HB 1110 and Middle Housing in King County What It Means for Zoning Infill Lots and ADUs
For decades, much of King County’s residential land has followed a simple rule: one house on one lot. Washington’s middle housing law changes that pattern. HB 1110 does not end local zoning, but it does require many cities to allow more homes in neighborhoods that were once limited mostly to detached single-family houses.
That shift matters in places like Seattle, Bellevue, Kirkland, Redmond, Renton, Shoreline, Bothell, Kent, and other King County cities where land is scarce, housing demand is high, and small infill projects can shape the next generation of neighborhoods.
The law opens the door for duplexes, triplexes, fourplexes, sixplexes in some locations, cottage housing, townhomes, stacked flats, and courtyard apartments. It also works alongside recent accessory dwelling unit laws that make backyard cottages and basement apartments easier to permit in urban areas.
This article is informational only and is not legal, planning, or design advice. Local codes still matter, and every lot needs its own review.
What HB 1110 changes in residential zoning
HB 1110 is Washington’s middle housing law. It requires certain cities to allow a wider range of housing types in residential zones that historically allowed only one detached home per lot.
The key idea is simple. A city cannot keep large areas of residential land reserved only for single detached houses if the law applies to that city. Instead, local zoning must allow a minimum number of housing units per lot, based on city size and location.
For larger cities, including many in King County, the law generally requires more housing capacity per lot than smaller cities. It also increases the required capacity near major transit stops and when projects include affordable housing. Cities still write their own development standards, but those standards cannot make the required housing types impossible in practice.
That means local governments can still regulate things such as:
Building height
Lot coverage
Tree retention
Setbacks
Stormwater
Design standards
Parking, within limits
Utility connections
Permit review procedures
The big change is that zoning rules must make room for middle housing. The phrase refers to home types that are between a detached single-family house and a large apartment building. These buildings often fit on existing residential lots and can look similar in scale to houses already on the block.
Common middle housing types include:
Duplexes
Triplexes
Fourplexes
Fiveplexes and sixplexes where allowed
Townhouses
Cottage courts
Courtyard apartments
Stacked flats
This is the core impact of HB 1110 and Middle Housing in King County What It Means for Zoning Infill Lots and ADUs. The law moves many neighborhoods from “one home by default” to “several homes may be allowed, if the site can meet the rules.”
Why King County feels the change more than many places
King County sits at the center of Washington’s housing pressure. Job growth, limited buildable land, high construction costs, and strong demand have pushed housing prices up across the region. At the same time, many neighborhoods have allowed only low-density housing even when they sit near schools, parks, transit, jobs, and utilities.
HB 1110 does not solve housing affordability by itself. A fourplex will not suddenly make a high-cost market cheap. Land, labor, financing, utility upgrades, and permitting all still affect what gets built.
But the law changes the supply options. It lets smaller builders, homeowners, and community-scale developers consider projects that were often blocked before zoning review even began.
In King County, that could mean:
A corner lot in Shoreline becomes a small cottage cluster.
A larger lot in Bellevue supports a duplex or fourplex instead of one replacement house.
A property near light rail in Seattle or Redmond allows more homes than a similar lot farther from transit.
An aging house in Renton becomes a townhome project.
A backyard cottage becomes part of a long-term family housing plan.
The impact will not look the same in every city. Seattle has its own layers of zoning reform. Eastside cities have different lot patterns and market pressures. South King County cities often have larger lots in some areas but different infrastructure constraints. North King County cities may see strong interest around light rail, bus rapid transit, and town centers.
Local implementation will decide many details. State law sets the floor, while cities update their codes to meet it.
What it means for single-family zoning
The phrase “single-family zoning” can be confusing after HB 1110. In many cities, the name of a zone may still look familiar. A city might keep residential zones with similar labels, maps, and neighborhood boundaries. What changes is what those zones must allow.
A lot that once allowed one detached house may now allow more than one dwelling unit. That does not mean every lot will be redeveloped. Most homes will remain as they are for years. Many owners will have no interest in building more units. Some lots will be too small, steep, constrained by trees, or expensive to redevelop in the near term.
