Defra publishes two biodiversity metrics, and the simpler one is optional rather than automatic. Qualifying as a small development only gets you to the door — four separate site conditions decide whether you are actually allowed through it.
Two tools, one of them optional
Defra publishes two biodiversity calculators: the statutory biodiversity metric and the Small Sites Metric (SSM). The GOV.UK guidance on calculating biodiversity value puts the relationship plainly — if a project qualifies as a small development, you can use the statutory biodiversity metric, or if you prefer, you can use the simpler Small Sites Metric.
That wording does two useful things. It confirms the SSM is a concession rather than an obligation, so a qualifying small site may always elect the full metric. And it establishes the order of the questions: qualifying as a small development is necessary but not sufficient.
Before any of this, establish whether BNG applies to the site at all. Since 6 August 2026 a site of 0.2 hectares or less is exempt where the development does not impact onsite priority habitat, which takes a large share of former SSM users out of the regime completely. There is no point choosing a metric for a site that does not need one.
Step one: does it qualify as minor development?
The metric borrows the definition of major development from the Town and Country Planning (Development Management Procedure) (England) Order 2015 and treats everything below it as minor. In numbers:
| Development type | Qualifies as minor when |
|---|---|
| Residential, dwelling count known | Between 1 and 9 dwellings inclusive, on a site of 1 hectare or less |
| Residential, dwelling count unknown | Site area under 0.5 hectares |
| Non-residential | Floor space created under 1,000 m², or total site area under 1 hectare |
Two details catch people out. The unknown-dwelling-count branch is stricter, not more lenient — not knowing how many homes you are building halves the area allowance from 1 hectare to 0.5. And the non-residential test is an or, so a 900 m² unit on a 3 hectare plot qualifies on floor space even though it fails comfortably on area.
Nationally Significant Infrastructure Projects are excluded from the SSM outright, regardless of the footprint of any individual element.
Step two: the four conditions that rule it out
This is where most sites that qualify on size still end up on the full metric. The SSM user guide — published by Natural England as JP040 — excludes the tool where any of the following is true on site:
- Habitats present are not listed in the tool. The SSM carries a reduced habitat list. A habitat it does not recognise cannot be assessed in it, and that alone sends you to the full metric.
- Priority habitat is present on site, with limited exceptions — certain hedgerows and arable field margins are generally carved out. Anything else on the section 41 list present inside the red line excludes the tool.
- A statutory protected site or habitat is present — an SSSI, SAC, SPA, Ramsar site or similar designation inside the boundary.
- European protected species are present — bats, great crested newts, dormice, otters and the rest of the Schedule 2 list.
Note the asymmetry between this and the 0.2 hectare exemption. For the exemption, the test is whether the development impacts priority habitat. For SSM eligibility, the test is whether priority habitat is present. A site can have a hedge it does not touch, be in scope for BNG because it is 0.4 hectares, and be eligible for the SSM because hedgerows are among the carve-outs — or be excluded, if the priority habitat present is something other than a hedge or field margin. These are different tests and they do not move together.
Species surveys do not go away. The fourth condition is about which calculator you may use. It is not a substitute for the protected species work itself. A small site with bat roost potential still needs the survey, the mitigation and, where relevant, the licence, whichever metric the biodiversity units are counted in.
The 500 metre question
The most frequently misstated rule in this area concerns habitat near the site rather than on it.
Priority habitat or a protected site outside the red line but within 500 metres does not automatically exclude the SSM. What the guidance does is warn you off: Natural England recommends the full statutory metric where high-value habitat is that close, and several local planning authorities say the same in their own validation checklists.
The practical answer is to raise it with the authority before you commit. An SSM assessment on a site 200 metres from an SSSI is not invalid, but it is the kind of submission a validation officer queries, and discovering that at validation costs more time than the conversation would have.
Who is allowed to complete it
The SSM must be completed by a competent person, and the significant point is that this person does not have to be an ecologist. That is the main reason the tool exists: it lets an architect, planning consultant or informed developer produce a defensible calculation on a straightforward site without commissioning a full ecological appraisal.
The full statutory metric sets a higher bar. Its condition assessments require judgements about habitat quality against published criteria, and the July 2025 revision of the user guide tightened the competency wording further alongside clarifying strategic significance and adding the recording of individual trees. In practice a statutory metric submission on anything other than a trivial site is ecologist work.
That difference is a genuine cost difference, which is why the eligibility question gets asked optimistically. It is worth resisting: a metric completed in the wrong tool has to be redone, and redone late.
What the simpler tool gives up
The SSM is simplified in specific, knowable ways. It uses a shorter habitat list, a reduced condition assessment, and fewer modifiers than the full metric. The consequence is that the two tools can return different unit figures for the same site.
Usually that is immaterial on a small site with uncomplicated habitat. Occasionally it is not, and the direction is not reliably favourable — the simplifications can understate a baseline as easily as overstate it. Where a site is marginal, where the 10% gain is going to be tight, or where off-site units are likely to be bought against the result, running the full metric even though you qualify for the simpler one can be the cheaper decision. Units bought against an inflated shortfall cost real money.
Common mistakes
- Treating size as the whole test. Qualifying as minor development is step one of two. The four site conditions do most of the exclusion work.
- Confusing the exemption test with the eligibility test. Impact on priority habitat decides the 0.2 hectare exemption; presence of priority habitat decides SSM eligibility.
- Reading the 500 metre rule as a hard exclusion. It is a recommendation, and a conversation with the authority, not an automatic bar.
- Assuming no ecologist is needed anywhere. The SSM relaxes who completes the metric. Protected species surveys and licences are untouched.
- Choosing the SSM to save a few hundred pounds on a site where units will be bought. A difference in the calculated baseline becomes a difference in the number of off-site units purchased.
- Using a stale tool version. Both metrics are versioned and local authorities check which version was used. Download the current tool for each new assessment rather than reusing the spreadsheet from the last job.
How this connects to your wider environmental reporting
Metric choice is the point where a biodiversity assessment either stays auditable or quietly stops being so. The tool version, the habitat parcels, the condition judgements and the person who made them are all things a local planning authority can ask about months later, when the Biodiversity Gain Plan is being discharged and nobody can remember which spreadsheet was the final one.
That is the same record-keeping problem as whole-life carbon assessments and Biodiversity Net Gain more generally: the calculation is the easy part, and the version control around it is what gets challenged. VerdCore runs both of Defra's metrics with an eligibility check in front of them, so the tool you used and the site conditions you used it under stay attached to the result.