Nothing in this Building Code shall require changes in the drawings, construction, or designated use of a building for which a lawful permit has been issued or otherwise lawfully authorized before the effective date of this Building Code, provided that work under such permit or authorization is under active prosecution upon such date, or is actively prosecuted within 90 days after such date, and that all work thereunder is completed, as so authorized, within one year after the date thereof or within an authorized extension of time; provided however, that if the work has been diligently prosecuted, an extension of time shall be granted for the completion of such work which shall permit its completion in accordance with the original permit.
(‘74 Code, § 150.28)