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Can You Build an ADU in Helena?

Yes — Helena is governed by Montana's statewide ADU law, which limits what the city can restrict. A 700 sq ft detached ADU here runs about $151,000–$247,000.

Construction costs in Helena track close to the Montana state average.

Detached ADU (700 sq ft)

$151,000–$247,000

Garage conversion (400 sq ft)

$47,000–$86,000

Statewide protection

Yes

What applies in Helena

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.

Because Montana has a statewide ADU law (Montana SB 528 (2023)), Helena must follow these minimums even where its older zoning code is stricter. Confirm fees and design standards with the Helena planning department.

Estimate your Helena ADU cost

Your property

ADU type

Estimated 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
Good news — building an ADU in Montana is very likely allowed.
  • 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.

Get your detailed feasibility report →

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.

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Other Montana cities

BillingsMissoulaBozemanGreat Falls

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.