We went through a similar conversation a few years ago, and had even picked out the portable building we were going to modify. In the end, though, we remodeled the entire house, and we both picked a room for our "studios." Mine is a room in the finished walk-out basement, and hers is upstairs across the hall from the new master bedroom. Mine is, of course, for music, and hers is a sewing room. We both refer to them as "I'll be in my studio today."

Good luck with the build. Always exciting watching someone build their new studio.