ADU rules in Gallatin County

Unincorporated Gallatin County has no countywide zoning: 22 separate zoning districts cover pockets of the county and most of its land area is unzoned, so the answer depends entirely on where the parcel sits. Detached ADUs are allowed with an administrative permit in the largest districts: one accessory dwelling per parcel in the Gallatin County/Bozeman Area (Donut) district (§15.02, internal, attached, or a standalone accessory building), one per single-family dwelling in Gallatin Canyon/Big Sky (§23.1.c), up to two in Four Corners, and one each in Bridger Canyon (within 150 feet of the house, shared electric meter), Reese Creek, Middle Cottonwood, South Cottonwood Canyon, Trail Creek (under 1,000 sq ft), and Bozeman Pass; North Gallatin Canyon allows all land uses by right. In the seven districts zoned under MCA Title 76 ch. 2 part 2 (Donut, Four Corners, East Gallatin, Middle Cottonwood, North Gallatin Canyon, Reese Creek, South Cottonwood Canyon), MCA 76-2-215 (SB 532, 2025) independently guarantees at least one attached, detached, or internal ADU by right on any lot with a single-family dwelling. Hyalite allows only accessory apartments (added onto or within the dwelling or a garage) by conditional use permit, Hebgen Lake allows only kitchen-less guest houses by CUP, and several small part 1 districts (Springhill, South Gallatin, Bear Canyon, River Rock, Sypes Canyon 1 and 2, Wheatland, Zoning Districts 1 and 6) list no ADU use at all, which under their permissive-use schemes means no independent second dwelling. In unzoned areas there is no zoning review of any kind.

Attached and internal ADUs ride the same district patchwork as detached ones. The Donut regulation expressly allows the accessory dwelling to be 'an independent living facility located within the principal Dwelling, attached to the principal Dwelling, or a standalone Accessory Building' (§15.02.2), and Four Corners, Reese Creek, Middle Cottonwood, and South Cottonwood Canyon use the same internal/attached/detached formula; Big Sky defines an Accessory Dwelling Unit as 'whether attached or detached' (§3.20.a). Hyalite's accessory apartment (conditional use) is the attached-or-internal form: 'an independent living facility added onto or created within a single-family dwelling unit' or added onto a detached garage (§9.3). Bridger Canyon allows the ADU inside the principal dwelling or in a standalone accessory building (§12.2.b). In the seven part 2 districts, MCA 76-2-215(1)(b) guarantees the attached and internal forms by right; in unzoned areas no zoning applies.

Each district writes its own cap. Donut (§15.02.3): the accessory dwelling may never exceed the principal dwelling's square footage, and standalone units are capped at 2,000 sq ft on parcels of one acre or more, 1,200 sq ft under one acre. Big Sky (§23.1.c.2): 1,000 sq ft of floor area on lots under 10 acres, 2,000 sq ft on lots of 10 acres or more, with additional floor area available through a conditional use permit. Middle Cottonwood mirrors Big Sky's 1,000/2,000 split at the 10-acre line (§4.8); Bridger Canyon caps ADUs at 1,200 sq ft of livable floor area (§12.2.c); Hyalite accessory apartments at 1,200 sq ft (§9.3.1); Trail Creek allows up to 1,000 sq ft as a permitted use and larger units only by CUP (§4.2.g, §4.3.d). Four Corners and North Gallatin Canyon set no numeric cap (subordinate-use test only), and unzoned land has no size limit. Because the county mandate in MCA 76-2-215 carries no size-cap sentence (unlike the municipal statute), these local caps stand even in part 2 districts.

No Gallatin County zoning district requires the owner to live in either the principal dwelling or the ADU: a sweep of all 22 district regulations found owner- or proprietor-residency language only in bed-and-breakfast rules (for example Big Sky §3.7, Springhill §8.8.1), never for accessory dwellings. In the seven part 2 districts the point is also preempted: MCA 76-2-215(2)(b) provides a county may not 'require that the single-family dwelling or the accessory dwelling unit be occupied by the owner.' The 15 part 1 districts (Big Sky, Bridger Canyon, Hebgen Lake, and others created under MCA 76-2-101 et seq.) sit outside that statute and could lawfully impose one, but none does today. Note the related but distinct restriction in Bridger Canyon: its ADU may not be rented separately from the main house, which functions as a household-unity rule rather than an owner-occupancy rule.

The Donut parking table lists 'Accessory Dwelling: 1 per bedroom, not to exceed 2' (§18.04.1). Big Sky's Table 27.1 requires 2 off-street spaces per Dwelling Unit, which includes an ADU. Reese Creek requires a minimum of two onsite spaces for an accessory dwelling (§3.01.2.1), South Cottonwood Canyon one (§3.01.2.1), and Hyalite one additional space for an accessory apartment (§9.3.3). Four Corners and North Gallatin Canyon impose no residential parking minimums, and unzoned areas have none. Unlike the municipal ADU statute (MCA 76-2-345), the county statute MCA 76-2-215 contains no parking prohibition, so counties may still require ADU parking and these district minimums are enforceable even in part 2 districts.

Where ADUs are allowed, they are almost always permitted or accessory uses approved through an administrative Land Use Permit: the Planning Director verifies conformance with the district regulation, with septic or sewer approval, an address, and access permits as prerequisites (Part 1 Zoning Administrative Regulation §6.2 to 6.3; Donut §25.01). Conditional use permits with a public hearing are required for Hyalite accessory apartments (§4.3.5), Hebgen Lake guest houses (§8.3.d), East Gallatin guest houses (§3.4.v, though as a part 2 district that CUP requirement is void as applied to a statute-compliant ADU under MCA 76-2-215(5) and (9)(b)), oversized units in Trail Creek (over 1,000 sq ft, §4.3.d) and Big Sky (above the size caps, or additional ADUs on lots over 40 acres, §23.1.c), and guest houses in Springhill and South Gallatin. At the permissive end, Bozeman Pass treats the accessory dwelling as an accessory use needing no land use permit at all (§3.3.a), North Gallatin Canyon requires no construction or use permits (§2.3.2, §3.2.1), and unzoned land needs no zoning approval of any kind (state-jurisdiction building codes, septic permits, and floodplain rules still apply).

Most districts place no restriction on renting an ADU long-term, and no district requires owner occupancy. The exceptions are explicit: Bridger Canyon's ADU 'may not be rented or sold separately from the Principal Single-family Dwelling Unit' (§12.2.f), Hebgen Lake guest houses 'shall not be rented' (§18.19.c), and Springhill and South Gallatin guest houses may not be offered for rent as individual dwelling units. For short-term rentals (under 30 days), the county's rule is that STRs are allowed only where a district's regulation lists them: permitted in most Big Sky sub-districts and in North Gallatin Canyon, conditional-use in Bozeman Pass (AR80, PL640, AR40), Bridger Canyon (B-2, B-3; permitted in B-4), Hebgen Lake (C, RP), South Gallatin (CC), and Four Corners (RR/A, LDRR/A, NC; principal use in MU and C); in every other district they are not permitted anywhere, and in unzoned areas zoning imposes no restriction. Every STR countywide needs a Gallatin City-County Health Department Public Accommodations License. Separately, renting out buildings implicates the county's Buildings for Lease or Rent Regulations (amended August 4, 2026): the first three or fewer rental buildings on a tract are exempt from full review but water/wastewater approval must be recorded, so a single rented ADU does not trigger BLR review.