Do I have to allow duplexes on single-family lots in Washington cities?

In most cities covered by the state's 2023 Middle Housing law (HB 1110), yes — lots zoned for single-family detached housing must also allow duplexes, and larger or transit-served cities must allow up to fourplexes or sixplexes. Smaller or less-urbanized cities may have longer compliance timelines or narrower thresholds.

What is the Growth Management Act and how does it affect zoning?

The Growth Management Act (RCW 36.70A) requires Washington's most populous and fast-growing counties, and cities within them, to adopt comprehensive plans that designate urban growth areas and keep development regulations consistent with those plans. It sets the planning framework within which local zoning ordinances must operate.

How do I find the zoning for a parcel in Washington State?

Zoning maps and codes are published by the county or city where the parcel sits — there is no single statewide zoning lookup. Most counties and larger cities maintain online GIS parcel viewers that show zoning designation alongside assessed value and ownership.

Can I build an ADU in Washington?

Yes. The 2023 statewide ADU law (HB 1337) requires most cities and counties to allow at least two accessory dwelling units per lot with a detached house, removes owner-occupancy requirements, and limits how much off-street parking a jurisdiction can mandate.

Washington · state zoning reference · Parcel data · lookup only

Washington zoning: how it works and what you can build

Washington pairs a mandatory statewide growth-management framework with some of the most aggressive state-level housing preemption in the country, requiring most cities to allow duplexes through sixplexes on lots once zoned exclusively for single detached houses.

How zoning works in Washington

Washington's land-use system runs on two tracks. The Growth Management Act (GMA) requires the state's most populous and fastest-growing counties, and the cities within them, to adopt comprehensive plans and development regulations that direct growth inside designated urban growth areas and protect resource and rural lands outside them. Zoning authority itself flows to municipalities primarily through Title 35A RCW (the Optional Municipal Code), which lets "code cities" exercise broad, home-rule-like land-use powers, alongside older general-law city and county planning statutes for jurisdictions that haven't adopted code-city status.

In recent years the state has moved to directly override restrictive local zoning rather than just require planning. House Bill 1110 (2023) mandates that cities meeting population and transit-access thresholds allow middle housing — duplexes, triplexes, fourplexes, and in some cases up to sixplexes — on lots zoned for single-family detached homes, with the required density scaling by city size and proximity to transit. A companion measure, House Bill 1337 (2023), requires most cities and counties to allow at least two accessory dwelling units per detached-house lot, removing owner-occupancy requirements and capping parking mandates. These laws phase in on a schedule tied to each jurisdiction's comprehensive plan update cycle under the GMA.

For developers, the practical takeaway is that Washington zoning is increasingly a floor set by state law rather than a ceiling set by each city. A parcel's local zoning designation still controls setbacks, height, and design review, but the underlying question of whether multi-unit housing is allowed at all is now substantially answered at the state level for most urbanized jurisdictions.

Who holds the zoning power

Land-use authority in Washington is delegated by statute, not assumed — these are the acts a zoning ordinance in Washington has to trace back to.

Growth Management Act Revised Code of Washington, Chapter 36.70A: Requires comprehensive planning, urban growth boundaries, and consistent development regulations in the state's most populous and fastest-growing counties and cities.
Optional Municipal Code Revised Code of Washington, Title 35A: Grants 'code cities' broad, largely self-executing authority to adopt zoning and land-use regulations.
Middle Housing law 2023 House Bill 1110 (amending RCW 36.70A): Preempts single-family-only zoning in covered cities, requiring duplexes through sixplexes to be allowed depending on city size and transit proximity.

Major cities and their codes

The enabling act sets the ceiling; the municipal code is what a project is actually reviewed against. The 7 largest Washington jurisdictions and the codes they administer:

Washington — municipal zoning codes
City	Zoning code
Seattle	Seattle Municipal Code, Title 23 (Land Use Code) Uses a Mandatory Housing Affordability program that requires affordable-unit contributions or payments in areas that received zoning capacity increases.
Spokane	Spokane Zoning Ordinance Eastern Washington's largest city; has been an early adopter of middle-housing-friendly zoning ahead of some state deadlines.
Tacoma	Tacoma Municipal Code, Land Use Regulatory Code Home In Tacoma initiative rezoned much of the city for middle housing in advance of the statewide HB 1110 mandate.
Vancouver	Vancouver Zoning Ordinance Fast-growing Portland-metro suburb where zoning updates are closely tied to cross-river transportation and growth-boundary planning.
Bellevue	Bellevue Land Use Code Eastside tech-corridor city with an intensively planned downtown core zoned for high-rise mixed use alongside large single-family neighborhoods.
Everett	Everett Municipal Code Puget Sound Gateway city balancing port/industrial zoning with growing multifamily development near light rail.
Kent	Kent Zoning Code South King County logistics and distribution hub with substantial warehouse and industrial-zoned land.

Recent reforms

State-level changes override or preempt what a local ordinance says — these are the Washington reforms that move the numbers on a real site.

2023 Middle Housing law (HB 1110) Requires most cities to allow duplexes through sixplexes on lots previously zoned single-family, with density scaled by city size and transit access.
2023 Statewide ADU law (HB 1337) Requires cities and counties to allow at least two accessory dwelling units per detached-house lot and removes owner-occupancy requirements.

Parcel & GIS data

There is no single unified statewide parcel portal; the Washington State Geospatial Portal aggregates state agency GIS layers while parcel-level zoning and assessment data remain published separately by each county assessor.

Real parcel geometry is already loaded for Everett — see the zoning-by-city section above for a live district-and-parcel lookup there. That is not the same as a cited feasibility report: Chicago is still the only jurisdiction with bulk standards, § citations and a generated massing scheme.

Zoning by city

The statute sets who may zone; the city's own code sets what you can build. One guide per Washington city below, explaining the district codes that appear on its zoning map — 102 district codes across Washington so far.

Washington zoning — common questions

Do I have to allow duplexes on single-family lots in Washington cities?: In most cities covered by the state's 2023 Middle Housing law (HB 1110), yes — lots zoned for single-family detached housing must also allow duplexes, and larger or transit-served cities must allow up to fourplexes or sixplexes. Smaller or less-urbanized cities may have longer compliance timelines or narrower thresholds.
What is the Growth Management Act and how does it affect zoning?: The Growth Management Act (RCW 36.70A) requires Washington's most populous and fast-growing counties, and cities within them, to adopt comprehensive plans that designate urban growth areas and keep development regulations consistent with those plans. It sets the planning framework within which local zoning ordinances must operate.
How do I find the zoning for a parcel in Washington State?: Zoning maps and codes are published by the county or city where the parcel sits — there is no single statewide zoning lookup. Most counties and larger cities maintain online GIS parcel viewers that show zoning designation alongside assessed value and ownership.
Can I build an ADU in Washington?: Yes. The 2023 statewide ADU law (HB 1337) requires most cities and counties to allow at least two accessory dwelling units per lot with a detached house, removes owner-occupancy requirements, and limits how much off-street parking a jurisdiction can mandate.

Working in Washington? Get on the list.

The parcel layer lands in the order demand does. Illinois went first because that's where the deals were; the next state is decided by who tells us what they're trying to underwrite. Tell us where you're building and what you wanted to run — you'll hear from us when Washington opens, and sooner if we need a test parcel.

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Want to see the finished shape of this work? Illinois is live at parcel level — Illinois zoning and the 53-district Chicago reference.

Washington zoning summaries on this page are provided for information, not legal advice. Confirm every project against the current adopted code for the jurisdiction and the statute as amended.