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ADU Permits in City of Mercer Island

Rules for detached (DADU) and attached (AADU) accessory dwelling units inCity of Mercer Island, 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 Mercer Island 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 Mercer Island

AADU — Attached

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

See AADU rules in City of Mercer Island

Standard Plan

Kolmo has not checked whether City of Mercer Island offers pre-approved DADU plans; ask the permit center.

City of Mercer Island permit-process facts

Permit department

Submit through the City of Mercer Island permit portal.

Permit portal

ADU permit fees

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

Fee schedule

Review timeline

Typical: 30–90 business days

HB 1337 conformance

City of Mercer Island 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 Mercer Island’s permit-rules table; per-parcel facts (your specific setbacks, easements, ECA buffers) are pulled live on the parcel page. Primary source: https://library.municode.com/wa/mercer_island/codes/city_code?nodeId=CICOOR_TIT19UNLADECO_CH19.02RE_19.02.030ACDWUN. Last verified 2026-05-16.