The Small Sites Metric (SSM) is a tool developed by the Department for Environment, Food and Rural Affairs (Defra) with input from Natural England and others to facilitate the calculation of biodiversity value for small-scale developments. It provides a simplified way to measure biodiversity net gain (BNG) on small sites. The SSM will not capture strategies relying on offsite BNG, so care should be taken when selecting this metric.

Small sites

"Small sites" are defined under the Town and Country Planning (Development Management Procedure) (England) Order 2015 and include:

• residential developments of 1-9 dwellings on sites of up to 1 hectare;
• where the number of dwellings is unknown, the site area is less than 0.5 hectares;
• commercial developments with floorspace under 1,000 square metres or the total site area is less than 1 hectare;
• developments that are not for the winning and working of minerals, or the use of land for mineral-working deposits; and
• developments that are not waste development.

A simplified biodiversity metric

The SSM is a simplified version of the statutory biodiversity metric tool and may not be appropriate for all small sites. The full statutory biodiversity metric tool should be used, even if the site is a small site, where:

• the site has habitat types which are not included in the small site;
• priority habitats are present onsite;
• statutory protected sites or habitats are present; and
• European-protected species are present.

Developers should also consider seeking ecological expertise if their small site is within 500 metres of their development site boundary.

The SSM does not work with offsite BNG strategies. If offsite BNG is relied upon to satisfy the statutory (10%) BNG requirement, applicants using the SSM must therefore provide supplementary documentation detailing how they plan to achieve the required BNG through offsite measures when submitting their biodiversity gain plan to satisfy the statutory BNG condition. If submitting a biodiversity gain plan on this basis, it should outline the offsite enhancements and whether any rules on trading have been breached. The biodiversity gain plan should include:

• Identification of the offsite biodiversity (with references to the Biodiversity Gain Site Register) or statutory BNG credits, if any;
• Detailed offsite strategy: Clearly articulate how offsite measures will compensate for any onsite deficiencies in meeting BNG targets (this includes specifying the location, habitat type and expected outcomes of offsite interventions); and
• Information on the mitigation hierarchy: Applicants should demonstrate that they have attempted to follow the mitigation hierarchy before seeking to rely on offsite biodiversity units or statutory credits.

Emerging from practice

In a recent planning appeal, an Inspector noted that the SSM provided did not account for individual trees lost on site. She had, however, been provided with some data (measured in square metres) on the loss of trees onsite and the proposed increase in the area of trees offsite. She ultimately concluded, without considering the species, age or habitat type, that an overall reduction in the area of trees (as measured in square metres) between onsite and offsite led to a net loss of trees. The SSM measures biodiversity by having regard to the habitat type, size and condition. By reducing the assessment of BNG in respect of trees to just size only, the approach is a significant departure from the SSM and statutory biodiversity metric approach.

While this appeal ultimately failed because the proposed BNG contribution fell foul of the Community Infrastructure Levy tests, the appeal demonstrates how Inspectors may approach the assessment of information outside the SSM and the care that should be taken when supplying information to demonstrate offsite BNG with the SSM.

Conclusion

Caution should be taken when using the SSM with a BNG strategy that relies on offsite units. As soon as material outside the SSM must be provided to demonstrate statutory BNG has been met, the assessment of that material may be outside the statutory methodology for measuring biodiversity. There is an increased likelihood that small sites relying on offsite BNG may be made on a case-by-case basis and with regard to the kind and quality of information a developer submits with its application or appeal.