Sites of Special Scientific Interest Site Units are divisions of SSSIs based on habitat, tenure and management, and are the basis for recording all information on SSSI Condition and management. They range in Area from 0.004ha up to 18,000ha and only overlap where SSSIs overlap.  A Site of Special Scientific Interest (SSSI) is the land notified as an SSSI under the Wildlife and Countryside Act (1981), as amended. Sites notified under the 1949 Act only are not included in the Data set. SSSI are the finest sites for wildlife and natural features in England, supporting many characteristic, rare and endangered species, habitats and natural features. The data do not include "proposed" sites. Boundaries are generally mapped against Ordnance Survey MasterMap.Full metadata can be viewed on data.gov.uk.