To defray the cost of installing and maintaining the system under the provisions of this part, the governing body shall adopt the following method of assessment:
(1) The governing body shall assess the entire cost of the improvements against the entire district.
(2) Each lot or parcel of land assessed in the district is to be assessed with that part of the whole cost that its area bears to the area of the entire district, exclusive of streets, avenues, alleys, and public places.