In all Commercial, Manufacturing, and Mixed Use Districts, the permitted obstruction provisions of paragraph (b)(2) of Section 33-23 and paragraph (b)(1) of Section 43-23 shall be modified such that, in any rear yard, any building or portion of a building used for any permitted non-residential use (except any building portion containing rooms used for living or sleeping purposes) shall be a permitted obstruction, provided that the height of such building, or portion thereof, shall not exceed two stories, excluding basements, nor in any event 30 feet above curb level. Any allowance for other permitted obstructions above a building in a rear yard or rear yard equivalent set forth in Section 33-23 or 44-23, as applicable, shall be permitted above such modified height limitations.