In the case of a single-family or two-family detached dwelling, there shall be two side yards, one on each side of the main building, together having an aggregate width of at least 20 feet, but neither side yard shall be less than eight feet wide, provided 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 60 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; and provided, further, that in the case of a single-family or two-family detached dwelling constructed with its greater dimension parallel with the front street, a one-story open or enclosed porch may project into one of the side yards, provided that the width of such side yard is not thereby reduced to less than the required eight feet.