Nothing herein contained shall require any change in plans, construction or designated use of a building for which a building permit has heretofore been issued and any construction which shall have been diligently prosecuted within three (3) months of the date of such permit, and the ground-story framework of which, including the second tier of beams, shall have been completed within six (6) months of the date of such permit, and which entire building shall be completed according to such plans, as filed, within one (1) year of the date of this Chapter.