Accessory Dwelling Unit (ADU) / Detached Accessory Structure Permit in Hamilton, Montana

Hamilton requires an accessory dwelling unit (adu) / detached accessory structure permit for: Only one ADU permitted per lot with a single-family home; Maximum ADU size: 1,000 square feet or 75% of the principal dwelling's size, whichever is less; New ADUs must meet all zoning-district setbacks; Detached ADUs must be a minimum of 10 feet behind the principal dwelling (existing accessory-building conversions are exempt from this specific setback); Detached ADUs are not allowed in front yards; ADUs must obtain a unique address from Ravalli County GIS; No additional off-street parking required for an ADU; Residential building permit required; plumbing, electrical, and/or mechanical permits required depending on scope of work; New water or sewer service lines require an additional Public Works permit; existing lines may need to be upsized if undersized for increased usage. If your project isn't listed, confirm with City of Hamilton Building Department at (406) 363-3316 before starting work.

In Hamilton, the published Impact Fee is: $0 — no impact fees are charged for ADUs. These are the jurisdiction's own published figures — see the fee schedule table and source citations on this page.

Plan review for an accessory dwelling unit (adu) / detached accessory structure permit in Hamilton typically runs 10–10 business days. This is an estimate, not a published figure — confirm current review times with City of Hamilton Building Department at (406) 363-3316.

You'll need: Residential Building Permit Application. Depending on your project, Hamilton may also ask for: ADU Fact Sheet (2025). See the required-documents checklist on this page for details on each.

Hamilton requires 3 inspection(s) for an accessory dwelling unit (adu) / detached accessory structure permit, in order: Foundation/Footing, Framing, Final. Schedule each through City of Hamilton Building Department ((406) 363-3316).

Apply through City of Hamilton Building Department at 920 New York Ave, Hamilton, MT 59840. Phone: (406) 363-3316, email: dpwsec@cityofhamilton.net. Office hours: City Hall: 223 S. 2nd Street, Hamilton, MT 59840, main line (406) 363-2101. Building Department / Public Works office: 920 New York Ave, (406) 363-3316 or (406) 363-6717. Building Inspector Tim Netzley: (406) 880-5832, inspector@constructmt.com. Inspection scheduling line: (406) 552-3238. Inspections scheduled Monday & Wednesday 1pm-4pm and Fridays 8am-12pm per the Roof Permit Application; 24 hours' notice required for plumbing inspections per the Plumbing Permit Application.. Official information: https://www.cityofhamilton.net/departments/building_department/index.php.

Hamilton, Montana has adopted: Montana has a mandatory statewide building code administered by the Montana Department of Labor & Industry (DLI) Building Codes Bureau, amended by the Administrative Rules of Montana (ARM) Title 24, Chapter 301. Cities, counties, and towns may become 'certified' to locally administer permitting and inspection for all or part of the code; a certified local program's adopted code 'may include only codes adopted by the Building Codes Bureau' (ARM 24.301.202). Non-certified areas fall under direct state administration by the DLI Building Codes Bureau. Source: Montana DLI Building Codes Bureau, Certified City, County and Town Programs (bsd.dli.mt.gov/_docs/building-codes-permits/certified-city.pdf, updated 04/22/2026).; Hamilton is a Montana DLI-certified jurisdiction for Building (B), Plumbing (P), Mechanical (M), Electrical (E), and Swimming Pool (SP) — full-trade certification — within Hamilton city limits. Certified program contact on file with DLI: Tim Netzley, (406) 880-5832. Source: Montana DLI Building Codes Bureau, 'Certified City, County and Town Programs' roster (bsd.dli.mt.gov/_docs/building-codes-permits/certified-city.pdf, updated 04/22/2026), row 'Hamilton — B, P, M, E, SP — Tim Netzley — 880-5832 — City Limits'. Directly verified by fetching and OCR-extracting the PDF text during this research pass (2026-07-03).; 2021 International Building Code (IBC) — adopted by City of Hamilton per the Building Department's Adopted Codes list on the Residential Building Permit Application; 2021 International Residential Code (IRC) — adopted by City of Hamilton (referenced on the permit application as '2021 International Residential Building Code'); 2021 International Existing Building Code (IEBC) — adopted by City of Hamilton; 2021 International Mechanical Code (IMC) — adopted by City of Hamilton; 2021 International Fuel Gas Code (IFGC) — adopted by City of Hamilton; 2021 International Energy Conservation Code (IECC) — adopted by City of Hamilton; 2021 Uniform Plumbing Code (UPC) — adopted by City of Hamilton (Montana uses the Uniform Plumbing Code, not the International Plumbing Code); 2020 National Electrical Code (NEC) — adopted by City of Hamilton; 2021 International Swimming Pool and Spa Code (ISPSC) — adopted by City of Hamilton; consistent with Hamilton's DLI Swimming Pool (SP) certification; 2017 ICC A117.1 Accessibility — adopted by City of Hamilton; All codes listed verbatim on the City of Hamilton 2023 Residential Building Permit Application, page 2 ('All construction shall comply with the minimum requirements of the following codes'). Source: City of Hamilton Residential Building Permit Application (PDF).. Local amendments apply — see the Hamilton overview page for the full list.

Yes, ADUs are allowed by right on single-family lots in the RS, RM, RH, and B zoning districts per Hamilton Municipal Code 17.16.060. They require a residential building permit, plus plumbing/electrical/mechanical permits depending on scope, and a Public Works permit for new water/sewer service lines. No impact fees apply. Source: City of Hamilton ADU Fact Sheet (2025).