All 1 1/2, two story and multi-level dwellings in a Residential R-1 district must have livable floor space of 1,500 square feet, with sidewall heights in a 1 1/2 or multi-level dwelling not to be less than eight feet in height. If the total livable square footage does not meet or exceed 1,500 square feet then said structure must have 1,050 square feet on the ground floor, exclusive of open or screened porches.