The answer is it depends. According to the American National Standard for Single-Family Residential Buildings, any finished areas not connected to the main residence by a finished hallway or stairway must be listed separately. The ceiling must also be a minimum of 7 feet high to be considered “livable space,” and if you have to leave the house to enter the structure, it cannot be included. However, if your casita is attached to the main house and you can enter it from inside, then you may include it.