Proceed with caution here. Listed buildings are listed for a reason, usually because they are historically significant or interesting - they will retain original features not only on the exterior of the building but also inside.
Features such as original floorboards, staircases and fireplaces - plus windows, ceilings and doors are all part of the reason why a building is Listed.
If you are looking to make home improvements to your Listed house, you should firstly consult your local council's Planning Department, who will advise you on your options. Indeed improvements can be made on a like-for-like basis, using salvaged original replacement doors, for example.
The most lawful and wise option would be to certainly put together a proposal of what you would like to do (considering the listed status) within the building to present to the planning office.
Remember no matter how small the alteration is, you cannot ignore the law which protects Listed property. Even if you own the house it is not your right to change it (Listed Buildings belong to English Heritage in preservation terms). You can be heavily fine or even imprisoned if you make changes without consent.
