Verify locally. Montana adopts the 2021 IRC (ARM 24.301.154) but MCA 50-60-102 exempts residential buildings with fewer than five dwelling units from the STATE building permit unless the local government has certified a building-code program. Billings, Missoula and about three dozen other cities (plus six counties) enforce locally; much of rural Montana has no structural building-permit requirement for single-family homes. State electrical (NEC), plumbing (UPC) and mechanical permits are required statewide regardless of location. Verify with your city building department or the MT DLI Building Codes Program. ADU rules below are drawn from official sources — always confirm with the issuing building department before designing or building.
ADU permit rule
ConditionalCase by casecity authority
Fee basis
Where a certified city/county enforces, fees follow that jurisdiction; in non-certified areas a single-family home is exempt from the state building permit (trade permits still apply). Verify locally.
Typical timeline
Where enforced, plan review typically 1-4 weeks depending on scope.
Setback
—
Area threshold
Not size-exempt
Exempt if
Not exempt
Go / no-go
case-by-case
ADU allowances are set by local zoning. In certified cities (Billings, Missoula) a permit is required; in non-certified areas, residential buildings with fewer than five dwelling units are exempt from the state building permit under MCA 50-60-102, though state electrical/plumbing/mechanical permits are still required.
Per Montana rules, an ADU is "conditional" for a permit and the authority is "city". ADU allowances are set by local zoning. In certified cities (Billings, Missoula) a permit is required; in non-certified areas, residential buildings with fewer than five dwelling units are exempt from the state building permit under MCA 50-60-102, though state electrical/plumbing/mechanical permits are still required. Always confirm with the issuing building department.
How big can an ADU be in Montana?
Size limits for an ADU in Montana are set by local zoning — verify with your city building department.
What are the ADU setbacks in Montana?
Setback rules vary by city in Montana; confirm with the local planning or building department.
How much does an ADU permit cost in Montana?
Fee basis: Where a certified city/county enforces, fees follow that jurisdiction; in non-certified areas a single-family home is exempt from the state building permit (trade permits still apply). Verify locally.
