Yep, that's what we did at the farm.
To many pluses to list them all, but here's a few. It's super insulated, decked above apt for less storage loss, no increase in taxes since new structure wasn't added, and exterior pretty much maintenance free.
Only real issue was plumbing. Since we already had the bathroom, we had to pretty much had to set the kitchen close to the existing water lines, because we didn't want to dig up the slab.
40X50 is the exact same size as the building we put it in, and we had room for 2 bedrooms, and still had enough room to pull a car in the back, and a shop.
If I remember, I'll take pics tomorrow when I go down.
