Since a garage conversion won't substantially increase your home's footprint they usually fall under permitted development and therefore you probably don't need a planning permission before beginning a garage to apartment conversion.

Do you need planning permission to convert a garage into a room?

Planning permission is not usually required to convert your garage into additional living space for your home, providing the work is internal and does not involve enlarging the building. ... A condition attached to a planning permission may also require that the garage remain as a parking space.

Is a garage conversion exempt from building regulations?

If you want to convert an integral or attached garage into habitable use, building regulations will normally apply, view specific information for garage conversions. In many cases, these structures will be exempt from requiring approval under the Building Regulations if they meet certain exemption criteria.