For a single-family dwelling, the minimum land area shall be 7,500 square feet and the minimum width of the lot at the front building line shall be not less than 60 feet. However, this shall not apply to prevent the construction of a single-family dwelling on a lot existing prior to the date of enactment of this chapter and not adjoined at the side by another unoccupied lot or other unoccupied land in the same ownership having an area of not less than 5,000 square feet.