Montana ADU and duplex laws

What state law lets you build

Montana's ADU law is MCA 76-2-345, passed in 2023 as SB 528. It says a city or town "shall adopt regulations" that allow "a minimum of one accessory dwelling unit by right on a lot or parcel that contains a single-family dwelling."

The law covers incorporated cities, towns and consolidated city-counties that do zoning. Your ADU can be attached to the house, detached, or internal (inside the existing house).

An ADU, as the law defines it, is a self-contained unit on the same parcel as a larger single-family home, with its own cooking, sleeping and sanitation facilities.

"By right" means the city approves it without a public hearing, a variance, a conditional use permit or other discretionary zoning decision. The only zoning check allowed is whether your site plan meets the zoning rules. [1]

What cities can and can't require

Size. If the ADU is attached or detached, it "may not be more than 75% of the gross floor area of the single-family dwelling or 1,000 square feet, whichever is less." Gross floor area means interior habitable area. For example, with a 1,200 sq ft house, 75% is 900 sq ft, so 900 sq ft is the cap.

Height, setbacks and lot size. A city may not set height limits, setbacks, minimum lot sizes, lot coverage limits or frontage rules for the ADU that are stricter than those for the house on the lot.

Parking. A city may not require extra parking for an ADU, or a fee in place of parking.

Design. A city may not require the ADU to match the house's exterior design, roof pitch or finishing materials.

Who lives there. A city may not require that the owner live in either unit. It also can't require a family, marriage or employment link between the people in the house and the people in the ADU.

Street work. A city may not require public street improvements as a condition of the permit, except to repair a street your construction disturbs.

What still applies. Building codes, fire codes and public health and safety rules still apply. A city may also require a will-serve letter from both the city water system and the city sewer system.

A city can be more generous than the state law, so your local rules may allow more. [1]

Fees

A city may charge a one-time fee to review your ADU application of "up to $250 for each accessory dwelling unit." It may also charge its usual building fees on top of that.

A city may not charge impact fees on building an ADU.

For what the rest of a project costs, see our ADU cost guide and ADU financing guide. [1]

ADUs in counties (new in 2025)

SB 532 (2025) added MCA 76-2-215. Where county commissioners have adopted zoning, the county's rules must allow at least one ADU by right on a lot with a single-family home.

As in cities, a county may not require owner occupancy, matching design, impact fees, or stricter height, setback, lot size or coverage rules for the ADU than for the house. It may charge a one-time review fee of up to $250 per ADU plus its usual building fees.

The county section does not include the city law's 1,000 sq ft cap or its ban on parking requirements. Ask your county planning office what it requires.

A county that had not updated its rules by October 1, 2025, must review and permit ADUs under the state law until it does. County rules in effect on or after that date that don't comply are void.

If your lot already has public water and wastewater service with capacity for the ADU, the Department of Environmental Quality must provide a 15-day expedited sanitation review. That part of the law ends September 30, 2029. [2][3]

Duplexes, parking reform and the Land Use Planning Act

Duplexes. Under MCA 76-2-304(3), from SB 323 (2023), a city with at least 5,000 residents must allow duplex housing as a permitted use on any lot where a single-family home is permitted. Its zoning rules for duplexes may not be stricter than its rules for single-family homes.

Parking. HB 492 (2025) took effect October 1, 2026. City zoning rules may not require more than one parking space per home, or more than one-half space per home under 1,200 square feet (accessible parking under the ADA is separate). You can still choose to build more parking.

Land Use Planning Act. The Montana Land Use Planning Act applies to cities of 5,000 or more people in counties of 70,000 or more. These cities must comply on a schedule set in the law. Their zoning must include at least five of 14 housing strategies. The list includes allowing at least a duplex where a single home is allowed, allowing at least one internal or detached ADU, cutting or ending impact fees for ADUs, and zoning for tiny houses.

The Act also says a local government that complies with it "is not subject to any provision of Title 76, chapters 1, 2, 3, or 8." The city ADU law and the duplex law are in chapter 2. How this plays out for ADUs is a question for your city: if you live in a larger city, ask its planning office which state law its zoning is adopted under and which ADU rules apply to your lot.

From October 1, 2026, cities under the Act also may not require any minimum parking for homes under 1,200 square feet. [4][5][6][7][8]

The court challenge, and where it stands

A group called Montanans Against Irresponsible Densification (MAID) sued over SB 528, SB 323 and other 2023 housing laws. A Gallatin County district court temporarily blocked SB 528 and SB 323, which were scheduled to take effect on January 1, 2024.

On September 3, 2024, the Montana Supreme Court reversed that injunction (2024 MT 200) and sent the case back.

On March 17, 2026, in a second appeal, the Montana Supreme Court (2026 MT 53) affirmed that the housing reform laws do not violate the right to equal protection. It reversed the lower court's conclusion that the Land Use Planning Act's public participation rules, on their face, violated the constitutional right to participate.

The court also threw out, as "nonjusticiable", a lower-court declaration about private restrictive covenants. It did not decide whether a specific covenant can block an ADU. [9][10]

HOA covenants, building permits and short-term rentals

Covenants. The state ADU law does not stop private parties from agreeing to restrictive covenants about ADUs. A city may not require a covenant, or make your permit depend on one. If your lot is in a subdivision with covenants, read them before you plan an ADU.

Building permits. Under MCA 50-60-102, the state building code does not apply to homes with fewer than five units unless your city, town or county adopts it for them. Whether you need a local building permit depends on where you live. Ask your city or county building office. The state energy code still applies to homes. For homes with fewer than five units that no local program covers, it is enforced through a builder self-certification program.

Short-term rentals. Both the city and county ADU laws let local governments regulate short-term rentals. Check local rules before you list the ADU on a rental site.

For a general walk-through of plans, permits and inspections, see our ADU permit process guide. [1][2][11]

If your city says no

Since January 1, 2024, a city that hasn't updated its rules must review and permit ADUs under the state law until it does. City ADU rules in effect on or after that date that don't comply with the state law "are void." For counties, the date is October 1, 2025.

If a city points to a rule that seems stricter than the state law, ask the planning office which law and section it is applying. Larger cities may have adopted zoning under the Land Use Planning Act, which has its own housing rules (see above).

See also our pages on Colorado and Arizona. [1][2][6]

Montana laws to know

• SB 528 (2023), MCA 76-2-345

  Requires cities and towns with zoning to allow at least one ADU by right on a single-family lot, and limits size caps, parking, owner-occupancy rules and fees. [1][9]

• SB 532 (2025), MCA 76-2-215

  Requires counties that have zoning to allow at least one ADU by right on a lot with a single-family home. [2][3]

• SB 323 (2023), MCA 76-2-304(3)

  Requires cities of at least 5,000 residents to allow duplexes wherever a single-family home is a permitted use. [4][9]

• HB 492 (2025)

  From October 1, 2026, limits city parking minimums to one space per home and one-half space per home under 1,200 sq ft. [4][5]

• Montana Land Use Planning Act (SB 382, 2023), MCA Title 76, ch. 25

  Sets a separate zoning framework for larger cities, which must adopt at least five housing strategies from a list that includes ADUs and duplexes. [6][7][10]

Questions people ask