St. Charles County, MO Environmental Rules: Stormwater Management (2026)

410 sits in the subdivision-regulations chapter and applies whenever a new subdivision is platted in the unincorporated county. Subsection (A) requires storm sewers alongside curbs and gutters, designed to the County's Design Criteria for the Preparation of Improvement Plans and built to the Metropolitan St. Louis Sewer District's 2009 Standard Construction Specifications, with the storm system kept entirely separate from the sanitary sewer. Subsection (B) allows an exception: subdivisions with one-acre-or-larger lots can use open drainage swales along the street instead of curb and gutter.

020, since the facilities become the subdivision's own maintenance burden, not the County's. Subsection (D) splits the substantive standard in two: water-quality treatment is required once a development disturbs one acre or more, including smaller phases of a larger common plan, but is waived outright for residential subdivisions built on three-acre-or-larger lots, unless the Director of the Division of Development Review knows of downstream stormwater problems or impaired waterbodies. 0 cfs or less. Any disturbance of 5,000 square feet or more also needs a separate land disturbance permit under Chapter 412, which is where the County's erosion and sediment control rules and performance guarantee requirements live.