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2026 Cost Guide · Updated September 2026

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

Real ADU pricing across Bozeman, Missoula, and Montana's other major markets, driven by heavy snow and wind load engineering, rural utility and septic capacity limits, and Bozeman's outsized housing demand.

$170K–$340K Detached ADU range
$85K–$180K Garage conversion (lowest cost)
SB 528 (2023) Upheld by MT Supreme Court, March 2026
1,000 sqft / 75% cap Detached/attached ADU size limit, still in effect
Montana mountain-modern ADU exterior
Montana ADU kitchen interior Montana ADU living room interior
Homeowners reviewing ADU permit documents in Montana
Montana's ADU Law

SB 528 — challenged in court, and now upheld.

Mont. Code Ann. § 76-2-345 (enacted as SB 528, 2023) requires municipalities to allow at least one ADU by-right on single-family lots, bans owner-occupancy and design-match requirements, and prohibits cities from requiring extra parking for an ADU. An advocacy group sued to block the law; the Montana Supreme Court lifted a lower court's injunction in September 2024, letting the law stay in effect during the case, and on March 17, 2026 the Court unanimously upheld SB 528 and two related housing bills against the constitutional challenge. The 1,000 sq ft / 75%-of-primary-home size cap remains in place. The detailed figures below are representative planning ranges, not a statewide fee schedule or review-time guarantee.

📄
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.
Verified September 16, 2026 · Upheld on appeal
Mont. Code Ann. § 76-2-345 (SB 528)

Requires municipalities to allow one ADU by-right per single-family lot. The Montana Supreme Court unanimously upheld the law against a constitutional challenge on March 17, 2026.

Governing frameworkMont. Code Ann. § 76-2-345
Size cap1,000 sqft or 75% of home
Supreme Court rulingUpheld, March 17, 2026
Representative permit fees$2,500 – $8,500
Local verificationRequired

Source: Montana Code — § 76-2-345 →

Verify the property before design

These are planning benchmarks, not a permit quote, eligibility ruling, or legal opinion. Confirm zoning, building, utility, site, private-covenant, fee, and timing requirements with the current local authorities. See the 51-jurisdiction law directory and national permit guide.

ADU Types

Which type fits your property?

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

A backyard cottage, converted garage, or newly built separate structure. Capped statewide at 1,000 sqft or 75% of the primary home's gross floor area, whichever is smaller — a limit the Montana Supreme Court left intact when it upheld the law in March 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
⚖️
Current ADU Legal Framework
Mont. Code Ann. § 76-2-345 requires municipalities to allow ADUs by-right, but the 1,000 sqft / 75%-of-home size cap remains binding statewide — the Montana Supreme Court left it untouched when it upheld the law in March 2026.
§ 76-2-345 (SB 528) · upheld March 2026
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
🚪
💡

Effective framework: Mont. Code Ann. § 76-2-345. State law requires covered local governments to allow ADUs under statutory standards; confirm current local administration and site rules. Permit fees and review timing remain planning estimates until confirmed for the specific property.

Calculate My ROI →
Return on Investment

Montana ROI planning inputs.

$900–$1,525
Adopted statewide 1-bedroom rent range
15%
National property-value planning input
10%–20%
Property-value uncertainty range
30%
Operating-expense planning reserve
Rent source
HUD FY2026 revised
One-bedroom FMR market basket
Exact HUD mappings
ROI-CM-1.4
Home-value source
Statewide Zillow ZHVI
June 30, 2026
One statewide input
Not a valuation
Payback period
Calculated live
Budget, ADU type, size and rent
Open calculator →
No static statewide promise
Full methodology
Transparent sources
HUD, Census PUMS, Zillow and FHFA
Review methodology →
Planning estimate, not a valuation
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
Property-Value Planning Input
15%
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. Mont. Code Ann. § 76-2-345 (enacted as SB 528, 2023) requires municipalities to allow at least one ADU by-right on single-family lots. An advocacy group sued to block the law; the Montana Supreme Court unanimously upheld it against that constitutional challenge on March 17, 2026, so the law is fully in effect statewide. Local zoning, building, utility, site, and private-covenant requirements may still apply; verify the current rules for the property before design or construction.
Did the Montana Supreme Court strike down or change the ADU size cap in 2026?
No. The March 17, 2026 ruling upheld SB 528 as written, including its 1,000 sqft or 75%-of-primary-home size cap for detached and attached ADUs — that cap remains in effect. The Court's decision addressed the law's constitutionality, not its substantive size or parking limits, which are unchanged from the original 2023 law.
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?
No. Municipalities cannot require additional off-street parking spaces, or fees in lieu of parking, as a condition of ADU approval under § 76-2-345. This restriction was part of the law the Montana Supreme Court upheld in March 2026, so it remains in effect statewide.
What is the ROI on an ADU in Montana?
The adopted Montana one-bedroom planning range is $900–$1,525 per month, derived from the HUD FY2026 revised FMR market basket under ROI-CM-1.4. Payback is calculated from the homeowner's budget, ADU type, size, rent range, and a 30% operating-expense reserve. Property value uses a 15% national planning input with a 10%–20% uncertainty range; it is not a valuation.
Montana ADU exterior at blue hour, warm interior light glowing
Imagine coming home to your own Montana ADU.

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Planning Disclaimer: Cost, fee, and timeline ranges are approximations for education and early planning; actual requirements and costs vary with the property, local authority, utilities, site conditions, design, and contractor. The Montana legal classification and framework shown here were verified on September 17, 2026, but they do not determine parcel eligibility and are not legal advice. Confirm current zoning, building, utility, private-covenant, fee, and timing requirements directly with the applicable authorities before design or construction. Obtain at least three contractor bids before committing to a project budget. Last updated September 2026.