Montana SB 528: ADU Law
Every municipality must allow ADUs by right — $250 max fee, no parking requirements, upheld by Montana Supreme Court.
$250
Max application fee
1,000
Max sq ft (or 75%)
Jan 2024
Effective date
All
Municipalities covered
What SB 528 Requires
Effective January 1, 2024, Montana SB 528 requires all incorporated cities, towns, and consolidated city-counties with zoning powers to allow at least one ADU by right on any lot with a single-family dwelling.
What Municipalities Cannot Require
Size Limits
For attached or detached ADUs:
75% of primary dwelling OR 1,000 sq ft
(whichever is smaller)
For internal ADUs (conversions within the existing home): no specific size limit.
Permitting Process
ADU approval is “by right” — no public hearing, variance, special use permit, or conditional use permit required.
Submit application with site plan to your local planning department
Pay application fee (maximum $250) plus standard building permit fees
Staff reviews for building code and setback compliance (objective criteria only)
Obtain water/sewer will-serve letters if required
Receive approval and begin construction
SB 532 (2025): Counties Now Included
Montana passed SB 532 in May 2025, extending similar ADU rules to counties with established zoning. Key additions:
- • Counties with zoning must allow ADUs on single-family lots
- • 15-day expedited sanitation review for parcels with existing public water/sewer
- • Same prohibitions on owner occupancy and design matching
- • Size limits retained (75% of primary or 1,000 sq ft)
Montana Supreme Court Upholds SB 528
In early 2026, the Montana Supreme Court upheld SB 528 and related zoning reform laws, overturning a previous injunction. The ruling requires approximately 10 cities (including Missoula) to update their regulations to comply by May 1, 2026.
This decision solidifies ADU rights across Montana and removes the last legal obstacle to statewide implementation.
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