ADU Guide · Verified 2026
Building an ADU in Idaho
Idaho requires cities over 10,000 residents to allow one ADU per lot, a brand-new law effective July 2026 that eliminated owner-occupancy requirements entirely. A homeowner has to choose between an internal ADU or a detached one, not both as a simultaneous right.
Quick Facts
Statewide floors set by SB 1354, signed March 31, 2026, effective July 1, 2026, Idaho Code 67-6541. Your city may be more generous but cannot go below them.
The Three Types of ADU
A very recent law with a real effective date
SB 1354 was signed March 31, 2026 and took effect July 1, 2026, making it one of the newest statewide ADU laws covered in this entire research, current as of just a few months before this writing.
A genuinely distinctive "choose one" structure
Unlike most states in this research, which protect both an internal and a detached ADU right simultaneously, Idaho's SB 1354 protects only one ADU per lot total, a homeowner must choose between an internal unit or a single detached structure, not claim both rights at once.
A size direction that runs the less common way
Cities cannot limit an Idaho ADU below the larger of 1,000 square feet or 75% of the primary home, the same "larger of" direction Iowa and Vermont use, rather than the more common "smaller of" rule found in states like Arkansas or Montana.
A real improvement over the prior law
The earlier HB 166 only addressed HOA restrictions on internal ADUs specifically, and still let cities require owner occupancy for those units, while leaving detached ADUs entirely up to local discretion. SB 1354 is a genuinely broader law that closes both of those gaps.
What people get wrong
The most common mistake is assuming SB 1354 lets a homeowner build both an internal ADU and a detached one. It protects only one per lot, the homeowner has to pick.
From Idea to Move-In: The Timeline
Who Administers It
Building It Yourself? Here's What You'll Need
Planning an ADU? Tools homeowners use in the design and layout phase:
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Check Your Local Rules
Your city may allow more than the state floor
State law sets the minimum; many Idaho cities and counties are more generous. Browse the full directory of 44 counties and every city in Idaho for local rules, fees, and building department contact details.
Browse the full Idaho directory →Not sure if your project needs a building permit, a zoning approval, or both? See our guide to building permits vs. zoning approval.
Official Sources
ADU law changes frequently and local ordinances add detail. This guide is general information, not legal advice, always confirm with your local building department. Reviewed by the StateDataIndex Editorial Team · Updated September 2026.