If a variance or special exception is granted or the issuance of a permit is approved, or other action authorized, the necessary permit shall be secured and the authorized action begun within six months after the date when the variance or special exception is finally granted, or the issuance of a permit is finally approved, or the other action authorized, and the building or alteration, as the case may be, shall be completed within 15 months of said date. For good cause, the Board may, upon application in writing stating the reasons therefor, extend either the six months or twelve-month period.