Can I Build an ADU (Accessory Dwelling Unit) in Montana?
ADU permit requirements. State law, internal vs. detached, basement apartments, and impact fee waivers.
Often yes — many states now require cities to allow at least internal ADUs.
State Law May Require It
A growing number of states have passed laws requiring cities to allow at least internal ADUs (within the existing footprint of the home — basement apartments, above-garage units, etc.). Many cities also allow detached ADUs, though rules on size and placement vary. Check whether your state has an ADU-enabling law before assuming your city can simply say no.
Types of ADUs
Internal ADUs include basement apartments, converted attic space, and attached additions. Detached ADUs include backyard cottages, converted garages, and purpose-built accessory structures. Each type has different permit requirements and setback rules.
Key Requirements
Most ADUs need: a separate entrance from the primary dwelling, adequate parking (usually one additional off-street space), fire separation (1-hour rating for internal ADUs), and compliance with all building codes for habitable space. Basement ADUs require egress windows in every bedroom.
Reduced Impact Fees
Some states limit impact fees for ADUs, making them cheaper to permit than a full second dwelling. Many cities waive or significantly reduce impact fees for ADUs. Ask your city about current fee schedules — this can save thousands of dollars.
In many states you can build an ADU thanks to state law requiring cities to allow it. Internal ADUs are the easiest path. Start by checking your state's ADU law and your city's specific ADU ordinance.
Permit fees by city in Montana
Verified from each city’s published fee schedule. Where a city has no separate fee listed for this project, open the city page for the current requirements and amount.
| City | Permit fee | Source |
|---|---|---|
| Anaconda-Deer Lodge | $12.50Verifiedvia Residential Building Permit (New Construction) | Residential Building Permit (New Construction) → |
| Belgrade | Valuation x 0.0024Verifiedvia Residential Building Permit (New Construction, ADU, Addition, Remodel) | Residential Building Permit (New Construction, ADU, Addition, Remodel) → |
| Billings | $30.00Verifiedvia Residential Building Permit (New Construction) | Residential Building Permit (New Construction) → |
| Bozeman | (Administrative Charges/Employee/Hour + Inspector's Hourly Pay Rate) x Table A factor x Building Square FootageVerified | Accessory Dwelling Unit (ADU) Permit → |
| Butte-Silver Bow | $75.00 per hour, one-hour minimumVerifiedvia New Residential Construction Permit | New Residential Construction Permit → |
| Choteau | See city page for the current Residential Building Permit (New Construction / Addition / Alteration) fee | Residential Building Permit (New Construction / Addition / Alteration) → |
| City of Deer Lodge | $20Verifiedvia Residential Building Permit (New Construction / Addition / Remodel) | Residential Building Permit (New Construction / Addition / Remodel) → |
| Colstrip | See city page for the current Residential Building Permit (New Construction / Addition / Alteration / Repair / Move) fee | Residential Building Permit (New Construction / Addition / Alteration / Repair / Move) → |
| Columbia Falls | Calculated at time of submission based on project valuation; the City's own application form does not print a standalone valuation-fee table (field left blank for Building Department completion). No dedicated Columbia Falls fee-schedule document was found published separately from the application form after exhaustive search of the City's DocumentCenter, site search, and Resolutions listings.Verifiedvia Residential Building Permit (New Construction / Addition / Remodel / Repair) | Residential Building Permit (New Construction / Addition / Remodel / Repair) → |
| Conrad | See city page for the current Zoning and Building Permit (New Construction / Addition / Alteration / Repair) fee | Zoning and Building Permit (New Construction / Addition / Alteration / Repair) → |
| Cut Bank | $23.50 (subject to the $100.00 minimum permit fee)Verifiedvia Residential Building Permit (New Construction / Addition / Alteration) | Residential Building Permit (New Construction / Addition / Alteration) → |
| Darby | $29.38Verifiedvia Building Permit (New Construction, Addition, Alteration, Repair) | Building Permit (New Construction, Addition, Alteration, Repair) → |
| East Helena | See city page for the current Construction Permit — Residential Building (New Construction / Addition / Remodel) fee | Construction Permit — Residential Building (New Construction / Addition / Remodel) → |
| Forsyth | See city page for the current Residential Building Permit (New Construction / Addition / Alteration) fee | Residential Building Permit (New Construction / Addition / Alteration) → |
