I've just bought a new house due to getting a new job in a different area, and my situation is similar to yours.
Typically, what people do is "add to the front" to double their garage space, but the house I'm buying into has no room out front to add on to it, hence I too am considering adding "to the back".
I'll share my info (if I decide to do it), like how much it's going to cost, or if city codes & laws are a problem, ect, and maybe it'll help out others looking for info in the future.
Seems to me that a garage is never big enough. I'm going from a fair sized 3 car garage to a smaller one the way things stand right now...
- Patrick