[Watsey]

ArthurKing I expect you've already checked the Planning Portal for your area, but this is what the Herts portal states with respect to garage conversions : "Planning permission is not usually required, providing the work is internal and does not involve enlarging the building. If your intention is to convert a garage into a separate house (regardless of who will occupy it), then planning permission may be required no matter what work is involved."

Building Regs approval is a 'must' for several reasons, but should be easy as long as the the requirements insulation, wiring, any plumbing (may not be part of your requirements), means of escape, etc, have all been satisfied.