Montana ADU Laws & Permit Guide (2026)
Montana passed sweeping zoning reform legislation in 2023, including SB 245, which requires cities to allow ADUs statewide and limits impact fees. Montana is now one of the most permissive states for ADU construction.
Montana ADU At a Glance
Permit Timeline
60–90 days
Average Build Cost
$90,000 – $220,000
Maximum Size
Up to 1,000 sq ft or 75% of primary home
Owner-Occupancy Required
Not required
Impact Fees
Capped under SB 245
Setbacks
Follows local standards with state minimums
Recent Law Changes in Montana
Required Montana cities and counties to allow ADUs on single-family lots statewide. Capped impact fees for ADUs and prohibited owner-occupancy requirements. Part of a broader package of zoning reform bills.
Eliminated single-family-only zoning in cities over 7,000 population, allowing duplexes and ADUs on all residential lots as a matter of right.
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Browse Montana Contractors →Frequently Asked Questions — ADUs in Montana
Are ADUs legal throughout Montana?
Yes. SB 245 (2023) requires cities and counties statewide to allow ADUs on single-family lots. Montana is now one of the more permissive states for ADU construction, particularly notable given its traditionally conservative land-use policies.
How much does it cost to build an ADU in Montana?
Montana ADU costs typically range from $90,000 to $220,000 for a detached unit. Bozeman and Missoula, which have seen significant growth, tend to run higher. Rural areas are generally less expensive. Material and labor costs have risen with the state's population growth.
How long does ADU permitting take in Montana?
Montana permitting is generally faster than coastal states — expect 60–90 days in most cities. Bozeman, which has seen a surge in ADU applications, may take slightly longer.
Do I need to live on my property to build an ADU in Montana?
No. SB 245 prohibits municipalities from imposing owner-occupancy requirements on ADU properties.
Are there impact fee limits for ADUs in Montana?
Yes. SB 245 caps impact fees for ADUs at levels proportional to their impact compared to a full single-family home. This was a significant win for ADU proponents, as impact fees in some Montana cities had been a major cost barrier.
Sources
- Montana SB 245 — Legislature ↗
- City of Bozeman — ADU Resources ↗
- MT Department of Commerce — Housing ↗
Disclaimer: The information on this page is provided for general informational purposes only and does not constitute legal, financial, or construction advice. ADU regulations change frequently — always verify current rules with your local planning department and consult a licensed professional before making decisions. ADUHomeResource.com is not liable for any errors, omissions, or outcomes resulting from use of this information.