Can I build a duplex on a single-family lot in Montana?

In cities with more than 5,000 residents, yes — Senate Bill 323 (2023) requires those cities to allow duplex housing anywhere a single-family home is a permitted use, and zoning regulations cannot impose stricter standards on the duplex than they would on a single-family home. Smaller towns and unincorporated county land are not covered by the mandate and follow local zoning instead.

Are accessory dwelling units legal in Montana?

Under Senate Bill 528 (2023), codified at MCA 76-2-345 and 76-2-215, at least one ADU must be allowed by right on any lot with a single-family dwelling, in any municipality or county that exercises zoning power — there's no population threshold for this mandate, unlike the duplex law. Local design and size standards still apply, so check the specific city's zoning ordinance for setbacks and unit-size limits, and note the law has faced legal challenges from local-control advocates that are worth confirming the current status of.

How do I look up zoning for a parcel in Montana?

Ownership and assessment data statewide is searchable through the Montana State Library's Cadastral application, but zoning district assignment is set locally — you'll need to cross-reference the parcel with the relevant city or county's own zoning map, since Montana has no single statewide zoning-district GIS layer.

Is Montana a home-rule state for zoning purposes?

Mostly no. Zoning authority for most local governments comes from a state enabling act (Title 76, Chapter 2, MCA), and since 2023 the state has actively preempted local zoning choices on housing types statewide. A handful of Montana's larger communities — including Missoula, Bozeman, Butte-Silver Bow, and Whitefish — have adopted constitutional self-government charters giving them broader home-rule-style authority, and that status is central to ongoing legal challenges to the state's housing mandates.

Montana · state zoning reference · Waitlist open

Montana zoning: how it works and what you can build

Montana ran one of the most aggressive statewide zoning-preemption experiments in the country — a 2023 package of laws forcing cities to allow duplexes and accessory dwelling units by right — though parts of that package have drawn constitutional challenges from local-control advocates that are still working through the courts.

How zoning works in Montana

Zoning authority in Montana is delegated to counties and municipalities through a state enabling statute (Title 76, Chapter 2 of the Montana Code Annotated), and most local governments may only zone within the bounds that statute sets. A subset of Montana's larger cities and consolidated city-counties — including Missoula, Bozeman, Butte-Silver Bow, Anaconda-Deer Lodge, and Whitefish — have also adopted constitutional self-government charters, giving them broader home-rule-style authority than Montana's general-government municipalities; that self-government status has itself become a point of contention in litigation over the state's 2023 housing mandates. In 2023 the legislature passed a package of bills — commonly nicknamed the "Montana Miracle" — that rewrote the zoning rulebook statewide: the Montana Land Use Planning Act (SB 382), codified at Title 76, Chapter 25, MCA, requires cities above a population threshold in larger counties to overhaul their growth policies and zoning regulations on a fixed timeline and to affirmatively plan for housing need; a companion bill (SB 323, codified at MCA 76-2-304) requires cities with more than 5,000 residents to allow duplexes anywhere single-family homes are permitted; and a separate bill (SB 528, codified at MCA 76-2-345 for municipalities and MCA 76-2-215 for counties) requires any municipality or county that exercises zoning power to allow at least one accessory dwelling unit by right on a lot with a single-family home — with no minimum-population carve-out for that particular mandate.

The duplex and ADU mandates have faced legal challenges from local-control advocates arguing they improperly override the authority of Montana's self-governing charter communities; the litigation's ultimate outcome should be confirmed against current court records before being relied on, since it may still be moving through appeal. Smaller municipalities and rural counties outside the SB 382 population threshold continue to zone under the traditional Title 76, Chapter 2 framework, which still requires a local planning commission and adopted growth policy before zoning regulations can be enacted or amended.

For a developer, the practical split is: in Billings, Missoula, Bozeman, Great Falls, Helena, Kalispell, and other covered cities, state law requires duplexes and ADUs to be allowed on single-family lots even where local code hasn't caught up — subject to the pending litigation noted above. Outside those cities — and in the state's many unzoned rural areas — county zoning is often thin or nonexistent, and land use is governed more by subdivision review than by conventional zoning districts.

Who holds the zoning power

Land-use authority in Montana is delegated by statute, not assumed — these are the acts a zoning ordinance in Montana has to trace back to.

