Nothing contained herein shall require any change in the plan, construction, size, or designated use of a building for which plans are filed with the commissioner of buildings and a building permit issued before the effective date of this zoning code; provided construction under such permit shall have been started within 6 months, and the ground story framework, including structural parts of the second floor, shall have been completed within one year, and the entire building completed within 2 years after the date of the issuance of such permit.
('73 Code, § 150.16)