Accessory Dwelling Unit (ADU) Permit in Great Falls, Montana
VerifiedRequired for construction of a new ADU on a residential lot in Great Falls. ADUs are allowed by right on any lot with an existing single-family dwelling under Montana's statewide ADU law (MCA 76-2-345). Requires sign-off from multiple city departments and utilities in addition to the standard building permit.
Verified 2026-07-02 · Source
When you need this permit
- 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))
Required documents
- Required
ADU Application
City of Great Falls Planning & Community Development Dept. ADU-specific application requiring multi-department/utility sign-off before submittal (Effective Date 3/2026)
- Optional
Attachment A — A Simple Explanation of Accessory Dwelling Unit (ADU) Rules
City-published plain-language summary of MCA 76-2-345 ADU rules, citing specific statute subsections
Fee schedule
| Fee type | Amount | Notes |
|---|---|---|
| ADU Zoning Application Fee | Capped at $250 by state law (exact city-charged amount not separately published in a fee table found; statutory ceiling confirmed) | MCA 76-2-345(4) — statewide statutory cap on one-time ADU zoning application fees; the City of Great Falls has not published a distinct dollar figure below this cap in any fee schedule located during this research |
| Building Permit Fee — valuation-based (same Resolution 10374 Exhibit A table as new residential construction) | Determined by declared ADU construction valuation against the Exhibit A table | Resolution No. 10374, Exhibit A — no separate ADU-specific building permit fee variance was found; ADUs are billed as ordinary residential construction |
Review timeline
~5–10 business days
Typical estimate — confirm current times with the Great Falls building department
Inspection process
- 1
Foundation
Footings and foundation before concrete pour
- 2
Framing
Structural framing before wall cover
- 3
MEP Rough-In
Electrical, plumbing, mechanical rough-in — separate trade permits required
- 4
Final
All work complete, ready for occupancy
Tips
- Montana's ADU-by-right law (MCA 76-2-345) took effect statewide after the Montana Supreme Court lifted a preliminary injunction against Senate Bill 323 — Great Falls now administers ADU permits under this law regardless of older local zoning restrictions.
- The ADU Application requires signatures from EIGHT separate departments/utilities (NorthWestern Energy, Engineering, Environmental, Fire Rescue, Addressing, Utilities, Floodplain Administrator, and Planning & Community Development Director) obtained BEFORE the application can be submitted — start this process early.
- State law caps any one-time ADU zoning fee at $250 and bars the city from charging impact fees or requiring extra parking for an ADU.
- Maximum ADU size is the lesser of 75% of the primary home's gross floor area or 1,000 sq ft — this is a hard statutory cap, not a local zoning choice.
Frequently asked questions
- Can I build an ADU on my property in Great Falls?
- 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.
- How much does an ADU permit cost in Great Falls?
- 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.
Sources & verification
Verified against official sources. Last reviewed 2026-07-02.
- City of Great Falls Building Division (Planning & Community Development Department) — official building department
- Accessory Dwelling Units and Duplexes | Great Falls, MT
- Attachment A — A Simple Explanation of ADU Rules (MCA 76-2-345)
- ADU Application (Effective 3/2026)
- Montana Code Annotated 76-2-345 (official statute text)
- Resolution No. 10374 — Building Permit Fees, Exhibit A
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 Great Falls building department before you submit. PermitBase is an independent reference and is not affiliated with any government agency.
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