ADU Pass
Jefferson County
ADU Pass helps homeowners in Jefferson County, Montana navigate the permit paperwork for building an accessory dwelling unit. We cover 6 cities and 6 ZIP codes in this county.
Map
County ADU details
County ADU ordinance
Jefferson County (Boulder county seat; southwestern Montana between Butte and Helena; ~12,800 residents across 1,656 sq mi; the Helena commuter shed pulls heavily on the northern county - Clancy, Montana City, Jefferson City, and Boulder are bedroom communities for Helena state-government workers; Whitehall and Cardwell anchor the southern county on I-90; the Elkhorn Mountains scenic and IRA wilderness study area cover the eastern county margin; the Boulder area carries 19th-century mining heritage with abandoned-mine and early-Superfund cleanup remnants from the historic Boulder Hot Springs / Comet / Wickes silver-mining district) does not maintain a comprehensive countywide zoning code. Jefferson County operates under the Montana Subdivision and Platting Act (MCA Title 76 Ch. 3) and state building code (IRC 2018, administered by the Montana Department of Labor & Industry, Building Codes Bureau). Citizen-initiated zoning districts under MCA § 76-2-101 et seq. cover portions of the county - notably the Montana City / North Jefferson area as a Helena commuter overflow district. Incorporated municipalities include Boulder (~1,200 residents, county seat) and Whitehall (~1,030); both are subject to Montana SB 528 (2023, Mont. Code Ann. § 76-2-345) ADU preemption inside their town limits. SB 528 applies to ALL incorporated Montana cities (NOT population-gated per the Montana AG interpretation underlying the MT-CW2 finding). At the COUNTY tier, SB 528 is municipal-only and does NOT reach unincorporated county land. Census-designated places include Clancy (Helena commuter), Montana City (Helena commuter), Jefferson City (mining-historic), Cardwell (I-90 / Lewis & Clark Caverns gateway), and Basin (mining-historic).
State-floor overlay: Montana SB 528 establishes a state ADU floor for incorporated Montana cities; Boulder and Whitehall are squarely subject inside their town limits. SB 528 does NOT extend to unincorporated Jefferson County or to the citizen-initiated zoning districts. ADU permissibility on unincorporated parcels is determined by local zoning district where applicable, otherwise by state-level oversight only.
Adopting body: Jefferson County Board of County Commissioners (administers citizen-initiated zoning districts; no countywide zoning ordinance)
County permitting (unincorporated parcels)
Jefferson County issues no countywide building permits for residential construction in unincorporated areas. Like most rural Montana counties without a full county building department, building-code enforcement is delegated to the State of Montana (Department of Labor & Industry, Building Codes Bureau) only for certain commercial or state-jurisdiction projects. Single-family residences and ADUs in unincorporated Jefferson County are typically constructed without municipal-style plan review. State-level oversight applies to on-site septic (DEQ Subdivision review for parcels created after 1973) and well construction (DNRC water rights). Floodplain development requires a permit through the county floodplain administrator under MCA Title 76 Ch. 5 - applicable along the Jefferson River, Boulder River, Prickly Pear Creek, and other regulated waters. Citizen-initiated zoning districts (Montana City / North Jefferson area) impose additional zoning verification through the Jefferson County Planning Office. Boulder and Whitehall operate their own town permitting processes inside their limits. Jefferson County is in IECC climate zone 6B; valley-floor parcels (Boulder, Whitehall, Clancy along I-15 / I-90) carry ground snow loads 30-50 psf; Elkhorn Mountains and high-elevation parcels run 60-90+ psf. SB 528 is municipal-only at the county tier - it does NOT reach unincorporated county land.
Impact fees: Jefferson County and its towns levy no impact fees. SB 528 prohibits ADU-specific impact fees in Boulder and Whitehall; neither has a general residential impact-fee schedule.
County assessor
Property assessment in Montana is centralized at the Montana Department of Revenue (DOR), not the county. The Jefferson County DOR field operations are administered through the Helena / Butte regional offices. ADU additions are captured as improvements to the host parcel on the next biennial residential reappraisal cycle - capture is delayed by the absence of a county building department feeding shared permit data. Mont. Code Ann. § 15-6-134 establishes Class 4 residential property and taxable-value calculation. MCA § 15-6-222 establishes a homestead exemption for principal residences. Montana operates on a two-year reappraisal cycle for residential property under MCA § 15-7-111. Jefferson County has experienced significant residential-value appreciation 2020-2024 driven by Helena commuter overflow into Clancy / Montana City and Bozeman/Butte spillover into Whitehall / Cardwell - reappraisal cycles have produced taxable-value increases that prompted state-level relief discussions in the 2025 legislative session, including the homestead-rate adjustment in HB 231 (2025).
