Detached and multiple dwellings. In the case of a single-family or two-family detached dwelling or a multiple dwelling, there shall be two side yards, one on each side of the main building, neither of which shall be less than six feet wide, provided that if such building is over 40 feet high, the width of each side yard shall be increased four feet for each 12 feet or portion thereof by which the building exceeds 40 feet in height; and provided further that in the case of a lot held in single and separate ownership at the time this chapter becomes effective, of a width less than 40 feet, a single-family or two-family detached dwelling may be built thereon with side yards of less width, when authorized as a special exception.