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2026 Cost Guide · Updated July 2026

ADU Cost in Montana — Bozeman, Missoula, Billings & Statewide

City-by-city ADU cost data across Montana's major markets. Built from permit records, contractor data, and regional labor rates.

$170K–$340K Detached ADU range
$85K–$180K Garage conversion (lowest cost)
SB 528 (2023) Upheld by MT Supreme Court, Sept. 2024
Size cap ends Oct. 2026 New revision strikes the current 1,000 sqft limit
Montana mountain-modern ADU exterior
Montana ADU kitchen interior Montana ADU living room interior
At a Glance

Montana ADU costs — by type.

Detached ADU
$170K – $340K
Bozeman skews highest of any Montana market
🏡
Attached ADU
$150K – $290K
Same size and setback rules apply as detached
🔗
Garage Conversion
$85K – $180K
Most affordable Montana option statewide
🚗
Basement Conversion
$95K – $190K
Additional utility/septic review common in rural areas
⬇️
Modular / Prefab
$155K – $280K
Must be engineered for Montana's snow and wind loads
📦
Junior ADU
$45K – $95K
Typically under 500 sqft, carved from existing living space
🚪
💡

Montana has one of the strongest ADU laws in the country — and it's about to get stronger. SB 528 (2023) requires every municipality to allow at least one ADU by right, with no owner-occupancy requirement, no parking mandate, and no design-match requirement. The Montana Supreme Court upheld the law in September 2024. A further revision effective October 1, 2026 strikes the current 1,000 sqft / 75% size cap entirely.

Calculate My ROI →
ADU Types

Which type fits your property?

01 / Most Common
Detached ADU
$170K – $340K

A backyard cottage, converted garage, or newly built separate structure. Currently capped at 1,000 sqft or 75% of the primary home (whichever is smaller) — a limit that's removed entirely starting October 1, 2026.

Detached ADU Guide →
02 / Same Rules as Detached
Attached ADU
$150K – $290K

A converted basement apartment or ground-floor addition sharing a wall with the primary residence. Montana law applies the same size limits and setback rules to attached and detached ADUs alike.

Attached ADU Guide →
03 / Most Affordable
Garage Conversion
$85K – $180K

A converted garage — attached or detached, both are protected equally under state law. No special use permit or public hearing required under the 2023 rules.

Garage Conversion Guide →
04 / Extra Review in Rural Areas
Basement Conversion
$95K – $190K

A converted lower level with independent entrance. Detached units and conversions may face additional review around utility connections and septic capacity in rural parts of the state.

Basement ADU Guide →
05 / Built for the Climate
Modular / Prefab
$155K – $280K

A factory-built unit installed on a permanent foundation, engineered for Montana's snow and wind loads. Several providers now serve the Bozeman and Missoula markets.

Prefab ADU Overview →
06 / Lowest Cost
Junior ADU (JADU)
$45K – $95K

Some Montana municipalities recognize a JADU category — typically under 500 sqft, carved out of existing living space within the primary home. The lowest-cost path to a legal second unit statewide.

Junior ADU Overview →
Cost by Region

Every Montana market is different.

Bozeman, Montana ADU regional photo
Bozeman
Gallatin County · Montana State University
$210K–$340K
Montana's most expensive rental & construction market
Highest cost
Missoula, Montana ADU regional photo
Missoula
Missoula County · University of Montana
$170K–$280K*
Mid-range Montana market
Moderate
Billings, Montana ADU regional photo
Billings
Yellowstone County · Montana's largest city
$140K–$230K*
Most affordable major Montana city
Most affordable
Helena, Montana ADU regional photo
Helena
Lewis and Clark County · state capital
$165K–$270K*
Comparable to Missoula
Moderate

*Total-cost ranges outside Bozeman are derived from regional construction-cost data at a representative ~650 sq ft build; verify against local bids before publishing final figures. Great Falls and Kalispell are notable additional markets not pictured here and are worth checking with local contractors directly.

Cost Factors

Why Montana ADUs cost what they cost.

