--- title: 'Statutory Metric or Small Sites Metric: Which One Do You Use?' description: "When you can use Defra's Small Sites Metric instead of the full statutory biodiversity metric, the four conditions that rule it out, the 500 metre question, and who is allowed to complete it." metaTitle: 'Statutory Metric or Small Sites Metric?' category: Compliance date: 2026-10-26 readingTime: 8 min read lead: 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. cardDescription: What counts as minor development, the four conditions that rule out the Small Sites Metric, why priority habitat and European protected species matter more than site size, the 500 metre question, and who is competent to complete an assessment. ctaHeading: Run both metrics on your own habitat data ctaBody: VerdCore includes Defra's statutory biodiversity metric and the Small Sites Metric side by side, with an eligibility check that tells you which one your site qualifies for before you start entering habitat parcels. footerNote: This guide is general information, not legal, planning or ecological advice. Metric eligibility conditions and tool versions change — check the current Small Sites Metric user guide and GOV.UK metric guidance before relying on either tool for a submission. ogTitle: Statutory Metric or Small Sites Metric? ogDescription: "When you can use Defra's Small Sites Metric instead of the full statutory biodiversity metric, and the four conditions that rule it out." faq: - q: What is the Small Sites Metric? a: A simplified version of Defra's statutory biodiversity metric, published by Natural England, designed so that small developments can calculate biodiversity units without the full habitat condition assessment the main metric requires. Using it is optional; a qualifying small site may still use the full statutory metric. - q: What counts as a small or minor development for the metric? a: Residential development of between 1 and 9 dwellings on a site of 1 hectare or less, or where the number of dwellings is unknown a site area under 0.5 hectares. For non-residential development, floor space created under 1,000 square metres or a total site area under 1 hectare. - q: When can you not use the Small Sites Metric? a: Where habitats present on site are not listed in the tool, where priority habitat is present on site other than certain hedgerows and arable field margins, where a statutory protected site or habitat is present, or where European protected species are present. Nationally Significant Infrastructure Projects cannot use it either. - q: Does priority habitat within 500 metres rule out the Small Sites Metric? a: Not automatically. Habitat outside the red line but within 500 metres does not by itself exclude the tool, but Natural England recommends using the full statutory metric where high-value habitat is that close, and it is worth agreeing the approach with the local planning authority first. - q: Do you need an ecologist to complete the Small Sites Metric? a: The assessment must be completed by a competent person, but that person does not have to be an ecologist. The full statutory metric expects a higher level of habitat condition assessment competence, which in practice usually does mean an ecologist. - q: Is the Small Sites Metric less accurate than the statutory metric? a: It is less granular by design. It uses a reduced habitat list and a simplified condition assessment, which trades precision for accessibility, and it can produce a different unit figure for the same site than the full metric would. - q: Which metric do you submit with a Biodiversity Gain Plan? a: Either, provided the site qualifies for the one used. The completed metric calculation tool is submitted alongside the Biodiversity Gain Plan, and the local planning authority will check that the tool used was one the site was eligible to use. --- ## 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](https://www.gov.uk/guidance/calculate-biodiversity-value-with-the-statutory-biodiversity-metric) 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](/resources/bng-small-sites-exemption.html) 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](https://publications.naturalengland.org.uk/publication/6047259574927360) — excludes the tool where any of the following is true on site: 1. **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. 2. **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. 3. **A statutory protected site or habitat is present** — an SSSI, SAC, SPA, Ramsar site or similar designation inside the boundary. 4. **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.
Choosing a metric
1
Is BNG in scope?
Red line over 0.2 ha, or priority habitat impacted. If not, no metric is needed at all.
2
Does it qualify as minor?
1 to 9 dwellings on 1 ha or less, under 0.5 ha if the count is unknown, or under 1,000 m² of floor space.
3
Do the four exclusions apply?
Unlisted habitats, priority habitat beyond the hedgerow and field margin carve-outs, protected sites, European protected species.
4
Choose deliberately
Eligible for the SSM is not the same as better off using it. Tight gains and bought units favour the full metric.
## 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](/resources/whole-life-carbon-assessments.html) and [Biodiversity Net Gain](/resources/biodiversity-net-gain-explained.html) 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](/bng-reporting-software.html) 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.