Geograph - photograph every grid square

A listed building in the UK is one which is on the Statutory List of Buildings of Special Architectural or Historic Interest.
A listed building may not be demolished, extended or altered without special permission from the local planning authority (who typically consult the relevant central government agency, particularly for significant alterations to the more notable listed buildings).
For a building to be included on the list, it must be a man-made structure that survives in something at least approaching its original state. Most structures on the list are buildings, but other structures such as bridges, monuments, sculptures, war memorials, and even milestones and mileposts may also be listed. Some structures can also be classified as Scheduled Ancient Monuments rather than Listed Buildings.
These notes from Link