Accessory Dwelling Unit (ADU) Permit in Great Falls, Montana

Great Falls requires an accessory dwelling unit (adu) permit for: ADUs are allowed by right on any lot with an existing single-family dwelling, per MCA 76-2-345(1)(a) — Great Falls cannot prohibit them in single-family-only zones; Maximum ADU size: the lesser of 75% of the primary dwelling's gross floor area or 1,000 sq ft, per MCA 76-2-345(1)(b-c); ADU setbacks in Great Falls match the city's standard accessory-structure setback requirements; May be attached to, detached from, or located inside the primary residence; State law bars the city from requiring extra parking, imposing impact fees, requiring architectural matching to the primary home, restricting occupancy based on ownership/relation, or setting stricter building/design rules than apply to regular houses (MCA 76-2-345(2)(a)-(i)); ADU Application requires sign-off from: NorthWestern Energy, City Engineering Dept., Environmental Dept., Great Falls Fire Rescue, Great Falls Addressing, Great Falls Utilities, the Floodplain Administrator, and the Planning & Community Development Director — all signatures must be obtained before submitting the application; A one-time ADU zoning application fee may be charged but is statutorily capped at $250 (MCA 76-2-345(4)); The city may require approval letters from water and sewer systems before an ADU is built (MCA 76-2-345(7)). If your project isn't listed, confirm with City of Great Falls Building Division (Planning & Community Development Department) at (406) 455-8430 before starting work.

Any one-time ADU zoning application fee is capped at $250 by state law (MCA 76-2-345(4)). The building permit itself is billed on the same valuation-based fee table (Resolution No. 10374, Exhibit A) used for standard residential construction.

Plan review for an accessory dwelling unit (adu) permit in Great Falls typically runs 5–10 business days. This is an estimate, not a published figure — confirm current review times with City of Great Falls Building Division (Planning & Community Development Department) at (406) 455-8430.

You'll need: ADU Application. Depending on your project, Great Falls may also ask for: Attachment A — A Simple Explanation of Accessory Dwelling Unit (ADU) Rules. See the required-documents checklist on this page for details on each.

Great Falls requires 4 inspection(s) for an accessory dwelling unit (adu) permit, in order: Foundation, Framing, MEP Rough-In, Final. Schedule each through City of Great Falls Building Division (Planning & Community Development Department) ((406) 455-8430).

Apply through City of Great Falls Building Division (Planning & Community Development Department) at Civic Center, 2 Park Drive South, Room 112, P.O. Box 5021, Great Falls, MT 59403-5021. Phone: (406) 455-8430, email: permit@greatfallsmt.gov. Office hours: Mon-Fri 8:00 AM - 5:00 PM. Official information: https://greatfallsmt.gov/627/Building-Division.

Great Falls, Montana has adopted: 2021 International Building Code (IBC) — adopted by the City of Great Falls September 9, 2022, effective per city adoption (state effective date June 11, 2022 under ARM Title 24, Chapter 301); city stopped accepting the prior 2018-cycle codes as of November 9, 2022; 2021 International Residential Code (IRC) — adopted September 9, 2022; 2021 International Existing Building Code (IEBC) — adopted September 9, 2022; 2021 Uniform Plumbing Code (UPC) — adopted September 9, 2022 (Montana uses the UPC, not the International Plumbing Code); 2021 International Mechanical Code (IMC) — adopted September 9, 2022; 2021 International Fuel Gas Code (IFGC) — adopted September 9, 2022; 2020 National Electrical Code (NEC) — adopted September 9, 2022 (Montana's statewide NEC edition is 2020, not 2021, per ARM Title 24, Chapter 301); 2021 International Energy Conservation Code (IECC) — adopted September 9, 2022; 2021 International Swimming Pool and Spa Code (ISPSC) — adopted September 9, 2022; 2021 International Wildland-Urban Interface Code (IWUIC) — part of the certified program scope ("W"); ICC A117.1-2017 Accessibility Code — statewide standard incorporated by reference; 2021 International Fire Code (IFC) — adopted separately by the City of Great Falls effective May 25, 2023 (amending OCCGF Title 15), administered by Great Falls Fire Rescue rather than the Building Division. Local amendments apply — see the Great Falls overview page for the full list.

Yes. Under Montana's statewide ADU law (MCA 76-2-345), every city must allow at least one ADU by right on any lot that already has a single-family home, up to the lesser of 75% of the primary home's floor area or 1,000 sq ft. Great Falls processes ADU applications through its Planning & Community Development Department, requiring sign-off from multiple city departments and utilities before submittal.