Montana homeowners and landlords now have one of the clearer statewide ADU frameworks in the country, thanks to legislation passed in 2023 that removed most local barriers to building. An ADU, or accessory dwelling unit, is a secondary home on the same residential lot as a primary residence. You may know them as granny flats, in-law suites, backyard cottages, or carriage houses. Landlords build them to add rental income, house family members, or increase property value without purchasing a separate lot.
What changed in 2023: the statewide framework
Senate Bill 245 (2023) is the foundation of Montana's current ADU policy. It allows ADUs on all residential lots statewide and, more importantly, preempts local governments from blocking them. Before SB 245, a city or county could effectively kill an ADU project through restrictive zoning, design mandates, or owner-occupancy requirements. Those local veto points are gone.
Senate Bill 528 (2023), also signed into law that session, added guardrails on how municipalities can regulate ADUs going forward. Together, the two bills created a statewide floor: every residential lot must be eligible for at least one ADU, approved by right, without discretionary hearings or special permits.
Core protections under the 2023 legislation include:
- No owner-occupancy requirements
- No family or employment relationship required between primary and ADU occupants
- No requirement that the ADU match the main home's exterior design, roof pitch, or materials
- No additional parking spaces required for ADU approval
- No impact fees on ADU construction
- Application review fees capped at $250
You can review the full text of Senate Bill 528 on the Montana Legislature's website.
Types of ADUs allowed in Montana
Montana law covers both attached and detached ADUs. An attached ADU shares a wall with the primary residence, such as a converted basement apartment or a ground-floor addition with its own entrance. A detached ADU stands alone on the lot: a backyard cottage, a converted garage, or a newly constructed separate structure.
The same size limits and setback rules apply to both types. Detached units may face additional review around utility connections and septic capacity in rural areas, but neither type requires a special use permit or public hearing under the 2023 rules.
Some Montana municipalities also have provisions for junior ADUs, or JADUs. A JADU is typically a smaller unit carved out of existing living space inside the primary home, often under 500 square feet. If your city or county has JADU-specific rules, they should align with the state's 2023 framework, but confirm locally before starting a conversion project.
Size limits
Montana law sets a clear ceiling on ADU size: the lesser of 1,000 square feet or the gross floor area of the existing single-family home on the lot. If your primary residence is 800 square feet, your ADU can't exceed 800 square feet. If your home is 2,000 square feet, you're capped at 1,000 square feet.
This rule applies regardless of whether the ADU is attached or detached. There's no minimum size requirement set at the state level, though local building codes may have their own minimums for habitable space.
Setbacks and development standards
Setback requirements for ADUs cannot be more restrictive than what applies to single-family homes in the same zone. The same principle governs building height limits and lot coverage rules. Municipalities can't create a separate, stricter tier of development standards just for ADUs.
Design standards are still permitted, but they can't require ADUs to match the primary residence's exterior materials, roofline, or architectural style. This matters for construction costs: it lets builders use modular construction or simpler finishes without triggering a variance process.
Permitting process
Municipalities still issue building permits and collect standard building fees, in addition to the capped $250 application review fee. Permit review must happen through a ministerial, by-right process, meaning your application is reviewed against objective standards without a public hearing or planning commission vote.
If your local planning department attempts to add requirements beyond the state floor, the 2023 legislation gives you grounds to challenge those conditions. In practice, most larger Montana cities have updated their ADU ordinances to reflect the new state rules.
For current permitting details, check directly with your local planning department. Helena's Community Development office and Bozeman's Community Development department both maintain updated ADU guidance online.
Owner-occupancy and rental rules
Montana's 2023 legislation eliminated owner-occupancy requirements entirely. You don't have to live on the property to rent an ADU. You can own the lot as a straight investment and rent both the primary residence and the ADU to separate tenants.
