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Accessory Dwelling Unit (ADU) in Unincorporated Yellowstone County, Montana

Montana has no ADU-specific state permit type; an ADU in unincorporated Yellowstone County is permitted the same way as any other residential structure. Because the state building-permit exemption applies to 'private homes and apartments with four or fewer units,' a detached or internal ADU that brings the total unit count on the property to four or fewer generally does not require a state building permit for the structure itself — but its electrical, plumbing, and mechanical systems each require their own state permits regardless.

Verified 2026-07-02 · Source

When you need this permit

  • No separate state ADU building permit exists; the standard residential building-permit exemption (four or fewer dwelling units, non-transient) determines whether a structural permit is needed
  • Electrical, plumbing, and mechanical permits are required for ADU systems (wiring, water heater, kitchen/bath plumbing, HVAC) regardless of the building-permit exemption
  • Zoning compliance for adding a dwelling unit is a SEPARATE matter from building permits — contact Yellowstone County/City-County Planning Division at (406) 247-8247 to confirm zoning allows an ADU on the property before building
  • Septic/well capacity for the added unit must be confirmed with Riverstone Health Environmental Health Services at (406) 256-2770

Required documents

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Fee schedule

Filing fee
Not published — contact the department

Review timeline

Plan reviewTypical estimate — confirm current times with the Unincorporated Yellowstone County building department
~1–5 business days

How long did your Accessory Dwelling Unit (ADU) permit actually take in Unincorporated Yellowstone County?

Anonymous — one question, no email. Answers are combined and only shown once at least 5 people have reported.

Inspection process

    See the full Unincorporated Yellowstone County inspection guide — scheduling & re-inspection fees →

    Tips

    • Before building an ADU, confirm zoning allowance with the City-County Planning Division ((406) 247-8247) — building-permit exemption does not mean zoning is automatically permissive.
    • If the ADU plus main house totals 4 or fewer dwelling units, no state building permit is likely required for the structure — but electrical, plumbing, and mechanical permits are each still required and billed per the individual fee schedules in this dataset.
    • Septic system capacity is a common blocker for ADUs in unincorporated areas — verify with Riverstone Health Environmental Health Services before design.

    Frequently asked questions

    No separate ADU permit exists in Montana. An ADU is permitted like any residential structure: the building/structural permit is exempt if total units on the property are four or fewer, but electrical, plumbing, and mechanical work for the ADU each require their own state permits. Zoning approval from City-County Planning is a separate requirement.

    Plan review for an accessory dwelling unit (adu) permit in Unincorporated Yellowstone County typically runs 1–5 business days. This is an estimate, not a published figure — confirm current review times with Montana Department of Labor & Industry — Building Codes Bureau (Business Standards Division, Building and Commercial Measurements Bureau) at (406) 841-2056.

    You'll need: Application for Electrical Permit. Depending on your project, Unincorporated Yellowstone County may also ask for: Application for Plumbing Permit; Application for Mechanical Permit. See the required-documents checklist on this page for details on each.

    Apply through Montana Department of Labor & Industry — Building Codes Bureau (Business Standards Division, Building and Commercial Measurements Bureau) at 301 South Park Ave, Floors 4-5, Helena, MT 59620 (mailing: Building Codes Bureau, PO Box 200517, Helena, MT 59620-0517). Phone: (406) 841-2056, email: buildingcodes@mt.gov. Office hours: Mon–Fri, state business hours; online permitting available 24/7 via https://aca-prod.accela.com/bcb/. Official information: https://bsd.dli.mt.gov/building-codes-permits/.

    Unincorporated Yellowstone County, Montana has adopted: 2021 International Building Code (IBC) — adopted statewide by ARM Title 24, Chapter 301; effective June 11, 2022; 2021 International Residential Code (IRC) — ARM Title 24, Chapter 301; effective June 11, 2022; 2021 International Existing Building Code (IEBC) — ARM Title 24, Chapter 301; effective June 11, 2022; ICC A117.1-2017 (Accessibility) — ARM Title 24, Chapter 301; effective June 11, 2022; 2021 Uniform Plumbing Code (UPC) — ARM Title 24, Chapter 301; effective June 11, 2022 (fixture table at ARM 24.301.351); 2021 International Mechanical Code (IMC) — ARM Title 24, Chapter 301; effective June 11, 2022; 2021 International Fuel Gas Code (IFGC) — ARM Title 24, Chapter 301; effective June 11, 2022; 2020 National Electrical Code (NEC) — ARM Title 24, Chapter 301; effective June 11, 2022 (NEC Technical Advisory NEC_TIA-23-3_210.8 issued August 12, 2022); 2021 International Energy Conservation Code (IECC) — ARM Title 24, Chapter 301; effective June 11, 2022; 2021 International Swimming Pool and Spa Code (ISPSC) — ARM Title 24, Chapter 301; effective June 11, 2022; 2021 Wildland Urban Interface Code (WUIC) — ARM Title 24, Chapter 301; effective June 11, 2022; NOTE: All codes listed are statewide codes administered directly by the Montana Building Codes Bureau for areas (like unincorporated Yellowstone County) that are NOT within a certified local-government building code jurisdiction..

    Sources & verification

    Verified against official sources. Last reviewed 2026-07-02.

    Fees, timelines, and adopted codes are researched from each jurisdiction's published records — see how we verify. Requirements change and vary by project, so always confirm the current details with the Unincorporated Yellowstone County building department before you submit. PermitBase is an independent reference and is not affiliated with any government agency.

    Other permits in Unincorporated Yellowstone County

    Residential Building PermitElectrical PermitPlumbing PermitMechanical / HVAC PermitRoofing Permit (Reroof)Solar Panel PermitWater Heater PermitElectric Vehicle (EV) Charging Station / EVSE Permit (State Electrical Permit)Sign PermitFire Sprinkler / Fire Alarm PermitChange of Use / Zoning Compliance Permit

    Accessory Dwelling Unit (ADU) in other cities

    Bozeman, MontanaGreat Falls, MontanaHelena, MontanaKalispell, MontanaMissoula, MontanaAlbuquerque, New MexicoArvada, ColoradoAurora, Colorado

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