Man cave. If it's not meant as living space, the code is far more relaxed. I have a semi-finished area above my garage. Someone could sleep there overnight if they visited, but it's not really meant for that. When I put in water/septic to the garage, I'll finish that area out better and do the proper fire breaking, but it'll still just be for someone to visit me and stay in rather than meant for continuous habitation.
If you want to rent it out, there will be a lot of code to be followed in most states to make it up to snuff, especially above a garage/shop. It would be cheaper and easier to add 20' to the length and build a townhome on the end instead of a 2nd story apartment.