Washington State ADU Laws, Costs & Permit Guide (2026)
Washington State expanded ADU rights significantly in 2023, requiring most cities to allow ADUs statewide and limiting fees and owner-occupancy requirements.
Washington ADU At a Glance
Permit Timeline
90–180 days
Average Build Cost
$140,000 – $300,000
Maximum Size
1,000 sq ft (varies by city)
Owner-Occupancy Required
Not required under HB 1337 (2023)
Impact Fees
Capped at 50% of single-family rate for ADUs
Setbacks
Varies by city; 5 ft minimum common
Recent Law Changes in Washington
Required cities planning under the Growth Management Act to allow at least two ADUs per single-family lot and eliminated owner-occupancy requirements.
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Browse Washington Contractors →Frequently Asked Questions — ADUs in Washington
How many ADUs can I build in Washington State?
Under HB 1337 (2023), cities planning under the Growth Management Act must allow at least two ADUs per single-family lot — typically one detached and one attached or JADU.
Do I need to live on my property to build an ADU in Washington?
No. HB 1337 eliminated owner-occupancy requirements for cities covered by the Growth Management Act.
Sources
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.