There's also no restriction on who the occupants can be. No family relationship is required between the person living in the main home and the person in the ADU. Short-term rental rules, however, may vary by city. Bozeman and Missoula both have short-term rental licensing requirements that apply separately from ADU approval, so check local ordinances if you're planning to list on platforms like Airbnb or Vrbo.
HOA considerations
State preemption applies to local government zoning, not to private homeowners association rules. If your property is subject to an HOA's covenants, conditions, and restrictions, those agreements may still limit ADU construction, design, or rentals. Review your HOA documents before starting any project. Montana law doesn't currently override private CC&Rs on ADUs the way it overrides municipal zoning restrictions.
Costs and construction considerations
Construction costs for Montana ADUs vary widely depending on type, size, and location. A basic detached studio or one-bedroom in a rural area might come in under $100,000 using modular or prefab construction. A fully custom attached unit in Bozeman or Missoula, where labor and material costs are higher, can run $150,000 to $250,000 or more.
A few cost factors specific to Montana:
- No impact fees: The 2023 legislation prohibits municipalities from charging ADU-specific impact fees, which can add $10,000 to $30,000 to ADU projects in other states.
- Utility connections: In rural areas, you may need a separate well or septic system, which adds significant cost. In urban areas, most properties can tie into existing infrastructure.
- No design-matching requirement: Using simpler finishes or prefab structures is legally permitted, which can reduce costs compared to states where ADUs must architecturally match the primary home.
- Standard building permit fees: These still apply and vary by jurisdiction. Contact your local building department for a fee schedule before budgeting.
Insurance for ADU rentals
Adding an ADU changes your insurance needs, especially if you plan to rent it out. A standard homeowner's policy typically won't cover a tenant-occupied unit. If you're renting your ADU, you'll likely need a landlord policy that accounts for the rental structure, liability exposure, and potential loss of rental income. Montana landlord insurance options can help you find coverage that fits an owner-occupied property with a separate rental unit, or a fully investment-owned lot with multiple renters.
Why Montana made these changes
Montana's population grew significantly through the late 2010s and early 2020s, particularly in Bozeman, Missoula, and Billings. Housing construction didn't keep pace, and rental costs rose alongside. ADUs let existing residential lots add housing without requiring new streets, new infrastructure, or large-scale development approvals. Montana's approach mirrors what California and Oregon did several years earlier, when both states stripped local governments of the authority to block ADUs outright. Both states saw measurable increases in ADU permit applications within two to three years.
Frequently asked questions
Can Montana cities still ban ADUs outright?
No. Senate Bill 245 (2023) preempts local prohibitions. Every residential lot in Montana is eligible for at least one ADU as a matter of state law.
Do I have to live on the property to rent out an ADU in Montana?
No. The 2023 legislation eliminated owner-occupancy requirements. You can own the property as an investment and rent both the primary home and the ADU to separate tenants.
How big can an ADU be in Montana?
The maximum is 1,000 square feet or the size of the primary dwelling on the lot, whichever is smaller.
Are there extra parking requirements for ADUs in Montana?
No. Municipalities cannot require additional parking spaces as a condition of ADU approval under the 2023 rules.
What does the $250 application fee cap cover?
It caps the fee a municipality can charge just to review your ADU application. Standard building permit fees still apply on top of that amount, but there's no open-ended ADU review fee that can be used to discourage applications.
Can a city require my ADU to match my house's exterior?
No. Under the 2023 legislation, municipalities can't require ADUs to match the primary residence's exterior design, materials, or roof pitch.
Do HOA rules still apply to ADUs in Montana?
Yes. State preemption applies to government zoning, not private CC&Rs. If your property is subject to HOA covenants, those private rules may still restrict ADU construction or rental use. Review your HOA documents before starting a project.
Can I build an ADU on a rural Montana property with a well and septic system?
Possibly, but it depends on the capacity of your existing systems. Your county building department and health department will review whether your well and septic can support an additional unit. A separate system may be required if capacity is insufficient.
.jpg)
.png)