Side by side · Montana
Compare ADU rules across Montana
Flexible on where you build? The same backyard cottage can be routine in one city and impossible next door. This table puts 13 Montana cities side by side so you can shortlist the friendly ones before reading a single ordinance, or narrow it to just the cities you are deciding between. Every value is summarized from the city's own code; open a city's page for the full notes, citations, and fine print.
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| City | Detached ADU | Attached / internal | Max size | Owner occupancy | Extra parking | Approval |
|---|---|---|---|---|---|---|
| Belgrade | Allowed | Allowed | 1,000 sq ft (Rural, Residential Suburban); 800 sq ft (Neighborhood Residential); no ADU-specific cap in other districts; max 2 bedrooms; state law guarantees up to the lesser of 75% of the house or 1,000 sq ft | Not required | None may be required under state law; the unamended 2023 code still prints 1 paved off-street space | Permitted use |
| Billings | Allowed | Allowed | 1,000 sq ft or 80% of the primary dwelling's gross floor area, whichever is less | Not required | None | Permitted use |
| Bozeman | Allowed | Allowed | 1,000 sq ft or 75% of the primary dwelling, whichever is LARGER; detached ADUs max 25 ft tall | Not required | None | Permitted use |
| Butte-Silver Bow | Allowed | Allowed | 1,000 sq ft or 75% of the house's gross floor area, whichever is less (state line; no local cap) | Not required | None for the ADU (state law forbids requiring it) | Permitted use |
| Columbia Falls | Allowed | Allowed | 75% of the primary dwelling's gross floor area or 1,000 sq ft, whichever is less | Not required | None | Permitted use |
| Flathead County | Allowed | Allowed | 1,000 sq ft of habitable space or 40% of the principal dwelling's habitable space, whichever is GREATER; one ADU per tract of record | Not required | Yes: 1 space per 400 sq ft of ADU floor area, maximum 2 spaces | Permitted use |
| 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. Gallatin County/Bozeman Area Zoning Regulation §15.02 (amended through March 2026); Gallatin Canyon/Big Sky Zoning Regulation §23.1.c (updated Nov. 13, 2025); MCA 76-2-215(1)Full rules for Gallatin County Ordinance read August 2026 | 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. Gallatin County/Bozeman Area Zoning Regulation §15.02.2; Hyalite Zoning Regulation §9.3; MCA 76-2-215(1)(b)Full rules for Gallatin County Ordinance read August 2026 | Varies by district, commonly 1,000 to 2,000 sq ft: Bozeman-area Donut standalone units 1,200 sq ft under 1 acre or 2,000 sq ft at 1+ acres and never larger than the main house; Big Sky 1,000 sq ft under 10 acres or 2,000 sq ft at 10+ acres; Bridger Canyon and Hyalite 1,200 sq ft; no cap in Four Corners or unzoned areas | Not required | Varies by district: 1 space per bedroom (max 2) in the Bozeman-area Donut; 2 spaces per dwelling unit in Big Sky; 2 spaces in Reese Creek; 1 space in South Cottonwood Canyon and Hyalite; none in Four Corners or unzoned areas | 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). Gallatin County Part 1 Zoning Administrative Regulation §6.2 (amended Aug. 10, 2021, Res. 2021-103); GC/Bozeman Area Zoning Regulation §25.01; Hyalite Zoning Regulation §4.3.5Full rules for Gallatin County Ordinance read August 2026 |
| Great Falls | Allowed | Allowed | 1,000 sq ft, or 75% of the primary dwelling's gross floor area, whichever is less (state cap; no separate city cap) | Not required | None | Permitted use |
| Helena | Allowed | Allowed | Detached or attached: lesser of 75% of the principal dwelling's square footage or 1,000 sq ft; internal: up to 40% of the principal building or an entire basement or attic | Not required | None | Permitted use |
| Kalispell | Allowed | Allowed | 1,000 sq ft of living space; single story and 18 ft tall unless built to principal-structure setbacks (then the district height cap, 35 ft in R zones, applies); ADU must be smaller than the main house | Not required | None: dwelling units under 1,200 sq ft need no off-street parking, and an ADU is capped at 1,000 sq ft | Permitted use |
| Livingston | Allowed | Allowed | 75% of the primary dwelling's gross floor area or 1,000 sq ft, whichever is less; no size cap for an ADU entirely inside the primary dwelling (e.g. a basement unit) | Not required | None | Permitted use |
| Missoula | Allowed | Allowed | No unit-size cap; a second unit is a full dwelling. Two-unit parcels are limited by floor area ratio instead: all buildings combined may not exceed 0.5x parcel area (LU-R1, LU-R2, U-R1) or 0.6x (U-R2 to U-R4); parcels of 4,000 sq ft or less are FAR-exempt; height 35 ft in most districts | Not required | None; the UDC has no minimum vehicle parking for any dwelling | Permitted use |
| Whitefish | Allowed | Allowed | 1,000 sq ft or 75% of the primary dwelling's gross floor area, whichever is less | Not required | None | Permitted use |
Summarized from each city's ordinance text, most recently read August 2026. Tap a value marked with an info icon to see exactly what it depends on, straight from the ordinance. Nearly every value has zone, lot-size, or process caveats beyond that; the per-city pages carry the full notes and code citations. Treat this as orientation, not a permit determination.
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Reading the table
- Detached ADU is the make-or-break column for backyard cottages: minimum lot sizes (often 8,000 to 14,500 sq ft) hide behind many of the "Allowed" values, so check your lot before celebrating.
- Attached / internal (basement apartments, additions) is legal almost everywhere and is usually the path of least resistance.
- Approval: "Permitted use" means staff-level sign-off; "Conditional use" means a public hearing, which adds time, cost, and uncertainty.
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