat affected by the development, with on-site or off-site delivery.
For a linear scheme that makes the baseline a corridor exercise: many parcels, many landowners and a long thin survey area. That is where a desk read of the whole route earns its keep.
Is your baseline ready to submit?
Net gain baseline check
Tick what is already in place and written down.
How Vireo helps
Vireo Nature offers an independent re-test: the statutory metric reproduced separately, parcel by parcel, with every difference explained.
For a site, the Site Evidence Pack reads habitat from satellite and LiDAR before the ecologist walks it. For a linear scheme, the Corridor Evidence Pack reads habitats and constraints along the whole route.
Frequently asked questions
Does biodiversity net gain apply in Scotland, Wales or Ireland?
Not in this form. The mandatory metric and unit market are English. Scotland and Wales work through planning policy, and Ireland is preparing its national nature restoration plan.
How long must gains be maintained?
At least 30 years, secured by a planning obligation or a conservation covenant.
Are statutory credits a sensible plan?
They are priced as a last resort. With two credits needed per unit and prices from £42,000 a credit, on-site and off-site units are almost always cheaper.
Is a small site exempt?
From 6 August 2026, development with a red line boundary of 0.2 hectares or less is exempt, unless priority habitat on the site is harmed.
What is the biodiversity gain site register?
The public record of land used for off-site gains. Defra said it held over 350 sites in late October 2026.
Sources
- Defra: Biodiversity net gain amendments and transitional arrangements published (14 July 2026)
- Defra: Biodiversity net gain for major infrastructure projects, new guidance (27 August 2026)
- Bristol Tree Forum: what the approved biodiversity gain plans show (13 August 2026, corrected 21 August 2026)
- GOV.UK: statutory biodiversity credit prices
Three details. The team checks coverage and fit, and replies within two working days.
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