| Glasgow | See city page for the current Residential Building Permit (New Construction / Addition / Alteration) fee | Residential Building Permit (New Construction / Addition / Alteration) → |
| Great Falls | Capped at $250 by state law (exact city-charged amount not separately published in a fee table found; statutory ceiling confirmed)Verified | Accessory Dwelling Unit (ADU) Permit → |
| Hamilton | See city page for the current Residential Building Permit (New Construction / Addition / Remodel) fee | Residential Building Permit (New Construction / Addition / Remodel) → |
| Hardin | See city page for the current Residential Building Permit (New Construction / Addition / Alteration / Re-Roof) fee | Residential Building Permit (New Construction / Addition / Alteration / Re-Roof) → |
| Havre | $23.50Verifiedvia Residential Building Permit (New Construction / Addition / Remodel) | Residential Building Permit (New Construction / Addition / Remodel) → |
| Helena | Per Ordinance 3134 Appendix A valuation table, based on the ADU's construction valuationVerified | Accessory Dwelling Unit (ADU) Permit → |
| Kalispell | $23.50 minimum (valuation $1-$500) scaling to $6,444.85 for the first $1,000,000 plus $5.25 per additional $1,000 over $1,000,000Verified | Accessory Dwelling Unit (ADU) Permit → |
| Laurel | See city page for the current Residential Building Permit (New Construction) fee | Residential Building Permit (New Construction) → |
| Lewistown | See city page for the current Residential Building Permit (New Construction / Additions) fee | Residential Building Permit (New Construction / Additions) → |
| Libby | $24Verifiedvia Residential Building Permit (New Construction / Addition / Alteration / Repair) | Residential Building Permit (New Construction / Addition / Alteration / Repair) → |
| Livingston | $23.50Verifiedvia Residential Building Permit (New Construction) | Residential Building Permit (New Construction) → |
| Manhattan | See city page for the current Residential Building Permit (New Construction, Addition, Remodel) fee | Residential Building Permit (New Construction, Addition, Remodel) → |
| Miles City | Set by a City of Miles City fee schedule adopted by council resolution under City Code § 5-4; the specific dollar-valuation table was not independently locatable as a standalone published document as of this review (see note)Verifiedvia Building Permit (New Construction / Addition / Remodel / Repair) | Building Permit (New Construction / Addition / Remodel / Repair) → |
| Missoula | $36 minimum (valuation $1–$500) up to $1,582 for the first $100,000 plus $9.94 per additional $1,000; see full valuation tableVerified | Accessory Dwelling Unit (ADU) Permit → |
| Missoula County (Unincorporated) | $26.00Verifiedvia Residential Building Permit (New Construction) | Residential Building Permit (New Construction) → |
| Polson | 35% of the Building Permit FeeVerifiedvia Zoning Conformance & Building Permit (New Construction / Addition / Remodel) | Zoning Conformance & Building Permit (New Construction / Addition / Remodel) → |
| Red Lodge | See city page for the current Residential Building Permit (New Construction / Addition / Alteration) fee | Residential Building Permit (New Construction / Addition / Alteration) → |
| Ronan | $35.00 for up to $1,999Verifiedvia Building Permit (New Construction / Addition / Alteration / Repair) | Building Permit (New Construction / Addition / Alteration / Repair) → |
| Shelby | $23.50Verifiedvia Building Permit (New Construction / Addition / Structural Alteration) | Building Permit (New Construction / Addition / Structural Alteration) → |
| Sidney | $23.50Verifiedvia Residential Building Permit | Residential Building Permit → |
| Stevensville | $35 (minimum fee)Verifiedvia Building Permit (New Construction / Addition / Remodel) | Building Permit (New Construction / Addition / Remodel) → |
| Townsend | See city page for the current Building Permit — Residential Building (New Construction / Addition / Remodel) fee | Building Permit — Residential Building (New Construction / Addition / Remodel) → |
| Troy | $24Verifiedvia Residential Building Permit (New Construction / Addition / Alteration / Repair) | Residential Building Permit (New Construction / Addition / Alteration / Repair) → |
| Unincorporated Yellowstone County | See city page for the current Accessory Dwelling Unit (ADU) fee | Accessory Dwelling Unit (ADU) → |
| West Yellowstone | See city page for the current Residential Building Permit (New Construction / Addition / Alteration / Repair) fee | Residential Building Permit (New Construction / Addition / Alteration / Repair) → |
| Whitefish | Based on a percentage of the estimated project cost (valuation-based); a deposit is required at time of permit submittalVerifiedvia Residential Building Permit (New Construction / Addition / Remodel) | Residential Building Permit (New Construction / Addition / Remodel) → |