Planning permission may not be required if it is only internal works. However, local interpretation can vary and you should contact your local planning authority for advice.
Permission is required where you extend or alter the roof space.
You should also check whether you own the roof space which you wish to convert. If you are a leaseholder, you may need to get permission from your landlord, freeholder or management company.
If your flat is a listed building it is likely you will need listed building consent and you should contact your local planning authority for advice before you start work. Works to a listed building that affect its special historic character without consent is a criminal offence.
If your flat is in a conservation area and your proposals involve an element of demolition then you may need to apply for planning permission for relevant demolition in a conservation area and you should contact your local planning authority for advice.
Building regulations approval is required to convert a loft or attic into a liveable space.