Straight repair and renovation of a habitable structure is generally exempt, but if the building has effectively ceased to exist as a dwelling, or you extend, alter the appearance or change its use, permission is normally required.
The critical question is whether the residential use survives. Where a cottage is roofless, has lost most of its structure or has been unused for decades, planning authorities often treat the residential use as abandoned, meaning a new permission is needed to create a dwelling again.
If so much of the original fabric is replaced that the result is effectively a new house, the proposal is assessed as a new dwelling and rural housing policy, wastewater treatment and access standards all apply.