Approval of the preliminary plat shall be effective for a period of eighteen (18) months unless upon written request of the subdivider the Council by resolution grants an extension of time. If the final plat is not filed with the Clerk within eighteen (18) months of the preliminary plat, all previous actions of the Council with respect to the plat shall be null and void. If the final plat is filed within eighteen (18) months of the preliminary plat and the final plat does not include all of the area of the preliminary plat, then all previous actions of the Council may not be null and void with respect to the balance of the area in the preliminary plat.