Seattle Zoning & ADU Law Guide: Washington State Housing Bills, 2023–2026
HB 1110 — The Middle Housing Act (E2SHB 1110)
Washington's landmark middle housing bill required cities to end exclusive single-family zoning in most residential areas, allowing duplexes, triplexes, and other middle housing types depending on lot size, city population, and proximity to transit. Seattle implemented this through the One Seattle Comprehensive Plan, converting most single-family (former RSL) zones to new Neighborhood Residential and LR-zone designations.
HB 1337 — Statewide ADU Legalization (EHB 1337)
This is the single most important bill for anyone considering a DADU. It requires cities and counties to allow at least two ADUs per lot (one attached, one detached), ends owner-occupancy requirements, caps parking mandates, and sets floors on minimum ADU size and height so cities can no longer regulate them out of practical existence. It passed April 2023 and took effect statewide by mid-2025 as jurisdictions updated their codes.
SB 5290 — Local Project Review Act Reforms
A permitting-process bill rather than a zoning bill. It streamlines how cities review project permit applications, clarifies timelines, and reduces the ability of local governments to indefinitely delay a determination of completeness on a permit application. It matters most for developers managing project timelines rather than for the DADU decision itself.
HB 1998 — Co-Living / Single Room Occupancy Legalization
Re-legalized single-room-occupancy and co-living buildings statewide, reversing restrictions that had effectively ended this housing type in cities like Seattle since around 2016. More relevant to multifamily and investor buyers than single-lot DADU projects.
SB 6015 — Statewide Parking Flexibility Standards
Washington's first stand-alone parking reform bill. It gives builders more flexibility in meeting local parking mandates, which directly benefits ADU and middle housing projects where a required parking space can be the difference between a project penciling out and not.
HB 2071 — Middle Housing Building Code Update
A technical follow-up to HB 1110, directing the state to adapt residential building code so it can apply to middle housing projects of up to six units, reducing the cost of building smaller multi-unit structures under the same code track as a single-family home.
HB 1217 — Rent Stabilization
Caps annual rent increases for tenants in buildings more than 12 years old. This is a tenant-protection bill rather than a construction or zoning bill, but it's worth knowing for any buyer evaluating an existing rental property, including one with an ADU already generating income.
SB 5184 — Parking Mandate Caps
Caps the number of off-street parking spaces local governments can require alongside new construction, building on SB 6015's flexibility standards from the year before with a firmer, statewide ceiling.
HB 1491 — Transit-Oriented Development (TOD)
Legalizes larger apartment buildings near major transit stops, with affordability requirements attached. Mostly relevant to larger multifamily development near light rail and bus rapid transit stations rather than single-lot DADU projects.
HB 1353 — ADU Self-Certification (SHB 1353)
Creates a voluntary program letting a registered architect self-certify that a detached ADU's building plans comply with code, rather than waiting for a full city review. The city still checks the application for completeness, but a self-certified, complete application can be issued without the standard plan review queue. Effective July 27, 2025. This is the bill most likely to shorten an actual DADU permitting timeline
HB 1757 — Commercial-to-Residential Conversion Expansion
Builds on a 2023 law that made it easier to convert existing commercial and mixed-use buildings to residential use, expanding it in 2025 to include existing buildings located in residential zones as well. Cities have until June 30, 2026 to update their regulations to conform.
HB 1345 and HB 2269 — County ADU and Middle Housing Expansion
These companion bills give counties new authority to allow detached ADUs outside urban growth areas and middle housing in certain rural areas served by large on-site sewage systems, provided conditions like water adequacy and Department of Health coordination on septic permitting are met. This mostly affects unincorporated Snohomish and King County parcels rather than properties inside Seattle city limits.
SB 6026 — Residential Use in Commercial and Mixed-Use Zones (E2SSB 6026):
Prohibits cities and counties with populations over 30,000 from excluding residential uses in areas zoned commercial or mixed-use, and bars them from requiring ground-floor retail as a condition of permitting residential or mixed-use development in those zones, with limited exceptions such as designated transit-oriented development areas. Signed into law and effective June 2026. This one applies directly to Seattle and opens up small commercial parcels, including some that might otherwise have been considered only for ground-floor-retail redevelopment, to full residential conversion.
Example 1: A Standard Lot in Ballard (Seattle, King County)
A buyer finds a single-family home on a typical in-city lot and wants to add a detached ADU in the backyard, both to house a family member now and to rent out later. Before assuming this is possible, the buyer or their agent checks the Seattle SDCI zoning map to confirm the parcel's zone designation and the SDCI Property Information Map for any critical areas, easements, or historic overlays that could restrict a detached structure.
Once zoning is confirmed, HB 1337 is what actually makes the project possible at all: it guarantees the right to build a second ADU on the lot (in addition to any existing attached unit), removes any requirement that the owner live on site, and sets a floor under how small the city can force the unit to be. SB 6015 and SB 5184 mean the city cannot require additional off-street parking as a condition of approval, which matters directly on a narrow city lot where a parking pad would eat into the buildable footprint. Finally, if the buyer hires a registered architect for the plans, HB 1353's self-certification option can let a complete application skip the standard plan review queue entirely, shortening the permit timeline that would otherwise be one of the slowest parts of the project.
Example 2: A Larger Rural Parcel in Unincorporated Snohomish County
A buyer is looking at a property outside a city's urban growth area, on a larger lot served by a septic system rather than sewer, and wants to add a detached ADU for long-term rental income. Because the parcel sits outside a UGA, HB 1337's baseline guarantee doesn't automatically apply the same way it does inside city limits. Here, the relevant bills are HB 1345 and HB 2269, which authorize (but do not require) the county to allow a detached ADU on rural parcels like this one, subject to conditions such as water adequacy and coordination with the Washington Department of Health on the large on-site sewage system permit.
The practical takeaway for this buyer is that the county's participation is optional under these bills, so the very first step is confirming with Snohomish County's planning department whether the county has actually adopted rules allowing this, and what the septic and water requirements are, before assuming the same ADU rights that would apply on an in-city lot.