Assessment policy: Improvement value for an ADU is added to the parcel record on the next biennial revaluation. Without a county building department, capture relies on field discovery, satellite imagery, owner self-report, or town-permit feeds (Boulder / Whitehall) - delays of 1-3 years are common. Montana applies a Class 4 residential rate (~1.35% of market value, adjusting per legislative session) to compute taxable value. Homestead / principal-residence treatment under MCA § 15-6-222 (and HB 231 2025 adjustments) shields the existing structure but new improvement value is fully captured. Appeals run first to the Jefferson County Tax Appeal Board (CTAB) and then to the Montana Tax Appeal Board.
County overlays (5)
Jefferson County's overlay landscape includes five material categories: (1) FEMA NFIP floodplain along the Jefferson River (the southern county spine through Whitehall and Cardwell), the Boulder River (through Boulder), Prickly Pear Creek (through Clancy and the Helena commuter corridor), and Cataract Creek. (2) Boulder area abandoned-mine Superfund remnants - the Basin Mining Area Superfund site, Comet, and Wickes carry residual heavy-metal contamination that affects soil disturbance and groundwater on a meaningful share of the central county. (3) Wildland-Urban Interface in the Elkhorn Mountains, the Tobacco Root Mountains gateway (Cardwell side), and the Helena-Lewis & Clark NF gateway. (4) Elkhorn Mountains Wilderness Study Area - federal IRA designation covering the eastern county margin; USFS does not regulate adjacent private land directly but federal-action triggers can apply. (5) Lewis & Clark Caverns State Park (Cardwell area) - a tourism overlay with karst-topography geological considerations.
- FEMA NFIP Special Flood Hazard Areas - Jefferson River, Boulder River, Prickly Pear Creek, Cataract Creek — Spring snowmelt drives flooding peaks. Helena commuter shed parcels in Clancy / Montana City face elevated floodplain compliance cost - the Prickly Pear corridor is densely-developed and elevation cost can add $20,000-$40,000 to ADU project budget. Jefferson River corridor through Whitehall is more rural with lower compliance cost.
- Boulder area abandoned-mine Superfund remnants (Basin Mining Area, Comet, Wickes) — ADU project site investigation cost on Comet / Wickes / Basin-area parcels can add $5,000-$15,000 to budget. Conventional financing and title insurance can require contamination disclosure. Some parcels carry institutional controls / Activity-Use Limitations (AULs) under MCA Title 75 Ch. 10 Pt 7.
- Wildland-Urban Interface - Elkhorn Mountains, Tobacco Root gateway, Helena-Lewis & Clark NF — Voluntary Firewise USA participation is widespread. Insurance markets are tightening for Elkhorn Mountains and high-elevation foothill parcels (15-30% premium versus valley-floor equivalent). Helena commuter shed parcels (Clancy, Montana City) face moderate WUI exposure on the Prickly Pear / Lump Gulch corridor.
- Elkhorn Mountains Wilderness Study Area / Helena-Lewis & Clark NF — Private inholdings within or adjacent to the WSA face access constraints. Federally-financed ADU projects (FHA, USDA RD, HUD-184) on adjacent parcels can face Section 7 ESA consultation timelines. Elkhorn elk hunting district draws sustained tourism with a small shoulder-season STR market.
- Comet NRHP mining ghost town; Boulder Hot Springs NRHP; Basin NRHP — ADU work on NRHP-listed contributing properties triggers Section 106 review only if federal financing or permitting (FHA, USDA RD, HUD-184) is involved. Comet and Wickes are tourism-historical destinations with modest STR potential.
Known county issues (4)
- policy-review — Northern Jefferson County ADU economics are strong (Helena state-government rental demand, ~5% vacancy, $1,400-$1,800/month for a one-bedroom rental). Southern Jefferson County (Whitehall, Cardwell) ADU economics are weaker - rural rental demand is modest. Whitehall on I-90 captures Bozeman/Butte spillover but at lower intensity than Three Forks (Gallatin County) or Manhattan.
- other — Site investigation cost on Boulder-area mining-historic parcels can add $5,000-$15,000 to ADU project budget. Conventional financing and title insurance can require contamination disclosure. ADU economics on Comet and Wickes parcels are constrained by remediation cost; tourism-historical ADU/STR positioning can offset some of this.
- other — Whitehall ADU rental yield is competitive (annual rent ~$10,000-$14,000 for a detached one-bedroom; STR shoulder-season nightly rates $80-$140). Cardwell parcel rentals are dominated by tourism cycling rather than long-term tenancy. Lewis & Clark Caverns shoulder season (April-October) drives most STR demand.
- other — Insurance premium adds 15-30% to operating cost for ADUs in WUI-mapped Elkhorn / Boulder canyon parcels; some new construction may face limited carrier choice. Whitehall and Cardwell valley-floor parcels face less constrained insurance market.
