Jefferson County, MO Accessory Structures: ADU Rules (2026)

2770(B) of the Unified Development Order permits an accessory dwelling unit (ADU) as an accessory use to a principal single-family dwelling, using a mother-in-law or guest cottage as its example, in both planned and non-planned residential zoning districts and in the "PM" Planned Mixed Use District. Eight conditions govern the unit. It requires its own building permit before construction. It must be a permanent structure, not a temporary trailer. It must connect to public water and sewer where available, or otherwise use on-site facilities that comply with County and State regulations.

It cannot be sold or conveyed separately from the principal dwelling and the rest of the property. It must meet every required building setback that applies to the principal residential use on that lot, so no separate, more permissive ADU setback exists. A mobile home may serve as the ADU only in the "RA-5" Rural/Agricultural and "LR-2" Large Lot Residential districts, meaning denser residential districts cannot use a mobile home for this purpose. Finally, the owner must provide sufficient on-site parking for the separate unit beyond the driveway serving the principal dwelling.

2770(A), which separately bars mobile homes, tractor trailers, and overseas shipping containers from serving as ordinary accessory buildings outside the ADU and conditional-use exceptions listed in that Article.