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 and 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 such permit and which entire building shall be completed
according to such filed plans within one year from the effective date
of this chapter.