Minimum depth of each side yard. One side yard, as defined and regulated by § 166-4 and § 166-113, shall have a depth of at least 10 feet, for principal buildings up to 25 feet high. For principal buildings greater than 25 feet high, one side yard, as defined and regulated by § 166-4 and § 166-113, shall have a depth of at least 15 feet. For all principal buildings, the second side yard shall have a depth of at least 20 feet. If the building varies in height, the setback requirement shall apply independently to each portion of the building, based upon the height of such portion.