Planning permission is not usually required, providing the work is internal and does not involve enlarging the building.
Sometimes permitted development rights have been removed from some properties with regard to garage conversions and therefore you should contact your local planning authority before proceeding, particularly if you live on a new housing development or in a conservation area.
Where work is proposed to a listed building, listed building consent may be required.
Sorce: Planning Portal
This is an introductory guide and is not a definitive source of legal information.