Accessory Dwelling Unit (ADU) in Unincorporated Yellowstone County, Montana
VerifiedMontana 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
- Required
Application for Electrical Permit
Required for ADU wiring — see Electrical Permit entry for fee schedule
- Optional
Application for Plumbing Permit
Required if the ADU has its own kitchen/bath plumbing — see Plumbing Permit entry for per-fixture fees
- Optional
Application for Mechanical Permit
Required if the ADU has its own HVAC/furnace system
Fee schedule
| Fee type | Amount | Notes |
|---|
Review timeline
~1–5 business days
Typical estimate — confirm current times with the Unincorporated Yellowstone County building department
Inspection process
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
- Do I need a special ADU permit in unincorporated Yellowstone County?
- 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.
Sources & verification
Verified against official sources. Last reviewed 2026-07-02.
- Montana Department of Labor & Industry — Building Codes Bureau (Business Standards Division, Building and Commercial Measurements Bureau) — official building department
- Montana Building Codes Bureau — Building Permits (exemption for 4-or-fewer-unit residential buildings)
- Yellowstone County — Construction Outside of City Limits notice (Planning/Riverstone Health/Public Works contacts)
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
Accessory Dwelling Unit (ADU) in other cities
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