Seattle, WA
Seattle ADU Permit: do you likely need a permit?
Seattle ADUs are not legal unless established through a permit process; attached, detached, and existing-unit legalization all need construction permits.
What the official sources say
Seattle SDCI says an ADU is a separate living space within a house or on the same property and is not legal unless established through a permit process. Adding an ADU within an existing house, building a detached unit, or legalizing an existing unit requires a construction permit. SDCI also points applicants to residential, building, mechanical, electrical, energy, land-use, environmentally critical area, and shoreline standards.
Likely permit path
- Construction addition/alteration permit for attached, detached, or existing-unit legalization.
- Electrical service changes or new services through Seattle City Light where applicable.
- Capacity-charge acknowledgement and checks against land use, shoreline, environmentally critical area, and residential code standards.
Watch-outs
- Tiny houses on wheels are treated like camper trailers and cannot be lived in on Seattle lots as ADUs.
- Pre-approved DADU plans may create a faster permitting path, but property feasibility still matters.
- Sewer treatment capacity charges are reported to King County.
Documents to gather
- Address, zone, ADU type, existing/proposed floor plans, and site plan.
- Utility/electrical service details, shoreline/ECA notes, and intended occupancy.
- If using a standard plan, the selected plan and property-fit information.
Questions to ask before applying
- Is this an ADU, JADU, attached ADU, detached ADU, garage conversion, or non-rental habitable-space conversion?
- What zoning district, overlays, coastal/historic/flood/fire conditions, HOA limits, or deed restrictions apply to the property?
- Will the unit include independent living, sleeping, cooking, and sanitation facilities?
- What building, zoning, trade, utility, addressing, parking, inspection, and certificate-of-occupancy steps apply before occupancy or rental use?
Official sources
Want this checked against your exact address and scope?
We are doing the first 25 beta reports free for Austin, Denver, and Raleigh residential projects. Send the address, scope, dimensions, and whether electrical, plumbing, mechanical, or structural work is included.
PermitCheckup provides informational permit-readiness guidance based on public sources. It is not legal, architectural, engineering, or contractor advice. Always confirm your project with the authority having jurisdiction before beginning work.