ADU Guide · Verified 2026
Building an ADU in Oregon
Oregon requires cities over 2,500 residents to allow at least one detached ADU per single-family lot, but only inside the urban growth boundary, a distinction a later bill had to clarify after the original law left it ambiguous. Portland was ahead of the state, dropping its own parking and occupancy requirements years before the statewide mandate existed.
Quick Facts
Statewide floors set by SB 1051 (2017), ORS 197A.425(1)(a); clarified by HB 4031. Your city may be more generous but cannot go below them.
The Three Types of ADU
A law that needed a fix to say what it meant
SB 1051 was ambiguous about geographic scope when first passed in 2017. HB 4031 later clarified explicitly that the ADU mandate applies only inside a city's urban growth boundary, not to rural land the city might otherwise annex or influence, a real legislative correction rather than a simple update.
Portland got there first, on its own
Portland eliminated its ADU parking requirement in 2013 and its owner-occupancy requirement in 2016, both years before SB 1051 created any statewide mandate, making Portland a genuine early mover rather than a city simply complying with state law.
Two different laws, easy to confuse
SB 1051 covers ADUs specifically. HB 2001 (2019) covers "middle housing", duplexes, triplexes, fourplexes, and cottage clusters, in cities of 10,000 or more. They overlap in spirit but are separate statutes with separate population thresholds and separate requirements.
What people get wrong
The most common mistake is assuming SB 1051 covers rural Oregon. It specifically applies only inside a qualifying city's urban growth boundary, HB 4031 corrected the ambiguity that used to make this unclear, so land just outside that boundary remains fully local.
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 Oregon cities and counties are more generous. Browse the full directory of 36 counties and every city in Oregon for local rules, fees, and building department contact details.
Browse the full Oregon 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.