There are over 500,000 listed buildings in the UK, ranging from grand castles and cathedrals to humble cottages and industrial relics.
They are graded according to their significance:
Grade I: Buildings of exceptional interest (2.5% of listed buildings)
Grade II*: Particularly important buildings of more than special interest (5.8%)
Grade II: Buildings of special interest warranting every effort to preserve them (91.7%)
Any work carried out on a listed building must preserve its special character and interest.
This includes both external and internal alterations, extensions, and sometimes even repairs.
Carrying out unauthorised works on a listed building is a criminal offence, punishable by unlimited fines and even imprisonment.