To defray the cost of installing and maintaining either sanitary or storm sewer systems under the provisions of this part, the board of county commissioners shall adopt the following method of assessment:
(1) The board shall assess the entire cost of the improvements against the entire metropolitan sanitary district.
(2) Each lot or parcel of land assessed in the district:
(a) may 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; or
(b) may be assessed by methods provided in 7-12-2151.