Any special or temporary use permit granted under the provisions of this subsection shall become null and void unless exercised within one year from the date when the decision of the Board granting said special or temporary permit becomes final, either by expiration of the time for review of the same by a court or by the final judgment of the court in the case of court review. However, the time to exercise said permit may be extended, for good cause shown, upon written application to the Director of Planning and Zoning made prior to the expiration of the time limit. The Director of Planning and Zoning, upon 30 days' notice to the Board which granted the original approval of said permit, may extend the time to exercise said permit for successive periods not to exceed six months each.