SSSIs are designated under the Wildlife & Countryside Act 1981 (WCA). In Wales, Natural Resources Wales (NRW) are responsible for identifying these important areas. SSSIs can be designated on land in private ownership as well as on land owned by public bodies and conservation charities. Protection of the SSSI is achieved by prohibiting damaging operations, unless permission has been granted by NRW. NRW are responsible for the condition of all SSSIs in Wales. See their website here for more information.
The Countryside and Rights of Way Act (CRoW) 2000 made some important amendments to the WCA and to the way SSSIs are protected.
Development of land around or adjacent to SSSIs can affect the quality of the SSSI. This is particularly true with regard to drainage and rivers or streams that are SSSIs.
NRW should be consulted before even considering development on or adjacent to a SSSI.