I did, as a matter of fact mine is my dedicated room now. My garage was attached only at the right rear corner of my home by a small breezeway off the side of the house.
I built a new garage in front of the old one with a new dinning room addition between the two. I now access the dining room from the new garage, I have an 8 foot wide access (two short steps) off of the rear of the new dining room into the new dedicated room.
I had the new garage built complete, the dining room ruffed in and did all the work on the new dedicated room. This has worked out great as all bedrooms are at the other end of the house so music and movies bother no one.
I was going to just move to a larger home at first..once I looked around $$$, I went with this plan. It was a lot of work on my part but I came out way ahead in both price and sq. footage.
The other thing was, by me doing all the design and build on the new room (I did not do the electric or heating) I got a much better room than a builder would have had time to do because I took my time (about six months) to research and implement things down to smallest detail. My advice is to go for it but be prepared for a mess for a while, give as much thought as you can before you start (just like building a good system).
I don't know about your state, I was lucky in that my old garage had footings that met code. You may want to check all codes for your area first.
Oh yea..I almost forgot, they raised my property taxes soon after I got done so now my system sounds much more expensive than it did in the old room.