The yard regulations of the underlying district shall apply, except as modified in this Section. In all districts, no rear yard regulations shall apply to any zoning lot that includes a through lot portion that is contiguous on one side to two corner lot portions and such zoning lot occupies the entire block frontage of the street. Where a rear yard equivalent is required by either Section 23-532 (Required rear yard equivalents) or Section 43-28 (Special Provisions for Through Lots), it shall be provided only as set forth in paragraph (a) of either Section, as applicable. However, in M1-5 Districts, a building existing prior to January 22, 2015, may be enlarged pursuant to Section 43-28, paragraph (b), provided that such building is on a zoning lot located entirely within 150 feet of the west side of the High Line. Where a rear yard equivalent is required by Section 23-533 (Required rear yard equivalents for Quality Housing buildings), the alternatives for through lots with a depth of 180 feet or less shall not apply.