We live in a house that as originally built in the 50s with a breezeway between the house and the garage.
The main house was originally (maybe) 650 sq ft (1 BR, 1 bath, small kitchen, living room). It the 80s it was upgraded with an additional 1389 sq ft. So, the house is listed with the county as 2039 sq ft (now a 3 BR, 2 Bath, larger kitchen, new family room and the same living room).
Then someone converted the breezeway into a room (18' x 15') and then someone else converted the garage into a carpeted and insulated room and sealed off the garage door into a solid wall.
Like Bitflipper, my studio and day job workstation is 400 sq ft. The wife works in the breezeway office.