Listed Building Consent is required for all works of demolition, alteration or extension to a listed building that could affect its character as a building of special architectural or historic interest.
Its an offence under the Planning (Listed Buildings and Conservation Areas) Act 1990 to carry out work without the necessary consent and you are liable to prosecution and enforcement action.
You should check first with the Conservation Officer whether or not consent will be needed for what you plan to do.
You should also get an outline of what might be acceptable and find out whether ideas need to be adapted to make them more likely to succeed. This simple step could save a lot of time and money.
Historic England have produced a guidance note which may assist owners in determining the type of works that would and would not normally require listed building consent - Historic England Advice Note 16: Listed Building Consent.