Official tools · England
Small Sites Metric for BNG explained
The Small Sites Metric (SSM) is the official simplified biodiversity metric tool for certain minor developments in England under mandatory biodiversity net gain. If your project meets GOV.UK’s small-site criteria and does not trigger the published exclusions (for example priority habitats, protected sites, or European protected species on site), you may use the SSM instead of the full statutory biodiversity metric calculation tool. Either way, planning submissions need the official metric outputs — not a third-party estimator alone.
What the Small Sites Metric is
The Small Sites Metric is Defra’s simpler official calculation tool for biodiversity net gain on qualifying small (“minor”) developments in England. It follows the same mandatory 10% net-gain framing as the full statutory biodiversity metric: you measure habitat value in biodiversity units before and after development, and the post-development total must deliver at least a 10% gain where BNG applies.
GOV.UK explains that if a project qualifies as a small development, you can use the full statutory biodiversity metric calculation tool, or — if you prefer and the exclusions do not apply — the Small Sites Metric. The SSM fixes condition values for habitats it assesses, so a separate condition assessment is not required as part of the existing-habitat survey when you use it. Download the official tools and user guides from GOV.UK; do not treat this hub page as a substitute for those documents.
Official starting points: Calculate biodiversity value with the statutory biodiversity metric — GOV.UK and Statutory biodiversity metric tools and guides — GOV.UK. For SSM-specific rules, see the Small Sites Metric user guide (July 2025 PDF).
Who may use it (eligibility in plain English)
GOV.UK defines a small (“minor”) development for this purpose as one that does not fall into the “major development” category in article 2(1) of the Town and Country Planning (Development Management Procedure) (England) Order 2015. In the calculate guidance, that includes:
- residential development of between 1 and 9 dwellings on a site of 1 hectare or less (or, if the number of dwellings is unknown, a site area less than 0.5 hectares);
- commercial development where the floor space created is less than 1,000 square metres or the total site area is less than 1 hectare;
- development that is not mineral working / mineral-working deposits, and not waste development.
Even when a project is a small development, GOV.UK states that the SSM cannot be used on sites where any of the following are present:
- priority habitats (excluding some hedgerows and arable field margins);
- protected sites;
- European protected species.
If you qualify to use the SSM, GOV.UK says you do not need an ecologist for the BNG metric calculation itself — though you may still choose ecological advice — and someone familiar with the site and its habitats (for example a project manager, gardener or landscape architect) may complete the simpler tool. That is GOV.UK’s framing for SSM eligibility, not a guarantee that your local planning authority will accept every submission without further input. Re-check the live GOV.UK page and the SSM user guide before you rely on these criteria; rules and page updates can change.
Separately, from 6 August 2026 GOV.UK records that BNG does not apply to planning applications for developments of 0.2 hectares or below. That is an exemption question, not the same as “use the SSM” — see the exemptions section below and our BNG exemptions overview.
Small Sites Metric vs full statutory metric
Both tools are official. The choice is about whether your small development is allowed to use the simpler SSM pathway. For the wider metric overview on this hub, see BNG metric.
| Topic | Small Sites Metric (SSM) | Full statutory metric tool |
|---|---|---|
| Who it is for | Qualifying small (“minor”) developments that also clear the SSM exclusions | Mandatory BNG calculations generally, including major developments and small sites that cannot or choose not to use the SSM |
| Complexity | Simplified official tool; fixed condition values (no separate condition assessment for existing habitat when using SSM) | Full calculation tool; competent person such as an ecologist expected except where SSM applies |
| When the other tool is required | Cannot be used where priority habitats (with stated hedgerow/arable-margin exceptions), protected sites, or European protected species are present | Use when SSM eligibility fails, for majors, or when you prefer the full tool on a small site |
| Off-site units | SSM has no off-site module; GOV.UK still allows off-site units with extra steps described in guidance | Includes on-site and off-site sections in the official tool |
| One application | Use one official tool pathway for the application’s metric calculation — do not mix SSM and full-tool outputs as if they were a single calculation. Follow the SSM user guide and GOV.UK calculate guidance for the submission package. | |
If you need off-site units while using the SSM, GOV.UK explains that you still submit the completed SSM with your biodiversity gain plan, summarise how the BNG requirement and trading rules are met, send the completed SSM to the off-site provider, and that allocation on the biodiversity gain site register uses the on-site and off-site sections of the statutory metric tool. That process detail lives on GOV.UK — not on this hub.
How this relates to the Hub calculator
The Hub’s biodiversity net gain calculator (non-statutory estimator) is an educational illustration of unit logic. It is designed to help applicants and agents understand how area, distinctiveness, condition and strategic significance interact before they commission surveys or open the official spreadsheet tools.
It is not the Small Sites Metric, not the full statutory biodiversity metric calculation tool, and not endorsed by Natural England or Defra. Do not paste Hub outputs into a planning submission as if they were official metric results. For real applications, download the official SSM or full metric from GOV.UK and use a competent person or ecologist where the rules require it. For the end-to-end process, read what a BNG assessment involves.
Exemptions are a different question
Some developments are exempt from biodiversity net gain entirely — for example certain householder applications, and (from 6 August 2026) planning applications for developments of 0.2 hectares or below, among other categories listed on GOV.UK. Being exempt means BNG does not apply to that application. That is not the same as “my site is small, so I use the SSM”.
SSM eligibility only arises when BNG applies and the development qualifies as a small site that also clears the SSM exclusions. If you are checking whether BNG applies at all, start with BNG exemptions (separate from SSM eligibility) and the official GOV.UK — Biodiversity net gain: exempt developments guidance.
Practical next steps
- Confirm whether mandatory BNG applies to your planning application (including any exemption that may remove the duty entirely).
- If BNG applies and the project is a small development, check SSM eligibility and exclusions on the live GOV.UK calculate page and in the SSM user guide.
- Download the official SSM or full statutory metric tools and guides from the GOV.UK tools publication — not from this hub.
- Get competent advice when habitat, protected-site or species flags appear, or when you are unsure which tool pathway applies.
- Use our directory only as a listings page for finding providers — listing is not an endorsement.
For how units and off-site options fit the wider picture, see biodiversity units and off-site options.
Common questions
What is the Small Sites Metric?
It is Defra’s official simplified biodiversity metric tool for qualifying small (“minor”) developments in England under mandatory BNG. It still measures biodiversity units toward the 10% net-gain objective, with fixed condition values and a simpler workflow than the full statutory tool.
When can a small development use it instead of the full statutory metric?
When the project meets GOV.UK’s small-development definition and the site does not have the published SSM exclusions (priority habitats with the stated exceptions, protected sites, or European protected species). Re-check GOV.UK before acting.
When is the full metric required instead?
For major developments, for small sites that fail SSM exclusions, and whenever you prefer or need the full statutory calculation tool. Except for qualifying SSM use, GOV.UK expects a competent person such as an ecologist to use the metric tool.
Is the Hub calculator the Small Sites Metric?
No. The Hub calculator is a non-statutory estimator for learning only. It is not Natural England–endorsed and must not be submitted as the official calculation. Use the GOV.UK SSM or full statutory metric for submissions.
Sources
Official guidance this page relies on. Always check the live GOV.UK page before acting. This hub is independent educational content — not Natural England, Defra or a local planning authority.