Mo. Rev. Stat. § 341.140
No person shall cause, authorize or permit the installation, construction, alteration or repair of any sewer or sewage treatment device, plumbing or drainlaying or installation relating thereto, except minor repairs, upon or about any building or premises owned, operated or controlled by any such person except by a licensed plumber or drainlayer and shall before such work is commenced procure a permit from the department of plumbing and sewer inspection authorizing such work to be done nor shall any licensed plumber or licensed drainlayer commence any such installation or construction until a permit shall have been obtained from said department authorizing such work to be done and until all inspection fees shall have been paid pursuant to the regulations adopted under the provisions of sections 341.090 to 341.220. The above provision requiring permits shall apply with equal force to plumbing, sewer or sewage treatment or drainlaying or installations or constructions relating thereto done in any private or public building, street, easement, lot or premises. All work shall be done by or under the direction or supervision of the plumber or drainlayer in whose name the permit has been issued.
§ 341.140, RSMo