What Washington's Accessory Dwelling Unit Law Means for You

Unlocking Your Property's Potential: What Washington's HB1337 ADU Law Means for You:

For homeowners and property investors across Washington, the residential landscape has permanently changed. The passage of House Bill 1337 fundamentally rewrote the rules for Accessory Dwelling Units (ADUs) and backyard cottages, effectively creating a new “right to build” on single-family lots.

Whether your goal is to build a beautiful backyard cottage for aging parents, create a separate space for adult children, or develop high-yield rental units, the state has removed the historical red tape that made these projects impossible.

Here is what you need to know about your new property rights—and why your local city still holds the keys to exactly how you can build.

The Statewide Guarantees: Your New Rights

HB 1337 established baseline minimums that local jurisdictions can no longer restrict. Here are the major wins for property owners:

• Two ADU's Per Lot: All lots in urban growth areas that allow single-family homes must now permit at least two ADUs. You can build two detached backyard cottages (DADUs), two attached basement units, or one of each configuration.

• True Living Space (1,000 Sq. Ft. Minimum): Cities can no longer force ADUs to be cramped tiny homes. Local governments must allow ADUs to be at least 1,000 square feet, which easily accommodates a spacious, highly livable two-bedroom layout.

• No Owner-Occupancy Required: This is a massive shift for property investors. Cities cannot require the property owner to live in the main house or the ADU. You can develop a lot and rent out all the units independently.

• Lower Fees and Relaxed Parking: Impact fees (for schools, traffic, etc.) for an ADU cannot exceed 50% of the fees for a principal unit. Additionally, cities cannot require off-street parking for the ADU if the property is within a half-mile of a major transit stop.

• Independent Sale: The state now requires cities to allow ADUs to be sold separately from the main house as condominiums, opening up entirely new equity opportunities.

 The Crucial Caveat: The State Sets the Floor, The City Writes the Code

While HB 1337 grants you the right to build, it does not provide a blank check to build whatever you want, wherever you want.

The state mandated that cities update their municipal codes to comply with these new rules, with most local jurisdictions facing a mid-2025 compliance deadline. Because these local codes are freshly updated, they vary wildly from one city to the next.

As a property owner it is critical to understand that your specific city's planning department still controls the following:

• Lot Coverage & Setbacks: Even if the state says you can build 1,000 square feet, your local city still dictates how close you can build to the property line and what percentage of your total lot can be covered by hard structures.

• Design and Façade Guidelines: While cities cannot ban ADUs, they can dictate that the new structure matches the roof pitch, siding style, neighborhood.

• How HB 1337 Interacts with HOAs While the state law aggressively strips away municipal barriers, its application to Homeowners Associations (HOAs) involves a specific legal boundary.

• Pre-Existing Bans: Covenants, Conditions, and Restrictions (CC&Rs) and HOA bylaws that strictly prohibited ADUs prior to the enactment of the law are generally permitted to remain in force if they were already active.   

Navigating the Next Steps

Whether you want to generate passive income or keep your family close, these relaxed laws drastically increase the intrinsic value of residential lots in Washington. We understand that every site comes with unique opportunities and site constraints—from topography and utilities to local municipal overlays.

Currently we are offering dedicated Lot Feasibility Assessments to evaluate what your specific property can legally and structurally support, even if adding new structural footprint to your lot isn't an option, the legislation includes a few critical provisions that still may offer you a path forward.

If you’d like to explore the possibilities for your lot, send us an email at wdgarchitects.com or call our office at 425 558-9330 and let us help you unlock the hidden possibilities of your property.