Yellowstone County, MT Accessory Structures: ADU Owner Occupancy (2026)

Yellowstone County allows accessory dwelling units conditionally in the Agricultural (A) district and by Special Review in the N4, RR1, and RR3 residential districts, under Section 27-1009.G.1 of the Zoning Code. Approval does not end the county's involvement: Section 27-1009.G.2, Ownership and Occupancy, ties every ADU to the property owner personally. The owner must live in either the primary dwelling or the accessory unit as their principal residence for at least six months of the year, and the code specifically forbids renting out the owner-occupied unit to a third party while the owner lives in the other one.

To make that binding beyond the current owner, the code requires the property owner to record a deed restriction with the Yellowstone County Clerk and Recorder that requires owner-occupancy of the property, and to give the Planning & Community Services Department evidence that the restriction was recorded. Any additional conditions the Director attaches to the approval must also be filed as deed restrictions with the County Recorder within 30 days of ADU approval, with proof of that filing submitted to the Director within the same 30 days. Because the restriction runs with the land, a buyer who purchases a property with an existing ADU inherits the owner-occupancy obligation; it does not expire when the property changes hands.

The rule has one further wrinkle in the N4 district: under Section 27-1009.G.2(d), an ADU built there cannot be sold separately or subdivided from the primary dwelling or lot, closing off the option of splitting the ADU into its own parcel to get around the occupancy requirement.