No building permit or construction, erection or alteration of any building or structure, or for any sign, shall be valid for more than six months from the date of issuance. Once a permit is issued, the Zoning Officer, for reasonable cause, may grant one or more extensions of time, each extension not to exceed 90 calendar days from the date of the expiration of the permit time period, or an extension thereof; provided, further, however, that a permit granted for demolition shall be valid for three months from the date of issuance.