ADU Rules in St. Charles, Missouri

St. Charles is the county seat of St. Charles County, located at the confluence of the Missouri River and I-70, approximately 20 miles northwest of downtown St. Louis. Missouri's first state capital from 1821 to 1826, the city retains its historic Main Street corridor — a popular destination with 19th-century commercial and residential architecture. St. Charles is also the county seat of one of Missouri's fastest-growing counties. ADU regulations are governed by the city's local Code of Ordinances. Missouri has enacted no statewide ADU preemption law.

Missouri State Law

Missouri has no statewide ADU preemption statute. St. Charles sets its own ADU rules through its local Code of Ordinances, zoning chapter.

Permitted ADU Types

St. Charles's zoning code addresses accessory dwelling units in residential districts. Permitted types may include:

• Detached ADU: A freestanding structure on the same lot as the primary dwelling, typically in the rear yard.
• Attached ADU: An addition to the primary dwelling that creates a separate dwelling unit.
• Garage or Accessory Structure Conversion: Converting an existing detached garage or outbuilding into a habitable ADU.
• Basement or Interior Conversion: Creating a separate unit from existing interior space in the primary structure.

Permitted types and applicable standards vary by zoning district. Contact the Department of Planning and Development to confirm what is allowed on your specific parcel.

Size, Setbacks, and Height

Dimensional standards are set by the applicable zoning district. General reference:

• Maximum size: Typically a square footage cap or a percentage of the primary dwelling's floor area per district standards.
• Setbacks: Rear and side yard setbacks for detached accessory structures are defined by district; commonly 3 to 5 feet.
• Height: Detached ADUs are generally limited to one story or a maximum height for accessory structures in the applicable district.
• Lot coverage: District-specific lot coverage maximums apply.

Always verify current standards with the Department of Planning and Development.

Historic District Considerations

St. Charles's historic Main Street area is a significant cultural and tourist destination. The city has local historic preservation standards for the designated historic district. If your property is within or adjacent to the historic district:

• Exterior alterations may require review and approval by the city's Historic Preservation Commission.
• Design standards for windows, doors, rooflines, and materials may apply.
• Contact the Department of Planning and Development early in your planning process to understand applicable historic review requirements.

Missouri River and Flood Zone Considerations

St. Charles sits on the south bank of the Missouri River. The Missouri River and its floodplain present significant flood risk in the region, particularly in lower-elevation areas adjacent to the river. Properties in or near the Missouri River floodplain may be in FEMA-designated Special Flood Hazard Areas. ADUs in flood zones must:

• Meet elevation requirements above the Base Flood Elevation.
• Comply with city floodplain development ordinance requirements.
• Obtain required floodplain development permits.

Check FEMA's Flood Map Service Center (msc.fema.gov) for your property's flood zone status. The Missouri River has experienced major flood events (1993, 2019) that affected properties in St. Charles County.

Permit Process

1. Verify zoning district: Check your property's zoning on the city's GIS mapping portal.
2. Confirm historic district status: Determine if your property is in or adjacent to a historic district.
3. Check flood zone: Verify flood zone status at FEMA's Flood Map Service Center.
4. Review Code of Ordinances: Identify ADU standards for your zoning district.
5. Pre-application consultation: Contact the Department of Planning and Development to discuss your project.
6. Prepare plans: Work with an architect or designer to prepare required documents.
7. Submit building permit application: File with the city's Building Division/Planning and Development Department.
8. Plan review and permit issuance: Obtain permit before starting construction.
9. Inspections and Certificate of Occupancy: Complete required inspections; obtain CO before occupying.

Contact Information

City of St. Charles Department of Planning and Development 200 N. Second Street, St. Charles, MO 63301 Phone: (636) 949-3227 Website: stcharlescitymo.gov/departments/planning-and-development

Disclaimer: Zoning regulations change frequently. Always verify current ADU rules directly with the City of St. Charles Department of Planning and Development before beginning any project. This page is for informational purposes only and does not constitute legal or zoning advice.