O'Fallon, MO Accessory Structures: ADU Rules (2026)

161(B)(4)(a) are the only place in the Zoning Code that expressly authorizes a secondary residential unit, called a "carriage house," on a single-family lot. " The supplementary standards limit the property to a maximum of one (1) accessory dwelling per lot, attached or freestanding; the unit's floor area, measured only under a solid roof, cannot exceed eight hundred (800) square feet unless the lot is at least one (1) acre, in which case it may reach one thousand (1,000) gross square feet. An open porch, patio, porte cochere or carport under the same roof adds up to five hundred (500) more square feet, provided it stays unenclosed, and the unit is limited to a single bedroom.

The carriage house must be "architecturally compatible in character and materials and be subordinate in size to the principal dwelling unit," and it "shall remain accessory to and under the same ownership as the principal single-family dwelling unit," so it cannot be subdivided or sold as its own condominium. Because these standards live inside the MUTDD overlay, they do not apply to a typical R-1, R-2 or R-3 lot; a carriage house is only available where a property has been developed or rezoned into the Mixed-Use Traditional Development District under an approved area plan.