Building area. No dwelling shall occupy more than 15% of its lot. Except for accessory dwelling units, every dwelling unit shall have a minimum ground floor area of 960 square feet, except that dwellings having living quarters on more than one floor above the basement may be reduced in ground floor area to 720 square feet, provided that a minimum living space of 960 square feet is provided therein. Open porches, garages, carports, barns, sheds, and unwalled covered areas shall not be included as ground floor area or living space. Dwellings plus open porches, decks, garages, carports, barns, sheds and other accessory buildings, plus patios, unwalled covered areas, impervious driveways, sidewalks, impervious walkways and other impervious surfaces, shall occupy no more than 15% of the lot.