thanks for your replies, i hadn't thought of the impact to myself/neighbour of becoming a terraced house.
my first step will be to draw up plans and get some quotes, as if its too expensive i won't need to apply for permission. i've no idea at present how much to expect, and it'll increase considerably if the garage has to be demolished and new foundations put in. I assumed i could build up from this (the garage was fitted professionally approx 15 years ago)