The enactment of this chapter or amendment shall not require a change in plans, construction or designated uses for which a zoning permit has been issued prior to this enactment or amendment if construction has commenced within six months, construction is diligently continued, and the entire building or use shall have been completed within the life of the permit or authorized renewals thereof. In the event that construction has not begun within six months or is not diligently prosecuted or is not completed within the life of the permit or any authorized renewal, the permit shall lapse and the construction shall be subject to the zoning regulations existing at the time of said lapse, unless otherwise provided under state law.