What Washington’s HB 1110 Means for Homeowners and Developers

Unlock the potential of your property under Washington’s new middle housing law

Introduction

Washington State is undergoing a major shift in how housing is planned and built. With the passage of House Bill 1110 (HB 1110) in 2023, many cities are now required to allow middle housing—such as duplexes, triplexes, and fourplexes—on land previously zoned for single-family homes. This change is opening the door to new development opportunities for homeowners, builders, and developers across the state. At Harmony Space Architecture, we help our clients take full advantage of these evolving zoning laws through thoughtful design, permitting, and development strategies. Here’s what you need to know about HB 1110 and how it could impact your property.

What Is HB 1110?

HB 1110 is a statewide housing reform law aimed at increasing housing supply and affordability. Signed into law in 2023, it requires many Washington cities to allow more housing types in areas that were previously limited to single-family homes.

The Goal:

To promote “middle housing”—which includes:

• Duplexes

• Triplexes

• Fourplexes

• Sixplexes (in some cases)

• Townhouses

• Cottage housing

• ADUs and DADUs (Accessory Dwelling Units)

Where Does HB 1110 Apply?

The law affects cities with populations of 25,000 or more, and smaller cities near large urban areas. These cities must update their zoning codes to allow middle housing on most residential lots by mid-2025.

That includes cities like:

• Seattle

• Bellevue

• Redmond

• Kirkland

• Renton

• Lynnwood…and many more.

At Harmony Space, we’ve already begun designing and submitting middle housing projects under these new rules in Redmond and surrounding cities.

What Can You Build Now?

Under HB 1110, most residential lots in qualifying cities can now legally accommodate:

• Up to 4 units per lot statewide

• Up to 6 units if the lot is near a major transit stop (bus or light rail)

• Plus ADU/DADU in many cases

Example: A typical 6,000 sq. ft. lot previously limited to one single-family home could now legally allow a fourplex or a combination like a triplex + DADU.

This is a game-changer for:

• Homeowners looking to add rental income or accommodate family

• Small developers seeking to build townhomes or cottage clusters

• Landowners looking to unlock more value from their property

Why It Matters

Washington State faces a housing shortage of over 1 million units in the next 20 years. HB 1110 helps close that gap by:

• Encouraging small-scale infill housing

• Reducing sprawl

• Making homeownership and rentals more accessible

• Supporting walkable, connected communities

For property owners, it means the chance to build more, earn more, and contribute to livable neighborhoods.

How Harmony Space Architecture Can Help

We specialize in turning these new opportunities into permittable, buildable, and profitable projects. Our services include:

• Property feasibility studies (What can I build under HB 1110?)

• Architecture design for middle housing: duplexes, fourplexes, townhomes, cottages

• Permit-ready drawing sets

• Unit Lot Subdivision coordination (so each unit can be sold individually)

• Sustainable design aligned with Washington energy codes

Whether you’re a homeowner thinking about a backyard cottage, or a developer exploring infill projects, we’ll help you navigate HB 1110 from idea to approval.

Conclusion

HB 1110 is more than a policy—it’s a powerful opportunity. For homeowners, it’s a way to grow with your property. For developers, it’s a path to profitable, community-focused projects. And for Washington’s future, it’s a step toward livable density and housing choice.

Have a property you’re curious about? Contact us at Harmony Space Architecture. We’ll help you understand what’s possible under HB 1110—and design a solution that works for you and your neighborhood.