❄️
Snow & Wind Load Engineering
Montana's climate demands structures engineered for real snow loads and wind exposure, especially in higher-elevation areas near Bozeman and Kalispell. This adds structural cost compared to milder-climate states.
$10K–$25K added cost
🚰
Rural Utility & Septic Capacity
Detached ADUs and conversions can face additional review around utility connections and septic system capacity outside dense urban cores — a meaningful cost and timeline factor in much of the state.
Varies significantly by property
👷
Bozeman's Outsized Demand
Bozeman's popularity, driven by Montana State University and its outdoor-recreation economy, has pushed labor and material costs meaningfully above the rest of the state. Billings remains the most affordable major market by comparison.
Significant Bozeman premium
📋
One of the Strongest ADU Laws — Getting Stronger
SB 528 (2023) already bans owner-occupancy requirements, parking mandates, and design-match rules statewide, upheld by the Montana Supreme Court in 2024. A further revision effective October 1, 2026 removes the current size cap entirely, while partially reinstating counties' ability to require parking.
Among the most ADU-friendly states, still evolving
Homeowners reviewing ADU permit documents in Montana
Permits & Fees

What permitting actually costs.

SB 528 caps the fee a municipality can charge just to review your ADU application — standard building permit fees still apply on top.

📄
Building Permit Fee
$2,500 – $8,500
Standard building permit fees apply on top of the capped ADU review fee.
💧
Utility Connection / Septic Review
$2,000 – $12,000
Detached units in rural areas may need septic capacity review.
📋
Plan Review
$1,000 – $4,000
Includes snow-load and wind-exposure structural review.
⚡
Expedited Review (SB 532)
15-day review
Available if your parcel is already connected to public water and wastewater service.
🧾
HOA Documents Check
Free – varies
Not a fee, but a required check — state preemption applies to zoning, not private CC&Rs.
Montana Law
Among the strongest ADU laws in the country.

SB 528 (2023) requires municipalities to allow at least one ADU by right on any lot with a single-family dwelling, with no special hearing, no owner-occupancy requirement, no parking mandate, and no design-match requirement. Montanans Against Irresponsible Densification sued to block the law; the Montana Supreme Court reversed a lower court's temporary block and upheld it in September 2024. A further revision takes effect October 1, 2026.

Statewide ADU lawSB 528 (2023), MCA 76-2-345
Legal statusUpheld by MT Supreme Court, Sept. 2024
Owner-occupancy requirementBanned statewide
Parking mandate (current)Banned — reinstated for counties Oct. 2026
Size cap (current)Lesser of 1,000 sqft or 75% of home
Size cap (from Oct. 1, 2026)Removed entirely
💡 If you can wait until after October 2026, you may get a bigger ADU

The current 1,000 sqft / 75% cap is set to disappear entirely under the 2025 revision, effective October 1, 2026. If your design is being limited by that cap today, it may be worth checking whether your local jurisdiction has already adopted the new rule or plans to shortly after the effective date.

Return on Investment

What a Montana ADU returns.

$950–$2,100
Monthly rental income range statewide (sourced)
10–18 yrs
Modeled payback period (detached)*
$100K–$270K+
Added property value*
No sales tax
Montana has no state sales tax
Bozeman
$1,675 – $2,100/mo
1-bed monthly rent (sourced)
11–17 years
Modeled payback*
Missoula
$1,250 – $1,700/mo
1-bed monthly rent (sourced)
10–16 years
Modeled payback*
Billings
$950 – $1,400/mo
1-bed monthly rent (sourced)
9–16 years
Modeled payback*
Helena
$1,200 – $1,750/mo
1-bed monthly rent (sourced)
10–17 years
Modeled payback*

1-bed rent ranges verified July 2026 against current listings (RentCafe/Yardi Matrix, Zumper, Rent.com, Rentometer). Bozeman commands the highest rents in the state, driven by Montana State University and its outdoor-recreation economy; Billings is consistently the most affordable major market. *Payback and added-property-value figures are modeled (rent ÷ construction cost) and should be checked against local bids and actual project outcomes before publishing as final.

Save Money

7 ways to cut your
Montana ADU cost now.

SB 528 already guarantees your right to build. These moves target the climate, rural infrastructure, and timing factors that remain within your control.

