Expiration of variances. Unless otherwise specified by the Board or by law, a variance shall expire if the applicant fails to obtain a zoning permit within six months from the date of authorization thereof by the Zoning Hearing Board or by the court if such variance has been granted after an appeal, or fails to complete any erection, construction, reconstruction, alteration or change in use authorized by the variance within one year from the date of authorization thereof by the Zoning Hearing Board or by the court if such variance has been granted after an appeal. The Zoning Hearing Board, upon written application and for reasonable cause shown, may extend the approval for an additional period of up to one year.