ADU Rules & Cost in Montana
A 700 sq ft detached ADU in Montana typically costs $151,000–$247,000 ($216–$353/sq ft). Here is what the rules allow.
Statewide ADU law: Montana SB 528 (2023)Montana ADU rules at a glance
Montana's SB 528 (2023) requires cities over 5,000 residents to allow at least one ADU per single-family lot by right, with limited parking and design standards.
- Statewide law
- Yes
- Max detached size
- Set locally
- Guaranteed minimum
- Set locally
- Setback
- Set locally
- Owner-occupancy
- Varies locally
- Approval time
- Set locally
Parking: Set by local ordinance — commonly 1 space per ADU unless near transit.
Estimate your Montana ADU
Pre-set to Montana. Adjust the type and size for your project.
Your property
ADU typeEstimated cost in Montana
$151,000 – $247,000
≈ $216–$353/sq ft turnkey · midpoint $199,000
- $135,300
- Construction
- $35,800
- Design & permits
- $27,900
- Site work
- iSize set locally. This state leaves ADU size limits to your city — typical caps run 600–1,200 sq ft. Check your local code.
- ✓No extra parking likely required. Set by local ordinance — commonly 1 space per ADU unless near transit.
- iOwner-occupancy varies. Owner-occupancy rules are set locally in this state.
- ✓Lot has room. A 700 sq ft ADU uses about 12% of your 6,000 sq ft lot — comfortably within typical coverage limits.
Rule basis: Montana SB 528 (2023)
If you rent it out
$1,050–$1,700
est. rent/mo
8.3%
gross yield
13 yrs
payback
~$15,345/yr after a 7% vacancy allowance, before financing & maintenance.
Financed over 20 yrs at 7.5%, the build is about $1,603/mo — vs ~$1,375/mo in estimated rent.
Estimates are for planning only and are based on regional construction-cost indices and published statewide ADU statutes. Local ordinances, lot conditions and contractor pricing vary — always confirm with your city planning department and a licensed contractor.
Cost by ADU type in Montana
| ADU type | 600 sq ft | $/sq ft |
|---|---|---|
| Detached new build | $129,500–$211,500 | $216–$353 |
| Attached addition | $117,500–$194,000 | $196–$323 |
| Garage conversion | $70,500–$129,500 | $118–$216 |
| Junior ADU (JADU) | $59,000–$106,000 | $98–$176 |
| Prefab / modular | $106,000–$176,500 | $176–$294 |
Financing your ADU
A handful of loan and grant programmes are built specifically for ADUs. These are informational links we don't currently earn a commission on — not paid placements.
RenoFi
Compare RenoFi ADU loan options →Renovation-specific loans (home equity loan, HELOC or cash-out refinance) that lend against your home's value after the ADU is built, so you can often borrow more than a standard HELOC allows.
Fannie Mae HomeStyle Renovation
See HomeStyle Renovation →A conventional mortgage that rolls ADU construction costs into a single loan, with down payments as low as 3% for qualifying first-time buyers.
CalHFA ADU Grant Program
Check CalHFA grant status →Reimburses up to $40,000 of pre-development costs (design, permits, soil tests) for California homeowners building an ADU.
Current round of funding is fully allocated — the page covers what to check for reopening.
Want financing options matched to your project?
Tell us roughly what you need to borrow and we'll point you to lenders who work with ADU projects.
ADU rules by city in Montana
Estimates are for planning only and are based on regional construction-cost indices and published statewide ADU statutes. Local ordinances, lot conditions and contractor pricing vary — always confirm with your city planning department and a licensed contractor.