Gallatin County

Gallatin County operates a full-service Department of Planning and Community Development plus a Building Department that together issue zoning permits, building permits, and inspections for unincorporated parcels throughout the county. Gallatin is one of only a handful of Montana counties (with Flathead, Yellowstone, Missoula, Cascade, and Lewis and Clark) operating a complete county building department. The Gallatin County Building Department administers the state-adopted Montana Building Code (currently 2018 IBC/IRC with Montana amendments) for unincorporated parcels. ADU intake follows a standard residential-permit track with separate zoning verification for compliance with the parcel's zoning district (within the Bozeman Area Donut, Gallatin Canyon, Bridger Canyon, Hyalite, Hebgen Lake, or other districts). Big Sky parcels go through Gallatin County permitting plus Big Sky Owners Association architectural review (covenant-based, not regulatory but contractually binding). Resort and high-elevation parcels face additional snow-load engineering review - Hyalite and Bridger Canyon parcels typically require 75-90 psf ground snow load with elevation calibration; Big Sky and the upper Gallatin Canyon push 90-120+ psf at higher elevations. Bozeman, Belgrade, Manhattan, Three Forks, and West Yellowstone operate their own city building departments inside city limits.

Impact fees: Gallatin County does not levy ADU-specific impact fees on residential additions. SB 528 prohibits ADU-specific impact fees inside Bozeman, Belgrade, Manhattan, Three Forks, and West Yellowstone. Bozeman levies general transportation, water, and sewer impact fees on new dwellings - these apply to ADUs but are not ADU-specific. The Big Sky Resort Area District levies a 4% resort tax that applies to construction sales but is not an impact fee.

Property assessment in Montana is centralized at the Montana Department of Revenue (DOR), not the county. The Gallatin County DOR field office in Bozeman maintains parcel records for the county. ADU additions are captured as improvements to the host parcel via shared permit data with the Gallatin County Building Department - capture is timely thanks to the county's full-service building department. 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. Gallatin County has experienced extreme residential-value appreciation 2020-2024 - Bozeman median home prices roughly doubled and Big Sky median sales pushed past $2M; reappraisal cycles have produced significant taxable-value increases that prompted state-level relief discussions in the 2025 legislative session, including the homestead-rate adjustment in HB 231 (2025) which provides modest tax relief for owner-occupied primary residences.

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), or sooner via shared permit-data feeds from Gallatin County Building Department. 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 Gallatin County Tax Appeal Board (CTAB) and then to the Montana Tax Appeal Board.

Gallatin County's overlay footprint is extensive, reflecting its mix of rapid-growth valley floor / Bozeman urban edge / Yellowstone NP gateway / Big Sky resort / high-elevation mountain WUI / multiple river floodplains. Six overlay categories are material: (1) Wildland-Urban Interface - Custer Gallatin National Forest surrounds the populated valley on every side; the Bridger Range, Gallatin Range, Madison Range, and Spanish Peaks all contain high-WUI parcels; multiple devastating wildfire seasons (2017 Sapphire Complex, 2020 Bridger Foothills, others) have driven county WUI hazard mapping but no countywide WUI building-code overlay has been formally adopted. (2) FEMA NFIP floodplain along the Gallatin River, East Gallatin River, Madison River, Jefferson River, Hyalite Creek, Bear Creek, Bridger Creek, and others. (3) Yellowstone NP gateway zoning - West Yellowstone gateway corridor along US-191 and US-287, plus the Hebgen Lake / Earthquake Lake corridor. (4) Big Sky Resort Area District - special-purpose 4% resort tax district covering the Big Sky community in Gallatin and Madison Counties; not a zoning overlay but a fiscal one. (5) Hebgen Lake / Earthquake Lake fault-zone / lakeshore - Earthquake Lake formed from the 1959 Hebgen Lake earthquake and is a USGS-monitored seismic hazard area. (6) Multiple NRHP historic districts (Bozeman, Manhattan, Three Forks, West Yellowstone) administered by city historic-preservation programs.

• Wildland-Urban Interface - Bridger Range, Gallatin Range, Madison Range, Spanish Peaks, Custer Gallatin NF — Voluntary Firewise USA participation is widespread (Big Sky, Bridger Canyon, Hyalite). Insurance markets are tightening: some carriers have stopped writing new homeowner policies in the highest-WUI zones (upper Gallatin Canyon, Big Sky upper parcels, Bridger Bowl-area). ADU economics need to factor in higher insurance cost (15-30% premium versus equivalent valley-floor parcel).
• FEMA NFIP Special Flood Hazard Areas - Gallatin River, East Gallatin River, Madison River, Jefferson River, Hyalite Creek, others — Spring snowmelt drives late-spring flooding peaks across multiple rivers. New ADU in a mapped SFHA must be elevated to or above the Base Flood Elevation; elevation cost can add $20,000-$50,000 in this market. Bozeman has had recurring floodplain controversies on East Gallatin and Bridger Creek.
• Yellowstone National Park Gateway / West Yellowstone / Hebgen Lake Corridor — Yellowstone NP gateway zoning is more restrictive than typical rural Montana zoning. STR economics are extreme in West Yellowstone and Big Sky but offset by zoning constraints. Hebgen Lake / Earthquake Lake corridor has additional seismic and lakeshore review.
• Big Sky Resort Area District (4% resort tax) — Big Sky is unincorporated and has no municipal government. Land use is regulated by Gallatin County (or Madison County for the southern slice) zoning plus Big Sky Owners Association covenants. The Big Sky Resort Tax District is the closest equivalent to a municipal authority.
• Hebgen Lake / Earthquake Lake Seismic Hazard Zone — Construction in the Hebgen Basin requires standard IRC seismic compliance plus lakeshore-protection permitting. The visible 1959 earthquake scarp is a tourist attraction (Quake Lake Visitor Center on US-287) and a routine reminder of regional seismic risk.
• Bozeman, Manhattan, Three Forks, West Yellowstone NRHP Historic Districts — 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. Bozeman has the most active historic-preservation program; review can add 30-60 days to ADU permitting on a contributing property.