Contrary to what has been stated here, Square footage for architectural purposes is the exterior dimensions of the frame of the house that is considered heated floor space. When you get into Lofts and balconies, that has to measured as well on an interior basis. A fireplace chase (box on the exterior) is counted as well along with bay and bow windows. Clerestory is not measured for determining the sq/ft of living space but builders many times factor it in because there is cost associated with it ( framing, insulation,drywall, molding, paint etc). Everything in the box of a home is included in square footage including stairs, closets etc. Basements are usually not counted unless it is legal living space. Garages are also not counted. If a house has a sun porch attached to a house but is not heated, it is not counted.
I hope this helps.
MJ Peterson Real Estate