Montana state — ADU law and programs
State ADU law
Montana has enacted strong statewide ADU preemption through Senate Bill 528 (2023), codified at Mont. Code Ann. § 76-2-345. SB 528 is part of the 2023 'Montana Miracle' housing reform package (also including SB 323 duplex preemption and SB 245 commercial-zone multifamily). SB 528 requires every Montana city to allow at least one ADU by right on any lot containing a single-family dwelling, with size capped at the lesser of 1,000 sq ft or the floor area of the primary dwelling. ADUs may be attached, detached, or internal. Cities are prohibited from requiring additional parking, fees, design-match to the primary dwelling, owner-occupancy, occupant-relationship limits, or impact fees specifically for ADUs. ADUs must still meet building, fire, and public-health codes. Effective 2024-01-01. The Montana Supreme Court (September 2024) lifted a Gallatin County district-court injunction obtained by Montanans Against Irresponsible Densification (MAID), allowing the law to take effect; a Daily Montanan March 2025 ruling confirmed the affordable-housing bills are a 'legitimate government concern.' The 2025 session continued the reform trajectory (HB 533 wildfire-insurance transparency); no rollback bills have advanced as of 2026-04-26.
State HOA preemption
Montana's HOA statute Mont. Code Ann. § 70-17-901 (https://mca.legmt.gov/bills/mca/title_0700/chapter_0170/part_0090/section_0010/0700-0170-0090-0010.html) prohibits an HOA from adopting or enforcing a covenant amendment that imposes more onerous use restrictions than those in place when a member acquired their interest, unless the affected member consents in writing. After 2019-05-09, an HOA also may not enforce a covenant in a way that limits uses that were allowed when the member acquired the property. This is a one-way ratchet protecting existing uses, not an affirmative ADU preemption: an HOA's pre-existing covenant prohibiting ADUs remains enforceable against owners who took title with that restriction in place. SB 528's text expressly addresses municipal zoning, not private covenants, and § 70-17-901(2) carves out covenants required to comply with applicable federal, state, and local laws — but a covenant banning ADUs is not 'required' by SB 528, so the carve-out does not flip pre-existing ADU bans. A homeowner subject to an HOA covenant prohibiting ADUs has no state-law remedy under current Montana law and must seek the consent or amendment process within the association.
State financing programs
Montana Board of Housing (MBOH) under the Montana Department of Commerce does not operate an ADU-specific loan or grant product as of 2026-04-26. Construction or rehab of an ADU on a primary residence may be financed indirectly through MBOH's Regular Bond Loan Program (30-year, low-interest first mortgage geared to first-time buyers under income and purchase-price limits) or the Bond Advantage / MBOH Plus Down Payment Assistance Programs (up to 5% / $15,000 in DPA). All MBOH first-mortgage products require qualifying for an underlying FHA, VA, RD, or HUD-184 loan; an existing ADU on the property is generally permissible. The Department of Commerce's Coal Trust Multifamily Loan Program funds rental housing developments which can include ADU-style units in multi-unit projects but is not a homeowner-facing instrument.
State housing programs
Montana does not operate a statewide pre-approved ADU plan catalog as of 2026-04-26. The 2023 Montana Miracle reform package (SB 528, SB 323, SB 245, SB 382) established the country's most aggressive state preemption of single-family-only zoning, but did not pair the preemption with a state plan library, impact-fee waiver fund, or per-ADU rebate. SB 382 (the Montana Land Use Planning Act, https://archive.legmt.gov/bills/2023/BillPdf/SB0382.pdf) restructures comprehensive-planning requirements for cities of 5,000+ residents toward a more streamlined, plan-then-permit model. Implementation by the Department of Commerce and the Department of Environmental Quality continues; the DEQ Energy Code office published an ADU best-practices guide in fall 2023.
Federal (United States) — ADU-relevant rules and programs
Federal ADU law
The United States has no federal statute that directly regulates accessory dwelling unit entitlement or design. Land-use authority over ADUs resides with states and local governments under the traditional police power. Federal engagement is limited to financing (Fannie/Freddie/FHA/VA/USDA), flood insurance (FEMA/NFIP), and discretionary housing programs (HUD), which are recorded in sibling sections of this file.
Federal financing programs
Federal housing-finance agencies and GSEs set nationwide underwriting rules that govern whether an ADU can be financed, appraised, and counted toward mortgage qualifying income. The relevant actors are Fannie Mae, Freddie Mac, FHA (HUD), VA, and USDA Rural Development.
Federal tax credits
There is no ADU-specific federal tax credit. ADUs may incidentally qualify for existing federal energy-efficiency and clean-energy tax credits when the ADU construction includes qualifying measures.
Federal housing programs
HUD administers several discretionary programs that can fund ADU-related activity at the grantee's election, but none is an ADU-specific program.