Mo. Rev. Stat. § 64.840
The county commission shall provide for a board of adjustment with powers to modify or vary the regulations, in specific cases, in order that unwarranted hardships, which constitute an unreasonable deprivation of use as distinguished from the mere grant of a privilege, may be avoided, the intended purpose of the regulations being strictly observed and the public welfare and public safety protected. If there is a county board of zoning adjustment as provided in section 64.870, that board shall be appointed to serve as the board of adjustment for the building or setback line regulations. If there be no county board of zoning adjustment, the personnel, length of terms, method of appointment and organization of the board of adjustment for the building or setback line regulations shall be the same as provided in section 64.870 for the board of zoning adjustment. Regulations authorized under section 64.835 shall not be adopted, changed or amended until a public hearing has been held thereon by the county planning commission, public notice of which shall be given in the same manner as provided for the hearing in section 64.815.
§ 64.840, RSMo