In Residential A Zones, each dwelling hereafter erected, together with its accessory buildings and structures shall be located on a lot having an average width of not less than seventy-five (75') feet, provided that, in calculating the average width of a lot, the portions thereof more than one hundred (100') feet distant from the street line shall be disregarded, it being intended that each building lot shall have a minimum area of 7,500 square feet. There shall be not more than one dwelling on any lot.