Minimum floor area of residential buildings. In order to ensure a reasonable ratio of open yard area and enclosed living space to provide for the health, safety and welfare of residents and further to preserve property values and promote purposes of zoning, every new dwelling unit shall have a minimum living space on the first floor of 7.2 square feet for every 50 square feet of lot area, up to the first 5,000 square feet of lot, said living space to be exclusive of porches, garages and accessory structures. Multistory dwellings shall have a minimum total living floor area of 12 square feet for every 50 square feet up to the first 5,000 square feet of lot, said living space to be exclusive of porches, garages and accessory structures.