After the hearing, the schedule or schedules, either as initially proposed or as modified or amended, may be finally adopted. A copy of the schedule or schedules of the rates, fees, rentals, or charges as finally adopted shall be kept on file in an office designated by the board and shall be open at all reasonable times to public inspection. The rates, fees, rentals, or charges so fixed for any class of users or property served shall be extended to cover any additional users or properties thereafter served which shall fall in the same class, without the necessity of any notice or hearing.
SDCL 7-25A-37