The Planning (Listed Buildings and Conservation Areas) Act 1990, as amended by the Historic Environment (Wales) Act 2016, requires the Welsh Ministers to draw up a list of buildings of special architectural or historic Interest. Assessment of structures for listing falls to Cadw's inspectors of historic buildings, who employ criteria set out in Annex B of Technical Advice Note 24: The Historic Environment.
Following a systematic survey of Welsh communities completed in 2005, over 30,000 buildings in Wales have been listed. All buildings constructed before 1700 in anything like their original condition qualify for listing, as do most buildings from between 1700 and 1840. Among buildings constructed between 1840 and 1914, only those of a definite quality and character qualify (especially significant works of principal architects). Some buildings from between 1914 and 1939 and a small number of post-war buildings have also been listed. A building of merit, whatever its age, is eligible for consideration for listing.
Listed buildings are graded under three categories:
Grade I — buildings of exceptional interest
Grade II* — buildings of particular importance
Grade II — buildings of special interest.
The great majority of listed buildings are grade II and less than 10% have been placed in the higher categories.