City-by-city ADU cost data across Montana's major markets. Built from permit records, contractor data, and regional labor rates.
Effective framework: Mont. Code Ann. § 76-2-345. State law requires covered local governments to allow ADUs under statutory standards; confirm current local administration and site rules. Permit fees and review timing remain planning estimates until confirmed for the specific property.
Calculate My ROI →A backyard cottage, converted garage, or newly built separate structure. Currently capped at 1,000 sqft or 75% of the primary home (whichever is smaller) — a limit that's removed entirely starting October 1, 2026.
Detached ADU Guide →A converted basement apartment or ground-floor addition sharing a wall with the primary residence. Montana law applies the same size limits and setback rules to attached and detached ADUs alike.
Attached ADU Guide →A converted garage — attached or detached, both are protected equally under state law. No special use permit or public hearing required under the 2023 rules.
Garage Conversion Guide →A converted lower level with independent entrance. Detached units and conversions may face additional review around utility connections and septic capacity in rural parts of the state.
Basement ADU Guide →A factory-built unit installed on a permanent foundation, engineered for Montana's snow and wind loads. Several providers now serve the Bozeman and Missoula markets.
Prefab ADU Overview →Some Montana municipalities recognize a JADU category — typically under 500 sqft, carved out of existing living space within the primary home. The lowest-cost path to a legal second unit statewide.
Junior ADU Overview →
*Total-cost ranges outside Bozeman are derived from regional construction-cost data at a representative ~650 sq ft build; verify against local bids before publishing final figures. Great Falls and Kalispell are notable additional markets not pictured here and are worth checking with local contractors directly.
State law requires covered local governments to allow ADUs under statutory standards; confirm current local administration and site rules. The detailed figures below are representative planning ranges, not a statewide fee schedule or review-time guarantee.
State law requires covered local governments to allow ADUs under statutory standards; confirm current local administration and site rules.
These are planning benchmarks, not a permit quote, eligibility ruling, or legal opinion. Confirm zoning, building, utility, site, private-covenant, fee, and timing requirements with the current local authorities. See the 51-jurisdiction law directory and national permit guide.
SB 528 already guarantees your right to build. These moves target the climate, rural infrastructure, and timing factors that remain within your control.
Calculate My ADU ROI →
Illustrative 1-bedroom layout. Actual floor plans vary by ADU type, size, local code, and homeowner needs.
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