For every principal building hereafter erected or used for any permitted use in this district, a lot without an approved public sanitary sewage system and an approved public water supply shall have an area of not less than 40,000 square feet and a lot width of not less than 140 feet at the building line. A lot size may be reduced to no less than 30,000 square feet provided that for each lot having less than 40,000 square, the developer shall be required to dedicate to open space use an area equivalent to the difference between the lot size and 40,000 square feet.