ADU Rules in Great Falls, Montana

Great Falls is the Cascade County seat and Montana's third-largest city, with approximately 60,000 residents. Nicknamed the "Electric City" for the series of Missouri River waterfalls that once powered its economy, Great Falls is home to Malmstrom Air Force Base — the US Air Force installation that operates Minuteman III intercontinental ballistic missiles (ICBMs) for the nation's nuclear deterrent. This military presence shapes land use planning around the base. Great Falls also serves as a regional agricultural and commercial hub for north-central Montana.

Montana SB 382 (2023) — Statewide ADU Reform

Montana Senate Bill 382 (2023) requires all Montana municipalities with a population of 5,000 or more — including Great Falls — to allow accessory dwelling units (ADUs) by right in residential zoning districts. "By right" means the city cannot deny a compliant ADU application through a discretionary process; the application is reviewed against objective standards only.

Montana HB 554 (2023) further expanded housing options by relaxing restrictions on missing middle housing types statewide.

Types of ADUs Permitted

• Detached ADU: A freestanding structure separate from the primary home, such as a backyard cottage or garage conversion.
• Attached ADU: An addition sharing a wall with the primary dwelling.
• Junior ADU (JADU): A smaller unit created within the existing primary home's footprint, often from a bedroom suite, basement, or attached garage interior.

Local Zoning Standards

The City of Great Falls regulates ADU development through its zoning ordinance, within the framework established by SB 382. General standards include:

• Maximum size: Detached ADUs are typically limited to 1,000 sq ft; verify the current limit with the Planning Division.
• Setbacks: Typically 5 feet from rear and side property lines for detached ADUs; verify current requirements.
• Height limit: Detached ADUs are commonly limited to one story or approximately 18–24 feet.
• Lot coverage: ADUs count toward maximum lot coverage for the zone; check your specific district's rules.
• Parking: SB 382 limits cities from imposing excessive parking requirements for ADUs; Great Falls generally follows state minimums.

Malmstrom AFB: AICUZ Overlay

Malmstrom Air Force Base is an active USAF installation on the eastern edge of Great Falls, home to the 341st Missile Wing and its Minuteman III ICBM missiles. The base operates an Air Installation Compatible Use Zone (AICUZ) program that identifies:

• Noise contour zones: Areas measured in Day-Night Average Sound Level (DNL) — zones over 65 DNL have recommendations against new residential construction; zones over 75 DNL discourage all residential use.
• Accident Potential Zones (APZs): Areas in the flight path where the risk of aircraft accidents is elevated; APZ-I and APZ-II have specific land use compatibility recommendations.

Properties within AICUZ noise contours or APZs may face restrictions on new residential units, including ADUs. The city coordinates with Malmstrom to maintain compatible land use near the base. If your property is near the base's flight paths or eastern perimeter, contact the City of Great Falls Planning Division before applying for an ADU permit.

Missouri River Floodplain

The Missouri River and its tributaries flow through Great Falls, and portions of the city lie within FEMA-designated Special Flood Hazard Areas (SFHAs). Properties in flood zones require:

• A Floodplain Development Permit from the City of Great Falls.
• Structures may need to be elevated to or above the Base Flood Elevation (BFE).
• An elevation certificate may be required.

Check flood zone status using Cascade County GIS or FEMA's Flood Map Service Center.

Permit Process

1. Parcel check: Use City of Great Falls GIS or the Planning Division to confirm your zone, any AICUZ overlay, and flood zone status.
2. Pre-application consultation: Contact the Planning Division to discuss your project before submitting.
3. Building permit application: Submit to the Great Falls Building Department with site plan, floor plans, elevations, and utility information.
4. Floodplain permit (if applicable): Required for properties in FEMA flood zones; submit concurrently.
5. Plan review: Staff review for zoning compliance, building code, and any AICUZ considerations.
6. Construction and inspections: Build per approved plans; complete required inspections.
7. Certificate of Occupancy: Issued after passing final inspection.

Contact

City of Great Falls Planning and Community Development 2 Park Dr S, Great Falls, MT 59401 Phone: (406) 455-8400 Website: https://www.greatfallsmt.net/planning

City of Great Falls Building Division 2 Park Dr S, Great Falls, MT 59401 Phone: (406) 455-8410

Disclaimer: ADU regulations change frequently. Montana SB 382 (2023) established statewide ADU rights, but Great Falls sets its own size, setback, and design standards. The AICUZ overlay near Malmstrom AFB may impose additional land use compatibility constraints. Always verify current rules with the City of Great Falls Planning Division before designing or building an ADU.