ADU Guide · Verified 2026
Building an ADU in Montana
Montana requires municipalities and, under a separate 2025 law, counties too, to allow at least one ADU per single-family lot, a mandate a citizen group sued to block, only to have the Montana Supreme Court uphold it in 2024. Montana is one of the few states in this research where an ADU law survived a real legal challenge all the way to the state's highest court.
Quick Facts
Statewide floors set by SB 528 (2023), MCA 76-2-345; extended to counties by SB 532 (2025). Your city may be more generous but cannot go below them.
The Three Types of ADU
A law that survived a real court fight
A group called Montanans Against Irresponsible Densification sued to block SB 528, and while a Gallatin County judge initially granted a temporary restraining order, the Montana Supreme Court upheld the law in September 2024 in MAID v. State of Montana, a genuine, documented legal test few other states' ADU laws in this research have faced.
Two separate laws, not one revised into the other
SB 528 (2023) covers municipalities. SB 532 (2025) is a genuinely separate law that extends the same core right to counties, with its own distinct additions, reinstated parking authority for counties, a 15-day expedited sanitation review, and a four-year sunset clause, rather than simply amending the existing city-level size cap.
Short-term rental rules stay local
While the ADU right itself is statewide, whether that ADU can be listed on Airbnb or a similar platform is left entirely to individual cities. Bozeman and Missoula both require their own separate short-term rental license on top of the ADU permit itself.
What people get wrong
The most common mistake is assuming SB 532 changed the size cap that SB 528 set for cities. It didn't, both laws share the same 1,000 sq ft/75% cap, SB 532's real changes are about counties specifically, parking, sanitation review, and its own sunset clause.
From Idea to Move-In: The Timeline
Who Administers It
Building It Yourself? Here's What You'll Need
Planning an ADU? Tools homeowners use in the design and layout phase:
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Check Your Local Rules
Your city may allow more than the state floor
State law sets the minimum; many Montana cities and counties are more generous. Browse the full directory of 56 counties and every city in Montana for local rules, fees, and building department contact details.
Browse the full Montana directory →Not sure if your project needs a building permit, a zoning approval, or both? See our guide to building permits vs. zoning approval.
Official Sources
ADU law changes frequently and local ordinances add detail. This guide is general information, not legal advice, always confirm with your local building department. Reviewed by the StateDataIndex Editorial Team · Updated September 2026.