Within two days after the engineer has filed a certificate that the work is half completed and within two days after the board of supervisors has accepted the completed improvement as in this subchapter, parts 1 through 5, provided, the county auditor shall notify the owner of each lot or parcel of land who has signed an agreement of waiver as provided in section 468.57, of such fact. Such notice shall be given by certified mail sent to such owners, respectively, at the addresses filed with the auditor at the time of making such agreement of waiver.