Nothing contained in this Zoning Code shall require any change in the plans, construction or designated use of any building for which a building permit was issued prior to June 6, 1949, provided that the construction thereof was diligently prosecuted within ninety days of the date of such permit, that the ground story framework thereof, including the second tier of beams, was completed within one year of the date of such permit and that such building was entirely completed according to such plans as filed within three years of June 6, 1949.
(1956 Code Sec. 1313.05)