Floor area shall be measured at each level from outside of wall to outside of wall, but for the purpose of determining minimum required floor area shall not include any area having an average height of less than seven feet, basements, attached garages, open porches, attics, public hallways, or storage areas. Basements and attics shall not be included in determining permitted FAR. For the purposes of minimum and maximum floor area regulations, in the case of floor levels built into a hillside, the floor area subject to the regulation shall be the area extending back in depth 1/2 the length of the exposed wall at grade.