Period of validity: A special permit shall become void within two years from the date of issue unless any construction work contemplated thereby shall have commenced and proceeded in good faith continuously to completion, or, if no construction work is contemplated by the special permit, the premises shall be open for business or in full use under said special permit. The two-year period shall not include time required to pursue or await determination of an appeal referred to in MGL Ch. 40A, §17. However, the special permit granting authority, in its discretion, may extend the time for exercise of such rights for a period not to exceed a total of one year upon a showing of good cause; and provided, further, that the request for such extension is filed with the special permit granting authority prior to the expiration of said two-year period. If the permit granting authority does not grant such extension, upon the expiration of the original two-year period, such special permit shall become void.