Small Sites Metric: BNG for Small Sites Explained

What is the small sites metric?

The small sites metric (often shortened to SSM) is a simplified version of the statutory biodiversity metric, published by Defra for use on smaller developments in England. Mandatory biodiversity net gain for small sites began on 2 April 2024, three months after the requirement started for major developments. Since then, small builders, self builders, architects and planning agents have needed a way to show a 10% biodiversity net gain without the cost and complexity of a full metric assessment. The small sites metric is that tool.

It is a spreadsheet calculator that uses a reduced list of habitat types and makes some simplifying assumptions about habitat condition. Most notably, the small sites metric assumes habitats are in a fixed, moderate condition rather than requiring a full condition assessment for every parcel. This keeps BNG for small sites proportionate, so that a scheme for three houses on a paddock does not need the same evidence as a 300 home urban extension.

Which sites qualify as a small site?

The simplified BNG metric can only be used where a development meets the small site definition and where the habitats present are suitable. In broad terms, a small site is:

• residential development of between one and nine dwellings on a site of less than one hectare
• residential development where the number of dwellings is not yet known, on a site of less than 0.5 hectares
• non-residential development where the new floorspace is less than 1,000 square metres, or the site area is less than one hectare

Size is only half the test. The small sites metric cannot be used where the site contains priority habitats (other than certain hedgerows and arable field margins), where the site lies within or affects a protected site, or where there are habitats the tool simply does not include. If any of these apply, the statutory biodiversity metric must be used, even though the development is small.

Before relying on the small site route, also check whether the scheme is exempt from biodiversity net gain altogether. The de minimis exemption, householder applications and genuine self build or custom build can take a development out of mandatory BNG entirely. Our guide to BNG exemptions explains these tests in detail.

Can I complete the small sites metric myself?

One of the main attractions of the small sites metric is that Defra intends it to be completed by a competent person, who does not have to be a professional ecologist. An architect, planning consultant or experienced developer can, in principle, record the baseline habitats, enter the proposals and produce the calculation.

In practice, the answer depends on the site. Doing it yourself works best where the habitats are obvious and low value: a garden, a hardstanding yard or a regularly mown amenity lawn. It becomes risky where there are hedgerows, scrub, ponds, rough grassland or trees, because misidentifying a habitat type changes the unit score significantly. Local planning authorities increasingly check small sites submissions, and a metric that understates the baseline can lead to a validation failure, a request for further information or a refusal.

You are likely to need an ecologist when:

• the site has any grassland that is not intensively managed, as it may be a higher distinctiveness habitat
• there are hedgerows, ditches or watercourses on or next to the site
• the site has been cleared or degraded recently, as the baseline may need to be taken from an earlier date
• other ecological issues such as bats, great crested newts or badgers are already in play
• the calculation shows you cannot reach 10% on site and need off-site provision

How the small sites metric works

The small sites metric follows the same logic as the full tool, in a simpler format.

Baseline. You record each habitat on the site before development, its area and, for hedgerows, its length. The metric applies fixed distinctiveness and condition scores.

Post-development. You record which habitats will be lost, retained, enhanced or created. Created habitats might include native hedge planting, a wildflower strip, a small pond, trees in gardens or a green roof.

Result. The tool compares the two and shows whether area habitat and hedgerow units each achieve the required 10% gain. Area and hedgerow units are calculated separately, so one cannot be traded for the other.

The habitat list in the small sites metric is deliberately restricted to habitats that a non-specialist can identify and create. This is also why habitats that are hard to create, or that need specialist management, push a site onto the statutory metric.

Typical small site BNG solutions

Most small sites biodiversity net gain strategies combine a few simple measures. Retaining existing hedgerows and trees protects the baseline. New native species hedgerows along boundaries can provide hedgerow units. Gardens count as vegetated garden habitat, and small areas of wildflower grassland, shrub planting or a pond can lift the area habitat score. Green roofs and street trees can also contribute on tighter urban plots.

Where on-site measures are not enough, a small site can still meet the objective using off-site units. Off-site provision brings registration, legal agreements and spatial rules into play, and in most cases a statutory metric calculation will be needed to support it. Our guide to BNG units explains how off-site units are bought and allocated.

What to submit with a small site planning application

A small site planning application subject to BNG should include the statement that the biodiversity gain condition applies, the completed small sites metric showing pre-development biodiversity value, a plan showing the on-site habitats, and the date used for the baseline. After permission is granted, a biodiversity gain plan must be approved before work starts. Where significant on-site gains are relied upon, the local planning authority may also require a habitat management and monitoring plan and a legal agreement to secure them for 30 years, though this is less common on small schemes.

Common mistakes with the simplified BNG metric

The most frequent problems we see on small sites are using the small sites metric where priority habitats are present, recording a cleared site as bare ground when vegetation was removed after January 2020, double counting gardens, forgetting hedgerows on the boundary, and proposing habitats that will not realistically survive, such as wildflower meadows in shaded back gardens. Each of these can delay a decision or leave a developer with a condition they cannot discharge.

How Collington Winter can help

Our ecologists complete small sites metric assessments for builders, architects and landowners throughout England. We can carry out a quick desk check to confirm whether the small sites metric is appropriate, visit the site to record the baseline, and suggest practical design changes that reach the 10% gain without losing plots. If the site does not qualify, we prepare the statutory metric instead and explain why. Speak to our BNG consultant team or get a quote for your small site.