HB 1110: What Washington's Middle Housing Law Means for Your Lot

For most of the last century, the majority of residential land around Puget Sound allowed exactly one thing: a single detached house. Washington’s middle housing law changed that, and Seattle has now written it permanently into its zoning. If you own a house here, the rules governing your land are meaningfully different than they were three years ago.

What the law actually did

House Bill 1110, adopted by the Washington State Legislature in 2023, requires many cities to allow a broader range of housing types on lots that previously permitted only detached homes. The statute names nine types — duplex, triplex, fourplex, fiveplex, sixplex, townhouses, stacked flats, courtyard apartments and cottage housing — and requires covered cities to permit at least six of them.

That is a structural change, not a tweak. The default unit of Washington residential land is no longer “one house.”

How Seattle implemented it

Seattle moved in two steps. In June 2025 the Council adopted Ordinance 127219, making interim changes to Neighborhood Residential (NR) zoning to satisfy the state requirement. The Council then adopted permanent legislation implementing HB 1110, replacing the interim rules with a broader update to NR zoning.

Other cities across Snohomish and King County are working through their own versions on their own timelines, which is exactly why rules of thumb are dangerous here — the answer depends on your city and your parcel.

What it means in practice for a homeowner

• Your lot may hold more than you think. A parcel that was capped at one house may now permit a duplex, a townhouse pair, or cottage housing — in addition to what ADU rules already allow.
• Zoning permission is not buildability. Setbacks, height, lot coverage, tree retention, parking, stormwater and utility capacity all still apply. A triplex that pencils on paper can fail on a narrow lot with a big protected tree.
• Cost per unit falls as units rise. Shared foundations, walls and utility runs make a duplex cheaper per door than two detached houses. It is also more complex to design, permit and finance than a remodel.
• The land value question changed. If your lot now supports more dwellings, that potential is part of what your property is worth — whether or not you ever build it.

Middle housing versus the ADU path

These are different tools and they stack. An ADU or DADU is accessory to your existing house — you keep living in the main home and add a secondary unit, which is why it is the most common path for families housing a parent or adding rental income. Middle housing adds primary dwellings, which usually means replacing or substantially reconfiguring what is there.

For most homeowners we talk to, the ADU route remains the practical one: lower cost, less disruption, and no need to move out. Middle housing matters most on larger lots, corner lots, and for owners thinking about the property as a long-term asset.

Getting a real answer for your parcel

Every one of these questions ends the same way: what does your lot allow, and what would it cost to build there? That is a site visit, a look at your city’s current code, and honest numbers — which is what the free estimate is for. We build additions, ADUs and DADUs across Snohomish and King County, and we will tell you plainly when the ambitious version does not pencil.

Find out what your lot allows →