ADU Rules in Butte, Montana

Butte-Silver Bow is a consolidated city-county government — one of the few in Montana — combining the former city of Butte with Silver Bow County into a single local government. With a population of approximately 35,000, Butte has a storied industrial history rooted in copper, silver, and zinc mining that made it one of the largest and most ethnically diverse cities in the American West during the late 19th and early 20th centuries. That heritage is also its most significant environmental liability: the Berkeley Pit — a massive open-pit copper mine filled with acidic, metal-laden water — is one of the largest Superfund sites in the United States.

Butte also boasts the largest National Historic Landmark District in the country by area, with thousands of Victorian-era homes and commercial buildings.

Montana SB 382 (2023) — Statewide ADU Reform

Montana Senate Bill 382 (2023) requires all Montana jurisdictions of 5,000 or more residents — including Butte-Silver Bow — to allow accessory dwelling units (ADUs) by right in residential zones. This means no discretionary denial, no mandatory public hearing solely for an ADU application; the city reviews applications against objective local development standards.

Montana HB 554 (2023) further expanded options for missing middle housing types statewide.

Types of ADUs Permitted

• Detached ADU: A freestanding structure separate from the primary home — a converted carriage house (Butte has many), new backyard cottage, or detached garage conversion.
• Attached ADU: An addition to the primary dwelling sharing at least one wall.
• Junior ADU (JADU): A unit created within the existing primary home — often a basement apartment, which is common in Butte's older multi-story homes.

Local Zoning Standards

Butte-Silver Bow regulates ADU development through its zoning code within the SB 382 framework. Typical standards include:

• Maximum size: Detached ADUs are generally limited to approximately 1,000–1,200 sq ft; verify current limits with Butte-Silver Bow Planning.
• Setbacks: Typically 5 feet from rear and side property lines; verify requirements for your specific zone.
• Height: Detached ADUs are commonly limited to one story or approximately 18–24 feet; historic district properties may have additional constraints.
• Lot coverage: ADUs count toward maximum lot coverage for the zone.
• Parking: SB 382 limits excessive parking requirements; verify current requirements.
• Alley access: Butte has an extensive alley network in older neighborhoods; detached ADUs accessed from alleys may have specific setback rules.

Special Considerations in Butte

Berkeley Pit and Superfund site: The Berkeley Pit is a former open-pit copper mine spanning approximately 1 mile wide and 1,800 feet deep, filled with roughly 50 billion gallons of acidic, metal-laden water. It is the centerpiece of the Silver Bow Creek/Butte Area Superfund site — one of the largest and most complex Superfund sites in the US under the EPA's National Priorities List. The Superfund boundary extends across large portions of Butte and the upper Clark Fork River basin.

Properties in or near the Superfund boundary may have contaminated soil from historic mining operations, smelter fallout, or mine waste. Renovation or excavation work that disturbs soil could:

• Trigger a review by the Montana DEQ or EPA Superfund program.
• Require soil testing before or during excavation.
• Require proper disposal of contaminated soil.

Before beginning any excavation for an ADU foundation, contact Butte-Silver Bow Planning and the Montana DEQ to determine if your property requires environmental screening.

National Historic Landmark District: Butte's historic district is one of the largest in the nation, encompassing thousands of buildings. Properties within the district may require a Certificate of Appropriateness from the Butte-Silver Bow Historic Preservation Office before constructing a new structure or making significant exterior alterations visible from public streets. Even if you are adding a rear-yard ADU, the design may be reviewed for compatibility with the historic character of the neighborhood.

Abandoned mine works: Butte's underground was riddled with mining tunnels and shafts. Some properties may be over abandoned mine workings; ground stability concerns are possible. The Montana Bureau of Mines and Geology (MBMG) maintains records of known mine locations.

Permit Process

1. Zoning and Superfund check: Contact Butte-Silver Bow Planning to confirm your zone and whether your property is within the Superfund boundary or any mine-related overlay.
2. Historic district check: If your property is in the historic district, contact the Historic Preservation Office about Certificate of Appropriateness requirements.
3. Environmental screening (if applicable): If in the Superfund area, consult Montana DEQ before planning any soil disturbance.
4. Building permit application: Submit to the Butte-Silver Bow Building Department with site plan, floor plans, elevations, and utility plans.
5. Plan review: Staff review for zoning and building code compliance.
6. Construction and inspections: Build per approved plans; complete all required inspections.
7. Certificate of Occupancy: Issued after passing final inspection.

Contact

Butte-Silver Bow Planning Department 155 W Granite St, Butte, MT 59701 Phone: (406) 497-6255 Website: https://co.silverbow.mt.us/157/Planning

Butte-Silver Bow Building Department 155 W Granite St, Butte, MT 59701 Phone: (406) 497-6260

Disclaimer: ADU regulations change frequently. Montana SB 382 (2023) established statewide ADU rights, but Butte-Silver Bow's local code sets specific development standards. Environmental legacy from the Berkeley Pit Superfund site and historic district preservation requirements may impose significant additional review processes for some properties. Always verify current requirements with Butte-Silver Bow Planning and, if applicable, the Montana DEQ before designing or building an ADU.