Billings, MT Accessory Structures: ADU Rules (2026)

Section 27-1008(G) of the Billings zoning code governs accessory dwelling units. Eligibility is broad: an ADU is allowed on any parcel where a single dwelling unit is permitted or currently exists, but the lot must be at least 3,000 square feet once it carries both a primary home and an ADU. Each residential lot gets one ADU, and it must sit on the same lot as the primary unit.

Size is capped two ways. An ADU attached to or inside the primary home cannot exceed the primary unit's gross floor area, and the overall ceiling is 80 percent of the principal dwelling's gross floor area or 1,000 square feet, whichever is less. A 1,500-square-foot house therefore tops out at a 1,000-square-foot ADU, while a 1,000-square-foot house tops out at 800.

The unit has to function as a home. It needs a kitchen or food prep area, a bathroom and a sleeping area for its sole use, plus water and sewer service, which the two units can share along with other utilities. Mobile homes, recreational vehicles, travel trailers and any other wheeled or transportable structure cannot serve as an ADU. A new street address is required for a new ADU to help emergency response, and the unit needs its own entrance with a clearly labeled address. A house with an incorporated ADU cannot add a second entrance facing the same street.

An ADU can be built inside the house, added onto it, placed in a garage or other accessory structure, or built as a detached structure. Unless Section 27-1008(G) says otherwise, the underlying zone district's setback, yard and building coverage rules still apply. An existing accessory structure with a nonconforming height or setback can be converted to an ADU, but it cannot be altered in any way that increases the nonconformity. Home occupations are permitted inside an ADU. Detached ADUs sit outside the 200-square-foot and 1,500-square-foot detached accessory building limits of Section 27-1008(D), which points to subsection G for ADU requirements.