Montana Planning and Zoning statutes Montana Code Annotated, Title 76, Chapter 2: Base enabling framework for county and municipal planning commissions and zoning districts.
Montana Land Use Planning Act Senate Bill 382 (2023 Session), codified at Montana Code Annotated, Title 76, Chapter 25: Requires larger cities/counties to adopt a coordinated growth policy and zoning update on a statutory timeline, addressing quantified housing need.
Statewide duplex mandate Senate Bill 323 (2023 Session), codified at Montana Code Annotated 76-2-304: Requires cities above 5,000 population to allow duplexes anywhere single-family homes are allowed.
Statewide ADU mandate Senate Bill 528 (2023 Session), codified at Montana Code Annotated 76-2-345 (municipal) and 76-2-215 (county): Requires any municipality or county exercising zoning power to allow at least one accessory dwelling unit by right on a lot with a single-family home; unlike the duplex mandate, it carries no population threshold.

Major cities and their codes

The enabling act sets the ceiling; the municipal code is what a project is actually reviewed against. The 6 largest Montana jurisdictions and the codes they administer:

Montana — municipal zoning codes
City	Zoning code
Billings	City of Billings Zoning Regulations State's largest city; now subject to the statewide duplex and ADU mandates.
Missoula	City of Missoula Zoning Ordinance Has been independently pursuing a citywide zoning rewrite alongside the new state mandates.
Bozeman	Bozeman Unified Development Code Fastest-growing mid-size market in the state; adopted a consolidated UDC ahead of the state reforms.
Great Falls	City of Great Falls Zoning Ordinance Covered by the SB 382 planning-update mandate given its population and county.
Helena	City of Helena Zoning Ordinance State capital; stood up a new planning commission specifically to implement SB 382.
Kalispell	City of Kalispell Zoning Ordinance Flathead Valley growth center directly affected by the 2023 housing bills.

Recent reforms

State-level changes override or preempt what a local ordinance says — these are the Montana reforms that move the numbers on a real site.

2023 Montana Land Use Planning Act (SB 382) Mandates coordinated growth-policy and zoning updates for larger cities, with quantified housing-need targets.
2023 Statewide duplex-by-right law (SB 323) Forces cities over 5,000 residents to permit duplexes wherever single-family homes are allowed.
2023 Statewide ADU-by-right law (SB 528) Forces any zoning municipality or county to allow at least one accessory dwelling unit on single-family lots.

Parcel & GIS data

The Montana State Library's Montana Cadastral application is the statewide parcel and ownership data source, aggregating county assessor records into a single searchable GIS layer.

Parcel-level feasibility in Montana waits on that data being ingested and the ordinances encoded — the waitlist below is what sets the order.

Zoning by city

The statute sets who may zone; the city's own code sets what you can build. One guide per Montana city below, explaining the district codes that appear on its zoning map — 84 district codes across Montana so far.

Montana zoning — common questions

Can I build a duplex on a single-family lot in Montana?: In cities with more than 5,000 residents, yes — Senate Bill 323 (2023) requires those cities to allow duplex housing anywhere a single-family home is a permitted use, and zoning regulations cannot impose stricter standards on the duplex than they would on a single-family home. Smaller towns and unincorporated county land are not covered by the mandate and follow local zoning instead.
Are accessory dwelling units legal in Montana?: Under Senate Bill 528 (2023), codified at MCA 76-2-345 and 76-2-215, at least one ADU must be allowed by right on any lot with a single-family dwelling, in any municipality or county that exercises zoning power — there's no population threshold for this mandate, unlike the duplex law. Local design and size standards still apply, so check the specific city's zoning ordinance for setbacks and unit-size limits, and note the law has faced legal challenges from local-control advocates that are worth confirming the current status of.
How do I look up zoning for a parcel in Montana?: Ownership and assessment data statewide is searchable through the Montana State Library's Cadastral application, but zoning district assignment is set locally — you'll need to cross-reference the parcel with the relevant city or county's own zoning map, since Montana has no single statewide zoning-district GIS layer.
Is Montana a home-rule state for zoning purposes?: Mostly no. Zoning authority for most local governments comes from a state enabling act (Title 76, Chapter 2, MCA), and since 2023 the state has actively preempted local zoning choices on housing types statewide. A handful of Montana's larger communities — including Missoula, Bozeman, Butte-Silver Bow, and Whitefish — have adopted constitutional self-government charters giving them broader home-rule-style authority, and that status is central to ongoing legal challenges to the state's housing mandates.

Working in Montana? Get on the list.

The parcel layer lands in the order demand does. Illinois went first because that's where the deals were; the next state is decided by who tells us what they're trying to underwrite. Tell us where you're building and what you wanted to run — you'll hear from us when Montana opens, and sooner if we need a test parcel.

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Want to see the finished shape of this work? Illinois is live at parcel level — Illinois zoning and the 53-district Chicago reference.

Montana zoning summaries on this page are provided for information, not legal advice. Confirm every project against the current adopted code for the jurisdiction and the statute as amended.