Individual buildings of special architectural or historic interest are given special protection under the Planning (Listed Buildings and Conservation Areas) Act 1990, which is supplemented by guidance in Welsh Office Circulars 61/96 and 1/98.
Section 1 of the 1990 Act 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. Properties are listed under three categories: The majority, of special interest, are in Grade II. A much smaller number of particularly important buildings are listed as Grade II*. Buildings of exceptional interest (2 per cent of the total) are in the top Grade I.
All buildings constructed before 1700, and which survive in anything like their original condition, qualify for listing, as do most buildings dating to between 1700 and 1840. Between 1840 and 1914, only buildings of a definite quality and character qualify (especially those which are the 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 protection by listing.
Buildings are assessed against criteria set out in Circulars 61/96 & 1/98. The principles of selection were originally drawn up by an expert committee of architects, antiquarians and historians and are still followed today, although the criteria are revised periodically. The following are the main criteria:
- Architectural interest: buildings important to the nation for the interest of their architectural design, decoration and craftsmanship; also important examples of particular building types and techniques (for example, buildings displaying technological innovation or virtuosity) and significant plan forms;
- Historic interest: buildings which illustrate important aspects of the nation’s social, economic, cultural or military history;
- Close historical associations with people or events of importance to Wales;
- Group value: especially where the buildings comprise an important architectural or historic unity or a fine example of planning (for example, squares terraces or model villages).
Local planning authorities have primary responsibility for the protection of listed buildings. However Cadw supports them in fulfilling that role and, in certain cases, may also act to ensure buildings are protected.
The 1990 Listed Buildings Act prohibits the carrying out of any works (either to the exterior or interior) which would affect the character of a building once it is listed unless listed building consent has been obtained from the appropriate planning authority. Works of routine maintenance or like-for-like repair are, normally, exempt from listed building controls. The relevant local authority will provide guidance in any particular case and should be consulted before any work starts.