Quick answer · Mercer Island ADU laws, 2026
In Mercer Island, King County, you can now build two ADUs on any single-family lot under Washington's HB 1337 (2023) — with no owner-occupancy requirement, no off-street parking mandate on smaller lots, and DADUs allowed up to 1,000 sq ft.
Mercer Island falls under HB 1337's two-ADU-per-lot rule with no owner-occupancy requirement and DADUs up to 1,000 sq ft. Shoreline and tree-retention rules apply on many island lots and are reviewed within the same permit.
Mercer Island is a premium custom ADU market: high land values, established neighborhoods, and I-90 access to both Seattle and Bellevue. Most projects here are architect-detailed cottages or attached suites for family rather than volume rentals.
Permit Office
Mercer Island CPD
Puget Sound ADU Rent (market range)
$1,600–$2,800/mo
NW ADU Builders — serving Mercer Island, WA
NW ADU Builders