You will need to apply for planning permission for most types of alterations to flats. However, since the introduction of a local development order, you do not need permission to replace windows and doors, providing certain conditions in the order are met. More about local development orders.
If your flat is a historically important building or is in a conservation area you will need planning consent.
Check if your flat is listed, in a conservation area, or on the local list
Sub-division of a house to flats
Planning permission is needed to sub-divide a house into flats. Converting a previously sub-divided house back to a single dwelling does not require planning permission unless the alterations affect the external appearance of the property and unless the property is a listed building where the alterations affect the character of the building.