What is an SSSI?

An SSSI, or Site of Special Scientific Interest, is an area of land that has been formally and legally recognised as having important flora, fauna, geology or physical features. They have a surprisingly long and complex history, being first described in the National Parks and Access to the Countryside Act 1949. 

Part 3 (Nature Conservation) Provision 23 stated,

Duty of Conservancy to inform local planning authorities of areas of special scientific interest
Where the Nature Conservancy are of opinion that any area of land, not being land for the time being managed as a nature reserve, is of special interest by reason of its flora, fauna, or geological or physiographical features, it shall be the duty of the Conservancy to notify that fact to the local planning authority in whose area the land is situated.

Source

This was superseded by the Wildlife and Countryside Act 1981 which provided greater detail on how an SSSI can be designated, how land owners are to be informed of the designation, and the penalties for damage or destruction on an SSSI. Further amendments were made in the Countryside and Rights of Way Act 2000, mostly relating to compulsory purchase of land, and the Natural Environment and Rural Communities Act 2006 which updates the notification methods to landowners and penalties for not following these instructions. 

The Nature Conservancy mentioned above was established in the National Parks and Access to the Countryside Act 1949 and was replaced by the Nature Conservancy Council via the Nature Conservancy Council Act 1973. This Act remained in force until the Environmental Protection Act 1990 was passed in 1991 when it was divided along country lines and in England became English Nature. In 2006 it changed its name again to Natural England following the passage of the Natural Environment and Rural Communities Act 2006.

So that's the boring legislative stuff, but what does it actually mean in practice? Well, mostly it means that the landowners have a responsibility to look after the land. They must inform Natural England of any work that may cause harm to the SSSI prior to carrying it out and must get permission for the work to take place (Wildlife and Countryside Act 1981 28.5). If someone is found to have caused harm they can not only be fined (Natural Environment and Rural Communities Act 2006 55.2) but made to undertake remedial works to repair the damage (Wildlife and Countryside Act 1981 31.1).

It should be noted that the fines are not limited to landowners, but to anyone who causes damage so make sure you treat the land with respect (as you should be doing anyway!).