ADU Pass
Carter County
ADU Pass helps homeowners in Carter County, Montana navigate the permit paperwork for building an accessory dwelling unit. We cover 3 cities and 3 ZIP codes in this county.
Map
County ADU details
County ADU ordinance
Carter County (Ekalaka county seat; far southeastern Montana corner; ~1,250 residents — the smallest-population county in Montana; badlands terrain including Medicine Rocks State Park and the Long Pines / Chalk Buttes / Ekalaka Hills uplift) does not maintain a comprehensive countywide zoning code. Land use in unincorporated Carter County is largely unregulated except for state-level subdivision review under the Montana Subdivision and Platting Act (MCA Title 76 Ch. 3). Montana SB 528 (2023, codified Mont. Code Ann. § 76-2-345) preempts municipal ADU restrictions but does NOT extend to unincorporated counties; Carter County is unaffected by the state ADU floor because it is overwhelmingly rural and contains no incorporated city of 5,000+ population (Ekalaka, the only incorporated town, has ~370 residents). ADUs may be built on most parcels with only state-level building/sanitation oversight.
State-floor overlay: Montana SB 528 establishes a state ADU floor for incorporated cities; Carter County's unincorporated territory is not subject to the preemption. Local rules (where they exist) govern.
Adopting body: Carter County Board of County Commissioners
County permitting (unincorporated parcels)
Carter County issues no countywide building permits for residential construction in unincorporated areas; like most rural eastern-Montana counties, 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 Carter 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.
Process overview: Typical workflow for an ADU on unincorporated land: (1) confirm parcel is outside Ekalaka town limits; (2) septic permit through Montana DEQ or county sanitarian if required; (3) well permit through DNRC if new water source; (4) floodplain permit if in mapped SFHA along the Little Missouri River, Box Elder Creek, or other tributaries; (5) construction with no county building inspections; (6) appraisal capture by Montana Department of Revenue on next revaluation cycle.
Impact fees: Carter County levies no impact fees. Septic and well permit fees are state-level and modest. Total permit-side cost for a rural ADU is typically under $1,000.
County assessor
Property assessment in Carter County is performed by the Montana Department of Revenue (state) through its regional field offices, consistent with all Montana counties. Residential property is appraised at market value with taxable-value calculation per MCA 15-6-134. ADU additions are captured on the next biennial reappraisal cycle once the improvement is recorded. The Carter County Treasurer collects property taxes once levied; the county does not perform independent appraisal.
Assessment policy: Improvement value for an ADU is added to the parcel record on the next biennial revaluation. Montana applies a residential class rate (currently ~1.35% of market value, adjusting per legislative session) to compute taxable value. No homestead exemption shields new ADU improvement value.
County overlays (3)
Carter County's overlay landscape is dominated by FEMA NFIP floodplain regulation along the Little Missouri River and Box Elder Creek corridors, plus Custer Gallatin National Forest WUI exposure in the Long Pines and Ekalaka Hills. There is no county-administered historic-district overlay. Medicine Rocks State Park is a state-managed natural area with internal restrictions on adjacent parcels.
- FEMA NFIP Special Flood Hazard Areas in Carter County — Most of Carter County's habitable parcels sit on uplands away from SFHAs; floodplain compliance is a localized concern.
- Wildland-Urban Interface (Long Pines / Ekalaka Hills / Chalk Buttes) — No county WUI ordinance; voluntary defensible-space practice recommended.
- No tribal trust land within Carter County
Known county issues (1)
- other — Carter County ADU projects face minimal regulatory cost but maximum construction-quality variance.
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.