Any internal or external works of alteration, extension or demolition, which affect the character of a listed building, require listed building consent. In practice, the position is less clear-cut. Repair and maintenance work carried out on a "like for like" basis is generally considered not to require Listed Building Consent.
Listed Building Consent will also be required for works to buildings or structures within the curtilage of the listed building, including walls, railings and outbuildings. Remember that planning permission may also be required and building regulations may also apply.
Carrying out unauthorised work to a listed building is a criminal offence, carrying penalties of up to three months imprisonment and/or a fine of up to £20,000. Enforcement action can also be taken requiring the reinstatement of a building or its features, the cost of which could run into millions.