ADU rules in Billings

Detached ADUs are allowed. Sec. 27-1008.G, as rewritten by the city's Montana Land Use Planning Act conformance ordinance (Ord. No. 26-5930, effective July 1, 2026, and now codified on Municode), permits an ADU 'on any parcel where a single dwelling unit is permitted or currently exists' and lets it be built 'as a separate, detached structure on a lot where a single-family dwelling exists or will exist'; the accessory use table lists ADUs as permitted in the residential districts. One ADU is allowed per residential lot, and a lot holding both a primary dwelling and an ADU must be at least 3,000 square feet. A detached ADU follows the underlying district's dimensional rules such as setbacks and building coverage. Mobile homes, RVs, travel trailers and other wheeled or transportable structures cannot serve as ADUs, and a new ADU needs its own street address.

Attached and internal ADUs are equally permitted: an ADU 'may be incorporated within or added onto a principal dwelling unit, garage, or other accessory structure,' and an existing accessory structure that does not meet dwelling height or setback rules may be converted as long as the conversion does not increase the nonconformity. An attached or internal ADU may not exceed the gross floor area of the primary dwelling. Every ADU needs a separate entrance with a clearly labeled street address, and a house with an incorporated ADU may not add another entrance facing the same street. The code definition covers units 'internal to or attached to the primary residential dwelling unit or in a detached structure.'

The code caps an ADU at 'eighty (80) percent of the gross floor area of the principal dwelling unit or one thousand (1,000) square feet, whichever is less,' and an attached or internal ADU also may not exceed the primary dwelling's gross floor area. The 2026 rewrite replaced the old 750 square foot cap on detached ADUs. A detached ADU otherwise follows the underlying district's setbacks and building coverage, and the lot holding both units must be at least 3,000 square feet. Because Billings completed its Montana Land Use Planning Act transition on July 1, 2026, the statewide size floor in MCA 76-2-345(1)(c) no longer binds the city directly (MCA 76-25-105(4)), so the city's own cap is the governing rule.

No owner occupancy is required. The ownership subsection of Sec. 27-1008.G now holds a single rule: 'The accessory dwelling unit shall not be sold separately or subdivided from the primary dwelling unit or lot.' The pre-2026 code required the owner to live in one of the units at least six months a year and to record a deed restriction; those provisions were void under MCA 76-2-345(2)(c) and (5) from January 1, 2024, declared unenforced in the city's March 11, 2025 code interpretation, and repealed by Ord. No. 26-5930 effective July 1, 2026. Municode now prints the amended text.

No off-street parking is required for an ADU: the off-street parking table lists 'Accessory dwelling unit' with 'No required minimum,' and Ord. No. 26-5930 deleted the old one-space rule. State law points the same way: since October 1, 2026, MCA 76-25-303(1)(j) (HB 492, Ch. 682, L. 2025), which governs Billings as a Montana Land Use Planning Act city, bars any minimum parking requirement for residential units under 1,200 square feet, and a Billings ADU cannot exceed 1,000 square feet. The primary house still needs one space under the table, which the statute allows, but the table's half-space minimum for dwelling units under 1,200 square feet conflicts with the same statute. Owners may always provide more parking than required.

ADUs are a permitted (by-right) accessory use: the accessory use table marks them 'P' in the residential districts, including N3, where the pre-2026 code demanded a discretionary special review. Review is administrative: the project needs a building permit and a staff zoning compliance permit, and the code's notice table says public notice is not required for permit review, so there is no hearing. Because Billings completed its Montana Land Use Planning Act transition, the statewide by-right mandate in MCA 76-2-345 no longer binds it directly (MCA 76-25-105(4)); by-right status now rests on the city code itself. Using an ADU as a short-term rental takes a separate annual STR permit, which is also administrative.

Long-term rental of an ADU is unrestricted: the old ban on third-party rentals and the owner-occupancy rule were repealed effective July 1, 2026, and the only remaining limit is that the ADU cannot be sold separately or subdivided from the primary dwelling or lot. Short-term rental (under 30 days) needs an annual STR permit from the zoning coordinator plus a city business license, and 'If a property contains both a principal single-unit dwelling unit and an accessory dwelling unit, only one unit may be permitted for use as a short-term rental.' Both owner-occupied guest homes and whole-unit tourist homes are allowed in the N1, N2, N3 and RMH districts, with a cap of two guests per bedroom, and a permanently deed-restricted affordable unit cannot get an STR permit. The city's posted planning fee schedule (covering fiscal years 2025 and 2026; no later schedule was posted as of October 2026) lists the STR permit at 300 dollars a year. Montana has no statewide STR preemption.