Eligibility
The SSM applies to sites under 1 ha that meet either: a residential threshold of 1 to 9 dwellings or, where the number of dwellings is unknown, an area of less than 0.5 ha; or a non-residential floorspace under 1,000 sqm on a site under 1 ha. The SSM is not available for priority habitat or designated sites; those revert to the full SBM regardless of area.[S2]
Interaction with the 31 July 2026 0.2 ha small-sites exemption
A site below 0.2 ha containing no priority habitat is now exempt from BNG entirely under the 31 July 2026 exemption rules; the SSM is therefore only material for the 0.2 to 1 ha band.[S4] A self-builder remodelling a 0.15 ha curtilage does not need either tool; a 0.6 ha infill on lowland modified grassland does.
Input set
- Site boundary area in hectares.
- Dominant baseline habitat (UKHab Level 3).
- Condition (poor, moderate, good) per the SSM condition card.
- Post-development habitat composition.
Strategic significance defaults to medium where the LPA has not adopted a Local Nature Recovery Strategy mapping; this choice is conservative and tends to slightly under-state the post-development units.[S11]
Feasibility cost build-up for a self-builder
A 0.4 ha self-build on modified grassland typically requires a PEA at £1,500 to 2,500, an SSM output prepared by the ecologist, and a delta of approximately 0.5 to 1.5 BU at low-to-medium distinctiveness. On-site delivery via species-rich grassland creation runs £8k to 18k including 30-yr management; off-site purchase clears around £10k to 20k; statutory credits would attach the punitive £84k floor and are unlikely to be the rational route.[S8]