Can I Build an ADU (Accessory Dwelling Unit) in Montana?
Quick Answer
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.
Bottom Line
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) → |
| 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 → |
| 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 → |
| 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 → |
| 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 → |
| Unincorporated Yellowstone County | See city page for the current Accessory Dwelling Unit (ADU) fee | Accessory Dwelling Unit (ADU) → |