Garage conversion rules in Yellowstone County, MT — sometimes called garage-to-ADU or accessory living unit conversions — govern permits, ceiling height, egress, and parking replacement.
In the county zoning area, a "private garage" is defined as an accessory building used only for storing motor vehicles. Converting it to a habitable rental unit creates a separate dwelling not authorized as an accessory use, and needs district, building and septic approval. Rural unzoned land sets no rule.
Yellowstone County's adopted Zoning Ordinance defines a "private garage" as "an accessory building or part of principal building used only for the storage of motor vehicles as an accessory use" (capped at three passenger vehicles for a single-family dwelling). Converting that garage into living space changes it from a storage accessory use into a dwelling. The county's accessory-living-quarters rules bar kitchen facilities and separate rental in an accessory building, so a rental conversion is not permitted in most residential districts. A legal conversion requires the district to allow the added dwelling density plus a building permit and RiverStone Health/DEQ septic sign-off. On unzoned rural land the county imposes no zoning rule, but state building code still applies.
An unpermitted habitable garage conversion in the zoning area is a zoning and building-code violation; Code Enforcement can require it be returned to permitted use. Building without required permits also risks state building-code enforcement.
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