St. Louis County, MO Accessory Structures: ADU Rules (2026)

Section 1003.020 packs three separate tests into one sentence, and a household has to satisfy all of them. The first is relationship: an individual alone qualifies, any number of people related by blood or marriage qualify, and an unrelated group qualifies only up to three persons. A fourth unrelated roommate puts the household outside the definition of family regardless of how the house is laid out.

The second test is how the household actually lives. The persons must be living together and subsisting in common as a single non-profit housekeeping unit. That phrase is doing real work in an accessory dwelling question: two households under one roof that keep separate finances, separate entrances, and separate cooking are not subsisting in common, so they are not one family even when the headcount is small enough.

The third test is physical and the easiest to inspect: not more than two kitchens. The cap belongs to the single family occupying the dwelling, so a house with a main kitchen and a finished lower-level kitchen is at the outer edge of what one family may use. A third cooking facility, or a second kitchen paired with the separate-household living pattern above, is the fact pattern that turns an accessory dwelling into a second dwelling unit in a district zoned for one.

Creating a genuinely separate dwelling therefore is not a matter of interior work alone. It generally means conditional use approval or a zoning change to a district that permits two dwelling units, handled through Planning at (314) 615-2520. Separately, §1110.1045 requires a re-occupancy permit whenever a new tenant moves in, which is the point at which an unpermitted second unit is most often discovered. Home-based work is governed by a different definition, home occupation at §1003.020.59, which is narrow and does not authorize a second residence.