If the stairs are the main problem, then perhaps a stair lift would work well for you. That's a lot cheaper than an addition.
Is the garage detached from your home? If so, and if it's not too far from the house, maybe you could put in an enclosed breezeway instead of a large addition. Put heated tile on the floors, an ac unit in a window, lots of windows and whatever else you want. Since it would connect to the house and the garage, you wouldn't have to go outside to get to the garage. If that's not an option, then perhaps you can just put an 36 inch wide enclosed walkway (with a few windows) to the garage.
Right now my DH and I are talking about making our dining room into a sewing room. That's where I do most of my sewing anyway, and it is not used for anything else.