One-story structure in side yards. In any One-Family Residential District R-C, R-1, R-2 or R-3, a portion of a dwelling or principal structure may occupy and be constructed upon and within the area otherwise set apart and required to be used as a side yard, provided that such portion shall be no more than one story in height, and provided that there shall remain open and unoccupied 75% of the width otherwise set apart and required to be used as a side yard, and provided further that no part of a side yard, the width of which shall have been computed in accordance with § 285-27C(2) as less than otherwise required in the district in which it may be, shall be occupied by a portion of a dwelling or principal structure as permitted by this subsection.