ADU Design in Columbus, MT

ADU Design in Montana: what governs your project

Montana's 2023 legislature passed HB 492, which requires municipalities to allow at least one accessory dwelling unit by right on any lot zoned for single-family residential use. The law bars cities from imposing owner-occupancy requirements, excessive minimum lot sizes, or design standards stricter than those for the primary home.

Energy code

Local jurisdictions set size caps and utility connection rules; state law limits the fees and review hurdles cities can impose. Expect straightforward administrative approval rather than conditional-use hearings in Montana's cities.

Permitting & plan check

Billings, Missoula, Bozeman, and other larger cities run their own building departments with published review processes. Many smaller jurisdictions rely on third-party reviewers or county-level review. Rapid growth in Bozeman and the Flathead has stretched review capacity in recent years.

Good to know

• The 1959 Hebgen Lake earthquake (M7.3) defines southwestern Montana's seismic design hazard.
• Montana DEQ administers the MPDES program for construction stormwater.
• Mountain snow loads in Montana are among the highest in the lower 48 states.

Every discipline, one team