Architecture, Multifamily Residential
Navigating the Challenges of Middle Housing
Washington State’s Middle Housing Bill creates a lot of opportunity, but it also presents some challenges for self-developers. Here, we explore the expanded choices HB1110 offers, and how having a multidisciplinary design team can help navigate the real-life outcomes of this new legislation.
December 4, 2025
In 2023, Washington’s Legislature passed House Bill 1110, known as HB 1110. The bill sets a new statewide baseline that compels many cities to allow Middle housing even in areas that were once zoned only for single-family homes. (Middle Housing is defined as duplexes, triplexes, fourplexes, townhomes, courtyard apartments, and more.)
Still, the law didn’t force an immediate, uniform change everywhere overnight. Rather, it set a schedule tied to each city’s individual, comprehensive plan update cycle. That means cities have and will adopt middle housing regulations at different times, depending on when their next planned update.
A statewide shift and the timeline of change.
Each jurisdiction has a different timeline. In the central Puget Sound region (King, Pierce, Snohomish, & Kitsap Counties) for example, many jurisdictions had a comprehensive plan deadline of December 31, 2024. That makes their HB 1110 compliance deadline June 30, 2025.
Some cities are already actively updating their municipal codes (or planning to) to integrate middle housing standards. For example, Seattle is rolling its changes into its One Seattle Plan, with intermediate and full zoning changes were first scheduled by June 2025, followed by full adoption in fall of 2025.
The Washington State Department of Commerce helped by publishing model middle housing ordinances and a user guide (delivered January 2024) to give cities a template and reduce the burden of drafting from scratch.
Because of this staggered approach, you’ll see some cities already adopting middle housing rules, while others further from their plan update cycle may not implement changes until 2026 or even later.
What this means for developers.
If you’re a homeowner or small-scale builder and/or developer, this is a pivotal time. Some cities in your area may already allow middle housing, while others may not yet have adopted the updated rules, which can affect timing, risk, and opportunity.
Under HB 1110, new options open. Where you once could build just one detached home, you might now be able to put in a duplex, a triplex, or even a fourplex outright, depending on your lot and your city’s implementation of the bill.
For first-time self-developers, this shift matters.
- Zoning clarity is improving, so you’ll increasingly see what can be permitted.
- By-right permissions (permitting that is automatically approved because it meets zoning and code requirements) reduce risk with fewer variances or appeals needed.
- Rental income becomes a tool you can use to help shoulder mortgage or carrying costs.
- Greater flexibility for development means you can better plan for future changes (selling, renting, multigenerational living) as your site evolves.
But again: the zoning map is just the starting point. The real test is figuring out how much you can realistically build given your site’s constraints.
Where unknowns can sneak in.
Even for someone who has done their homework, hidden constraints can be the difference between a project that works on paper and one that actually gets built. This is where bringing in an integrated, multidisciplinary design team becomes deeply valuable to your project.
Utilities and Site Capacity
More units equal more demand on water, sewer, and electrical. You might need trenching, new lines, or transformer upgrades that could yield 8-12 months added to the project schedule before building construction can fully begin in earnest.
Right-of-Way and Frontage
Sidewalks, street trees, and stormwater improvements can cause rethinking driveway or unit layout in ways the zoning texts won’t flag.
Tree Protection & Landscaping: Mature trees or tree canopy rules might restrict your building footprint more than setbacks do.
Environmental Conditions: Wetlands, steep slopes, or shoreline buffers can eat away at buildable area unless they are spotted early.
Parking & Circulation: Even when minimums are lowered, maneuvering cars, guest spaces, and green space must fit on the lot.
Permit Sequencing: “By-right” doesn’t always mean “fast.” Different departments such as public works, utilities, environmental, and planning may hold up approvals unless coordination is tight.
Neighborhood Fit: Cities and town can, at times, impose design standards or compatibility checks. Good design strategies can help your project feel like it belongs rather than feeling jarringly new.
Construction costs right now.
One of the first questions we hear is “What will it cost me to build?” Here’s what we’re seeing in Washington currently, always with the caveat that local site conditions, finishes, and timing can push numbers up or down.
- Relatively simple duplexes or triplexes: $375 – $425 per square foot hard costs.
- Fourplexes and more complex middle housing: $375 – $450+ per square foot depending on structural complexity, shared walls, etc.
- Site and infrastructure costs: Especially for frontage, utility extensions, stormwater systems, these can easily add $50,000 to $150,000+ depending on how far services need to be run or upgraded.
- Soft costs (design, permitting, fees): Typically, 15%–25% of the total project cost but can vary widely.
The balance between construction cost and rent or sales value is where the return on investment (ROI) lives. A strong design team helps you test this balance early on so you don’t overbuild or under-optimize.
Moving from vision to reality.
Washington’s middle housing laws aren’t just policy ideas, they’re already reshaping neighborhoods across the state. Some cities have adopted new codes, while others are in the middle of updates and more will follow in the coming years.
For self-developers, this means timing matters: the opportunity is real, but the window and requirements may vary depending on where your property is located.
Turning that opportunity into a successful project takes more than knowing what’s technically allowed. The real challenge is understanding what’s actually feasible once site conditions, infrastructure, and market dynamics come into play.
That’s where working with an experienced, integrated design team like Board & Vellum becomes invaluable. The right partners will give you tools and insights to anticipate and resolve potential issues before they become expensive setbacks, shape design choices that respond to both site realities and market expectations, streamline permitting by reducing rework and delays, balance construction costs against long-term return on investment, and create housing that serves your goals while adding lasting value to your community.
By approaching middle housing with both vision and practicality, you can move forward confidently — maximizing the potential of your property while navigating complexity with fewer surprises.
