I think you are pretty close to accurate, depending on your location. I would call a few realtors and ask their opinion. If you get extravagant you may get less. I looked at 1 house that had added a detached garage with office above. They spent like 70k when they were all finished. No way was it adding that kind of value to the home. I am adding one though and think I am doing all of it myself, I should get at least all my material money out of it if not more.

I wasn't planning on anything extravagant, just a garage...probably 20 wide by 24 deep.