Does City of Seattle allow detached ADUs (DADUs)?

Yes. Washington's HB 1337 (RCW 36.70A.681) requires City of Seattle to allow at least two accessory dwelling units on a lot in a single-family zone within the urban growth area, attached or detached in any combination, except on lots with critical areas or their buffers.

What is the maximum ADU size in City of Seattle?

State law bars a city from setting a maximum ADU size below 1,000 sq ft of gross floor area (RCW 36.70A.681(1)(f)). City of Seattle's own maximum may be higher — the per-parcel page shows it with the city's code citation.

Does Seattle have pre-approved DADU plans?

Yes. Seattle SDCI maintains a library of pre-approved DADU plans. SDCI says: "In most cases, you can get your permit in just 2-6 weeks."

ADU Permits in City of Seattle

Rules for detached (DADU) and attached (AADU) accessory dwelling units inCity of Seattle, King County.

Free lookup. We’ll resolve your zoning, setbacks, and which ADU types are allowed on your specific parcel.

Choose an ADU type for your parcel

State law (RCW 36.70A.681) requires City of Seattle to allow two ADUs on a lot in a single-family zone — one attached and one detached, two attached, or two detached — except on lots with critical areas or their buffers. The per-parcel canvas checks your specific zoning.

DADU — Detached

Backyard cottage with its own foundation and roof. Most flexibility on layout.

See DADU rules in City of Seattle

AADU — Attached

Basement, garage, or addition conversion inside the primary residence footprint.

See AADU rules in City of Seattle

Standard Plan (Expedited)

Seattle’s pre-approved DADU plans. SDCI: “In most cases, you can get your permit in just 2-6 weeks.”

See standard-plan options

City of Seattle permit-process facts

Permit department

Submit through the City of Seattle permit portal.

Permit portal

ADU permit fees

Fee schedule not yet aggregated. Check the City of Seattle fee schedule directly.

Fee schedule

Review timeline

Timeline not yet aggregated — see the per-parcel page for the live estimate.

Standard-plan DADUs cut this in half.

HB 1337 conformance

City of Seattle permits at least two ADUs per single-family lot, with no owner-occupancy requirement and parking limits that respect the HB 1337 ceilings.

Building code: 2021 IBC w/ WA amendments · Residential: 2021 IRC w/ WA amendments · Energy (WSEC): 2021 WSEC

Rules above are aggregated from City of Seattle’s permit-rules table; per-parcel facts (your specific setbacks, easements, ECA buffers) are pulled live on the parcel page. Primary source: https://www.seattle.gov/sdci/permits/common-projects/accessory-dwelling-units. Last verified 2026-09-24.