ADU Pass
Fergus County
ADU Pass helps homeowners in Fergus County, Montana navigate the permit paperwork for building an accessory dwelling unit. We cover 8 cities and 9 ZIP codes in this county.
Map
County ADU details
County ADU ordinance
Fergus County (Lewistown county seat; central Montana; ~11,500 residents; bounded south by the Big Snowy / Little Snowy / Judith Mountains; spans into the Charles M. Russell National Wildlife Refuge / Missouri River Breaks to the north) does not maintain a comprehensive countywide zoning code. Like most rural central-Montana counties, Fergus operates under Montana's permissive default: parcels in unincorporated territory have no county-imposed zoning unless a specific zoning district has been formally created under MCA Title 76, Chapter 2 Part 1 (county zoning) or Part 2 (planning-and-zoning districts). Montana SB 528 (MCA § 76-2-345) preempts MUNICIPAL ADU zoning to require ADUs by-right inside city limits — but only Lewistown (population ~5,800) clearly clears the SB 323 5,000-resident duplex-preemption threshold. SB 528 ADU preemption applies to all incorporated cities regardless of size, including Lewistown, Denton, Grass Range, Moore, and Winifred. For unincorporated parcels — the bulk of the county by area — the only land-use constraints are typically subdivision review under the Montana Subdivision and Platting Act, on-site sewage rules under MCA Title 75 Ch. 5, and any voluntary covenants on the parcel. An ADU on an unincorporated Fergus County parcel is generally permissible without county zoning approval, subject to building-code compliance and septic permitting through the local sanitarian. The Charles M. Russell National Wildlife Refuge (CMR) covers ~1.1 million acres along the Missouri River in northern Fergus County and adjacent counties (Petroleum, Phillips, Valley, McCone, Garfield); private inholdings within the CMR are rare but can carry refuge-coordination requirements for new construction.
State-floor overlay: Montana SB 528 (MCA § 76-2-345) establishes a state floor for ADU permissibility in incorporated cities; the county's lack of a comprehensive zoning code in unincorporated territory means ADUs face no county zoning bar there.
Adopting body: Fergus County Board of County Commissioners
County permitting (unincorporated parcels)
Fergus County does not operate a county building department in the traditional sense. For unincorporated parcels, building permits are generally not required by the county for residential construction outside any formally-created zoning district. The state-adopted Montana Building Code applies as a technical floor; enforcement in counties without a building department falls back to the State Building Codes Bureau on complaint or to point-of-sale inspection requirements. The single permit most likely to be required is the on-site wastewater (septic) permit issued by the County Sanitarian / Public Health Department under MCA Title 75 Ch. 5 and ARM Title 17 Ch. 36. Subdivision review by the County Planning Office applies if creating a new parcel; placing an ADU on an existing parcel does not normally trigger subdivision review. Lewistown city (the only meaningful population concentration) operates its own building permits inside city limits.
Impact fees: Fergus County does not levy impact fees on residential additions in unincorporated areas. SB 528 prohibits ADU-specific impact fees inside Lewistown, Denton, Grass Range, Moore, and Winifred city limits.
County assessor
Property assessment in Montana is centralized at the Montana Department of Revenue (DOR), not the county. The Fergus County DOR field office maintains parcel-level assessment records for all real property in the county. ADU additions are typically captured as improvements to the host parcel via shared permit data with the building department — but in counties without a county building department, capture is delayed and often happens at the next regular reappraisal cycle. 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.
Assessment policy: Improvement value for an ADU is added to the parcel record on the next regular reappraisal cycle (every two years for residential under MCA § 15-7-111). Homestead / principal-residence treatment under MCA § 15-6-222 shields the existing structure but new improvement value is fully captured. Appeals run first to the County Tax Appeal Board (CTAB) and then to the Montana Tax Appeal Board.
County overlays (4)
Fergus County's overlay footprint is shaped by three distinct geographies: (1) the Lewistown-area FEMA NFIP floodplain along Big Spring Creek and the Judith River (relatively narrow but covers downtown Lewistown reaches), (2) the WUI footprint of the Big Snowy / Little Snowy / Judith Mountains (forested parcels at elevation), and (3) the Charles M. Russell National Wildlife Refuge boundary along the Missouri River — a federal overlay with refuge-coordination implications for any private inholding (rare) or for new development on adjacent parcels. No coastal or seismic overlays. Lewistown has a NRHP commercial historic district downtown.
- FEMA NFIP Special Flood Hazard Areas - Big Spring Creek, Judith River, Missouri River — A new ADU in a mapped SFHA must be elevated to or above the Base Flood Elevation; elevation cost can add $15,000-$40,000.
- Wildland-Urban Interface - Big Snowy, Little Snowy, Judith Mountains — Voluntary defensible space and ignition-resistant construction recommended; not formally required by county code.
- Charles M. Russell National Wildlife Refuge — Refuge boundaries are mapped on USFWS GIS. Most Fergus County residential parcels are far enough south of the refuge that this overlay is not material; the relevance is concentrated in the Hilger / Roy / Winifred area approaching the refuge.
- Lewistown Central Business District (NRHP) — ADU work on a NRHP-listed contributing property triggers Section 106 review only if federal financing or permitting (FHA, USDA RD, HUD-184) is involved.
Known county issues (2)
- other — Lower permit cost and timeline (typically septic-permit only); reduced inspection oversight; resale/insurance may require post-hoc inspection or affidavit.
- other — Material only for the small set of refuge-inholding parcels (Hilger/Roy/Winifred area approaching the Missouri River); most county parcels unaffected.
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.