Area per dwelling unit and lot dimensions. The minimum land area per dwelling shall be 15,000 square feet. For subdivided islands, the minimum width of the lot at the front property line shall be 100 feet. However, this shall not apply to prevent the construction of a one-family dwelling on a subdivided lot existing prior to the date of enactment of this chapter and not adjoined at the side by other unoccupied land in the same ownership having an area of not less than 7,500 square feet and a width at the front property line of not less than 50 feet.