Calculate My ADU ROI →
Compact detached ADU exterior with mountain-modern landscaping
🏡
Compact Detached ADU
Montana — statewide range
Added Property Value
$100K–$270K+
1
Ask about the 15-day expedited review
If your parcel is already connected to public water and wastewater service, SB 532 qualifies you for a 15-day expedited review — a meaningful timeline advantage over a standard plan review.
Cuts weeks off your permitting timeline
2
Convert a garage — attached or detached, both are protected equally
Montana law treats attached and detached ADUs the same way for size and setback purposes, so a garage conversion is one of the most cost-efficient and legally straightforward paths available.
Saves 40–55% vs. new construction
3
Check your septic capacity early if you're outside a dense urban core
Detached ADUs in rural areas can face additional review around utility connections and septic capacity. Confirming this before finalizing your design avoids a costly mid-project surprise.
Avoids a costly mid-project redesign
4
Consider a Junior ADU (JADU) if your budget is tight
Some Montana municipalities recognize a JADU category — typically under 500 sqft, carved out of existing living space. This is often the lowest-cost legal path to a second unit.
Often 60–75% less than new detached construction
5
Budget for real snow-load and wind-exposure engineering
Montana's climate requires genuine structural engineering for snow and wind loads, especially at higher elevations. Pricing this into your initial budget avoids a mid-project change order.
Avoids mid-project change-order costs
6
Design freely — Montana bans exterior design-match requirements
Unlike Colorado, Montana municipalities can't require your ADU to match the primary home's exterior design, materials, or roof pitch — giving you real flexibility to choose lower-cost materials without a compliance fight.
Opens up lower-cost material choices
7
Review your HOA documents — state law doesn't reach private covenants
Montana's ADU preemption applies to government zoning, not private CC&Rs. If your property sits in an HOA, review those documents separately before starting a project.
Prevents a legal dispute after you've started building
Frequently Asked Questions

Montana ADU answers.

Does Montana have a statewide ADU law?
Yes. Senate Bill 528 (2023), codified at MCA 76-2-345, requires municipalities to allow at least one ADU by right on any lot with a single-family dwelling, with no special hearing or discretionary review. The Montana Supreme Court upheld the law in September 2024 after a legal challenge from a group opposing the state's zoning reforms.
Is Montana's ADU law changing in 2026?
Yes. A 2025 revision effective October 1, 2026 strikes the original 1,000 sqft / 75% size cap entirely and partially reinstates counties' ability to require parking, which the 2023 law had banned. As of this writing, that change has not yet taken effect.
Do I have to live on the property to rent out my Montana ADU?
No. Montana is one of the states that prohibits owner-occupancy requirements for ADUs under the 2023 law, so you can rent both the primary home and the ADU.
Can my HOA block my Montana ADU even though state law allows it?
Yes, potentially. 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.
Does Montana require an ADU to match the main house?
No. Under the 2023 legislation, municipalities can't require ADUs to match the primary residence's exterior design, materials, or roof pitch — a real point of contrast with Colorado, where most cities do require a design match.
What is the cheapest ADU to build in Montana?
A Junior ADU (JADU) — typically under 500 sqft, carved from existing living space — is usually the cheapest option at $45,000–$95,000. Garage conversions are the next most affordable at $85,000–$180,000.
Does Montana require additional parking for an ADU?
Not currently — municipalities cannot require additional parking spaces as a condition of ADU approval under the 2023 rules. That changes partially on October 1, 2026, when a 2025 revision reinstates counties' ability to require parking for ADUs.
Montana ADU exterior at blue hour, warm interior light glowing
Imagine coming home to your own Montana ADU.

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Cost Disclaimer: All cost ranges and estimates are approximations based on industry contractor data, published reports, and regional market research as of 2026. Actual ADU costs vary significantly based on lot conditions, city requirements, contractor selection, material specifications, and design complexity. Montana's Senate Bill 528 (2023, MCA 76-2-345) requires municipalities to allow at least one ADU by right, upheld by the Montana Supreme Court in September 2024, but a 2025 revision effective October 1, 2026 will remove the current size cap and partially reinstate county parking authority — figures on this page reflect rules current as of July 2026 and should be re-verified after that date. Private HOA covenants are not preempted by state law and should be reviewed separately. 1-bedroom rent ranges shown were verified against current rental-market listings as of July 2026, though Great Falls and Kalispell were not independently verified this round due to a lack of dedicated photography for those markets. Permit fee figures, regional construction-cost totals outside Bozeman, and all payback-period and added-property-value estimates are modeled figures rather than verified project outcomes and should be checked against current local bids before publishing as final. Always verify current zoning and permit requirements directly with your local authority before beginning design. Always obtain a minimum of three contractor bids before committing to a project budget. Last updated July 2026.