The practical shift is in development rights.
Before HB 1110, an owner might have had only a few basic options:
Keep the house.
Remodel the house.
Tear it down and build one larger house.
Add an ADU if local rules allowed it.
After a city updates its code, that same owner may have more choices:
Convert a house into multiple units.
Build a duplex or triplex.
Add detached units through cottage housing.
Combine an ADU strategy with other allowed housing, where local code permits.
Sell to a builder who can create several homes instead of one expensive replacement house.
This could reduce the pressure to replace modest houses with very large single homes. When zoning allows only one primary home, the market often favors the biggest home that a high-end buyer can afford. When zoning allows multiple homes, the economics can support smaller units on the same land.
That tradeoff is not automatic. Construction costs are still high, and new homes may still be expensive. Yet middle housing creates more price points than a single luxury teardown.
How infill lots could be developed under the new rules
Infill lots are properties within already developed areas. They may be vacant lots, underused lots, oversized lots, corner lots, or sites with older homes that need major repairs. HB 1110 makes these lots more flexible.
The best infill candidates often share a few traits:
Reasonable access from a street or alley
Enough lot width or depth for multiple entries
Manageable slope
Space for stormwater facilities
Utility capacity or a clear path to upgrades
Few critical area constraints
A layout that protects significant trees where required
A flat corner lot may work well for a duplex or fourplex because separate entrances can face different streets. A deep lot with alley access may fit townhomes or cottages. A larger lot near transit may support more units if local code allows the higher capacity.
But zoning permission is only one piece of feasibility. Builders still have to solve practical site questions.
Parking can shape the design. State law limits some local parking requirements, especially near frequent transit, but parking still affects market demand and site layout. A project with no required parking may still need bike storage, waste pickup access, and safe pedestrian connections.
Stormwater can take serious space. King County’s wet climate means redevelopment often triggers drainage review. Rain gardens, vaults, dispersion trenches, or other systems can affect where buildings can sit.
Trees and slopes matter. Many King County neighborhoods have mature tree canopy and steep terrain. Tree protection rules, critical area buffers, and grading limits can reduce the buildable area.
Utilities may need upgrades. Water, sewer, and power connections can add cost, especially when a project adds several kitchens, bathrooms, and meters.
The result is that two lots with the same zoning may have very different outcomes. One may support four homes with a simple permit path. Another may struggle to fit two homes because of access, grade, or utility issues.
How ADUs fit into the middle housing picture
Accessory dwelling units, or ADUs, are not new in King County. Many cities already allow attached ADUs, such as basement apartments, and detached ADUs, often called backyard cottages or DADUs. What changed recently is the state’s stronger push to make ADUs easier to build in urban growth areas.
ADUs sit next to HB 1110 rather than replacing it. Middle housing laws increase the number and types of primary homes allowed on residential lots. ADU laws focus on smaller secondary homes on lots that have a primary residence.
In practice, the two reforms overlap. A homeowner may ask whether to build a backyard cottage, convert a basement, or pursue a duplex or fourplex. The answer depends on goals, budget, lot conditions, and local code.
ADUs may make sense when the goal is:
Creating space for family members
Adding a small rental home
Keeping the existing main house
Building in phases
Avoiding a larger redevelopment project
Middle housing may make more sense when the goal is:
Creating several separately owned or rented homes
Replacing a failing structure
Using a larger lot more fully
Building near transit where more units are allowed
Creating a project that supports construction financing
HB 1110 does not mean every ADU project should become a fourplex. ADUs can be simpler and less disruptive. They can also help households age in place, support multigenerational living, or add income while keeping neighborhood scale.
At the same time, new middle housing rules may give owners more options than an ADU alone. A detached backyard cottage might work well on one lot. On another, a pair of cottages or a small courtyard project could create better homes with better light, access, and open space.
