The other issue that comes up with some local authorities with respect
to doing anything to a garage is if it impacts off-road parking.

Yes, I had issues with this in my last house. I was told that an application
to convert my garage wouldn't be open and shut because a house of that size
required 2 spaces according to the local plan. A space had to be 2.4m x
4.8m. Unfortunately, my drive was 60cm too short to park two cars, so the
garage was officially part of my allowance. The fact that the garage (as all
garages are) was full of junk and there was tonnes of off road parking
wasn't important.

I'm not complaining, though. They said there was a reasonable chance of
success and no fee was payable. I just never got round to it before moving.

Christian.