Unless otherwise specified by the Board or by law, a special exception shall expire if the applicant fails to obtain a zoning permit within one year from the date of authorization thereof by the Board or by the court if such special exception has been granted after an appeal, or fails to complete any erection, construction, reconstruction, alteration or change in use authorized by the special exception approval within two years from the date of authorization thereof by the Board or by the court if such special exception has been granted after an appeal. The Board, for reasonable cause shown, may extend the approval for an additional period of up to one year.