Lot Area Per Family. Every dwelling hereafter erected, moved, or altered shall provide a lot area of not less than seven thousand five hundred (7,500) square feet per family for each single-family dwelling, or three thousand seven hundred fifty (3,750) square feet per family for two-family dwellings, provided that where a lot has less area than herein required in separate ownership at the time of the passage of this Chapter this regulation shall not prohibit the erection of a single-family dwelling.