The important step is to compare options before spending heavily on design. A feasibility review should look at both ADU rules and middle housing rules, not one or the other.
What cities still control
HB 1110 sets statewide requirements, but it does not create one identical zoning code for every city. King County cities still play a major role in how middle housing appears on the ground.
Local code updates may address:
Which middle housing types are allowed in each residential zone
Maximum building size and height
How many units are allowed on different lots
Design standards for entries, windows, and facades
Tree retention and replacement
Open space requirements
Parking rules
Short plat and unit lot subdivision standards
Permit review timelines
Infrastructure and impact fees
This local control matters. A city can make middle housing attractive and workable, or it can make projects difficult through complicated standards. The state law limits local restrictions that conflict with required housing capacity, but gray areas will still exist.
For example, a city may allow four units on paper but require site conditions that make four units hard to build on many lots. Over time, state guidance, local code updates, permit decisions, and possibly legal challenges will clarify what works.
For property owners and builders, the safest approach is to check the current city code, not just the state law. Timing matters too. Some cities may update their zoning before others, and interim rules may apply during the transition.
What to check before planning a project
Middle housing creates opportunity, but early due diligence matters more than ever. A promising zoning change can lose value if the lot cannot handle access, drainage, or utility needs.
Before planning a duplex, fourplex, cottage court, or ADU, review these basics.
City and zoning jurisdiction
Confirm whether the property is inside a city or in unincorporated King County. HB 1110 applies to cities that meet its criteria. County rules may differ for unincorporated areas, especially outside urban growth areas.
Lot size and dimensions
Unit count is only useful if the site can physically fit the homes. Lot width, depth, setbacks, and easements can decide the project.
Transit proximity
Some higher unit allowances apply near major transit. The exact definition and mapping matter. A lot that seems close to transit may or may not qualify under the adopted local rules.
Critical areas
Wetlands, streams, steep slopes, erosion hazard areas, and buffers can reduce buildable space. King County has many sites where environmental review is central to feasibility.
Tree rules
Tree canopy is a major issue in many cities. A project may need to preserve certain trees or plant replacements.
Utility capacity
More homes mean more demand on water, sewer, electricity, and sometimes frontage improvements. Early utility review can prevent costly surprises.
Ownership plan
Rental units, condominiums, fee-simple townhomes, and unit lot subdivisions each raise different design, financing, and legal questions.
Permit timing
State law creates the framework, but local adoption schedules and permit backlogs can affect when a project can move forward.
The likely neighborhood impact
The visible impact of HB 1110 will be gradual. Zoning reform does not rebuild a county overnight. Most lots will not change soon, and many will not change at all.
The first wave will likely appear where incentives line up:
Lots near high-capacity transit
Aging homes on valuable land
Larger parcels with simple access
Areas where local codes are clear
Places where builders already understand small multifamily construction
Some blocks may see very little change. Others, especially near transit and town centers, may see more duplexes, cottages, and small apartment-style buildings.
The policy tradeoff is real. More homes may mean more construction, more street activity, and changes to neighborhood character. It may also mean more housing choices near jobs, schools, parks, and transit. For a region with severe housing pressure, keeping every low-density block frozen carries its own cost.
The best outcomes will come from careful local implementation. Good middle housing should be livable, durable, and well connected to the street. It should make room for trees, manage stormwater, and avoid designs that maximize unit count at the expense of basic quality.
The takeaway for King County
HB 1110 marks a major turn in Washington housing policy. It does not ban single-family homes. It does not force homeowners to redevelop their property. It does change what many residential lots are allowed to become.
For King County, the practical message is clear. Single-family zoning is no longer the fixed boundary it once was. Infill lots may support more homes. ADUs remain useful, but they are now part of a wider set of housing choices. Cities still control many details, so local code updates will decide how much of the law’s potential becomes real housing.
Anyone thinking about a project should start with the site, not just the headline. Check the city’s adopted rules, measure the lot, review constraints, and compare ADU and middle housing paths side by side. The new law creates more doors, but each property still needs the right key.
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