Single-family dwellings. In the case of a single-family dwelling, there shall be two side yards, one on each side of the main building, together having an aggregate width of at least 25 feet, but neither side yard shall be less than 10 feet wide, provided that in the case of a lot held in single and separate ownership at the effective date of this chapter, of a width less than 70 feet, a single-family 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 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 hereby reduced to less than the required 10 feet.