The current policy and legislation relating to listed buildings are the Planning (Listed Buildings and Conservation Areas) Act 1990, the National Planning Policy Framework and the National Planning Practice Guide. These are supported by Historic England's Good Practice and advice notes 1 to 3 and Historic England advice notes 1 to 8.
It is not the purpose of the legislation to make it impossible to alter a listed building - rather to ensure that particular care will be taken over decisions affecting its future. Clear justification therefore must be provided for any proposals, and this needs to be included in a heritage statement to accompany and listed building consent application. Advice on this is provided in:
The special architectural or historic interest of the building and the case for its preservation must be considered when determining the merits of any development proposals. Extensions to such buildings may be acceptable, but they must be very carefully designed and must preserve the character of the original building.
Listed Building Consent (which is separate to any planning permission required) must be obtained from the Local Planning Authority to alter or extend any part of the building, any object or structure fixed to the building and any object or structure within the curtilage of the building which dates from before 1 July 1948, which in any way affects its special character. Consent is required for all extensions or alterations (whether internal or external), although repairs may not require consent if they are carried out strictly to match the original materials and construction techniques.