For every single-family detached dwelling there shall be two side yards on the lot, which yards shall be not less than 30 feet in aggregate width, and neither of which shall be less than 12 feet in width; and for every main building, other than a dwelling, there shall be two side yards on the lot, neither of which yards shall be less than 15 feet in width; provided, however, that on a corner lot, which is subject to Subsection C of this section, there shall be one side yard of not less than 15 feet in width.