I consider square footage to be living area, less bathrooms. Some do not add in halls either. Finished basements are not common here, so the "walk-out" the others mentioned does not often come into consideration. But I have seen substantial variation on what others measure. If you take the exterior dimensions, that gives a higher figure than if you measure inside (as you should) - on an average home just that could affect the total by 80ft2. Inclusion of bathrooms and hall could change it easily another 150ft2 or more. Do not just assume there is standardization to these figures.