Construction on a site with an approved site plan must be started within one year from the date of the Planning Board's approval of the site plan. Site plan approval may be extended for one year at the discretion of the Board after receipt of a written request from the landowner, showing good cause. If one year has elapsed from the date of approval, and no extensions have been granted, or if one year has elapsed since the granting of said extension, the final site plan approval shall become null and void without requiring any further action by the Planning Board. This time period shall not include delays resulting from litigation.