I would consider putting it on TOP of it as opposed to one end. It costs a whole lot less to go up versus what it does to add on to the end.
I have several friends who have done this. They purchased their ideal lot, built a garage with apartment upstairs and then later built their house and converted the upstairs apartment to a game room / man cave.
Nothing like have somewhere to go with heat/air, running water and cable if the little lady gets sideways with ya ... and all your tools are within walking distance is just an added bonus.
