Washington State’s recent housing legislation has significantly expanded what can be built on residential lots, including lots previously zoned for single-family use only. Here is what Seattle homeowners and investors need to know about ADUs on multifamily-eligible lots in 2026.
What Changed Under HB 1110
Washington State’s HB 1110 (effective 2023-2024) requires cities with more than 25,000 residents to allow at minimum:
- Duplexes on all residential lots previously zoned single-family
- Fourplexes within a quarter mile of frequent transit
- Six units within a half mile of major transit
Seattle has been implementing these changes through its One Seattle Comprehensive Plan update, which affects how ADUs layer onto newly-permitted density.
ADUs on Duplex Lots
On lots now eligible for duplex development, ADUs may still be permitted as additional units depending on the zone, lot size, and total unit count allowed. Seattle is working through the details of how ADU rules interact with the new density allowances, this is an area where an experienced architect is essential to understand what is actually achievable on your specific parcel.
Planning a project?
Wondering what an ADU on your lot would cost?
Send David the address and a few lines about what you want. The first basic feasibility check is free and usually takes about a day.
Architect since 1996, member of the AIA and NCARB. Three Masonry Excellence Awards for commercial projects; the Carr residence was featured in Above The Lake and Lux magazines.
Impact on ADU Strategy
For investors and homeowners with larger lots, the new density rules create opportunities to build more units than previously allowed. The combination of a duplex plus two ADUs (four units total) on a single lot is becoming viable on more Seattle parcels. This fundamentally changes the economics of ADU development.
Piper Cole Architects stays current on Seattle’s evolving housing code and can assess what is buildable on your specific lot. Contact us for a free feasibility consultation. Call 425-753-6452.
Authoritative sources
Regulatory requirements referenced in this guide are set by the official agencies below. Rules change, always confirm current requirements with the source:
