Nothing herein contained shall require any change in plans, construction or designated use of a building, for which a building permit has been heretofore issued; provided, however, the construction of which shall have been diligently prosecuted within three 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 months of the date of the permit, and which entire building be completed according to such plans as filed, within 12 months of the date of this chapter.