I turned my attached garage into my sewing room. It had heat in there already and I just added a lot more lights, outlets, did the drywall and threw a new floor in there. I still have to work on the ceiling. I don't mind if that takes a long time to get done. It's so nice to have the extra space to work.
If you put a room in there that is completely enclosed, I don't see there being much of an issue.