Nothing herein contained shall require any change in the plans, construction or designated use of a building or which a building permit has been heretofore issued or for which plans are on file with the Construction Official at the time of passage of this chapter and for the erection of which a permit is issued within one month from the passage of this chapter and the construction of which, in either case, 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 plans as have been filed, within one year from the date of the passage of this chapter, except if construction has been delayed as a result of a national emergency.