Mo. Rev. Stat. § 249.570
In all cases where lateral sewers have already been constructed of suitable materials and manner of construction the county commission shall provide regulations authorizing the connection of the same with the main or submain sewers and shall have the same authority to regulate house and building connection with said sewers as with sewers constructed under sections 249.430 to 249.660. The ground served by any such lateral sewer already constructed may be established as a sewer district by the county commission and shall not be assessed with the cost of construction of any sewer provided for in sections 249.430 to 249.660 except submain or main sewers, unless it is deemed necessary by the county commission that additional sewers be constructed in said district or districts, in which case the county commission shall have the right to construct additional sewers and levy assessments to pay the cost thereof, the same as in the case of other districts.
§ 249.570, RSMo