A listed building is a building which has been placed on the Statutory List of Buildings of Special Architectural or Historic Interest.
It is a widely used status, applied to around half a million buildings. A listed building may not be demolished, extended or altered without special permission from the local planning authority.
The listed status covers not only the listed building but also everything with the curtilage of that listed building ie. within the boundary.
The Listed status of a building creates both limitations and opportunities. Improving or extending a Listed building can be done and you shouldn't shy from it. It does require sensitivity and skill on the part of the architect and client to make it work.