Yellowstone County, MT Accessory Structures: Tiny Homes (2026)

Accessory dwelling units are permitted conditionally in the county's Agricultural district and by Special Review in the N4, RR1, and RR3 residential districts under Section 27-1009.G.1, and Section 27-1009.G.5, Construction, sets the physical requirements an ADU must meet. Paragraph (c) of that section is unambiguous: mobile homes, manufactured housing, recreational vehicles, travel trailers, and other wheeled or transportable structures cannot be used as accessory dwelling units, full stop. A wheeled tiny home, park model, or converted travel trailer parked in the side yard does not qualify no matter how it is used, because the ban is written around the structure's wheeled or transportable nature rather than its floor plan or size.

The same paragraph pairs that ban with affirmative requirements for the ADUs the county will approve: the unit must contain a kitchen or food-prep area, bathroom, and sleeping area for its sole use, and water and sewer service must run through an individual well or septic system or a water and sewer district, with the district itself deciding whether the primary home and the ADU can share utilities. A new ADU also needs its own street address so emergency responders can find it. Read together, the section tells residents that a tiny home can become a legal ADU only after it stops being a vehicle: it has to be built on a permanent foundation and tied into the same water, sewer, and utility infrastructure the code expects of any other dwelling.