If after a zoning variance or conditional use permit has been authorized by the Planning Board, such permit is not lifted and executed by the applicant within a period of six months from the date of the authorization, and if construction under the permit does not proceed diligently and is not completed within one year after the date of the lifting of the permit, then such authorization shall be null and void, no further permit shall be issued thereunder and no further construction shall proceed under any permit which may have been issued.

At the time of application, the applicant may request a period of time in excess of one year to construct the proposed structure. The Board having jurisdiction thereof, after considering the nature of the structure, may include such longer period of time as it deems necessary and reasonable in the permit.