Side yards. In the case of a single-family detached dwelling, there shall be two side yards, one on each side of the building. The aggregate width of such yards shall be not less than 20 feet, and neither side yard shall be less than 10 feet. However, in the case of any building other than a single-family detached dwelling or a building accessory thereto, if such building is not over 30 feet high, the width of each of the two side yards shall be at least 10 feet, and if such building is over 30 feet high, the width shall be increased five feet for each 10 feet or portion thereof by which the building exceeds 30 feet in height.