House Bill 1337 is the most significant piece of ADU legislation Washington state has ever passed. Signed into law by Governor Jay Inslee in May 2023, this bill fundamentally reshaped the rules around accessory dwelling units across the state. If you're a homeowner, investor, or builder in Washington, understanding HB 1337 is essential — it directly affects what you can build, where you can build it, and how much value your property can generate.

What HB 1337 Actually Does

At its core, HB 1337 requires all cities in Washington to allow ADUs on lots zoned for single-family residential use. But the bill goes much further than simply allowing ADUs. Here are the key provisions:

Two ADUs Per Lot

Under HB 1337, cities must allow at least two ADUs on any lot where a single-family home is permitted. That means you can have one attached ADU (like a basement conversion) and one detached ADU (a backyard cottage) on the same property, or two detached units if your lot can accommodate them. Before HB 1337, most Washington cities only allowed one ADU per lot, and many had additional restrictions that made even that difficult.

Key Takeaway A single-family lot in Washington can now support up to three dwelling units: your main house, an attached ADU, and a detached ADU. That's a massive shift in how residential property can be used.

No Owner-Occupancy Requirement

This is one of the biggest changes. Before HB 1337, many cities required the property owner to live in either the main house or the ADU. This effectively killed the investment case for ADUs, since you couldn't buy a property, build an ADU, and rent both units without living on-site. HB 1337 prohibits cities from imposing owner-occupancy requirements for ADUs, opening the door for rental investors and out-of-state property owners.

No Parking Minimums

Cities can no longer require off-street parking for ADUs. This is a practical game-changer for homeowners with smaller lots. Previously, some cities required one or two additional parking spaces per ADU, which consumed valuable yard space and sometimes made a DADU physically impossible to build. Under HB 1337, parking cannot be a barrier to ADU construction.

Streamlined Permitting

The law requires cities to establish clear, objective design and development standards for ADUs rather than subjective design review processes. Cities can still impose reasonable standards around setbacks, height limits, and lot coverage, but they cannot use discretionary review processes to block or delay ADU permits. The intent is to make ADU permitting as straightforward as any other residential building permit.

Impact Fee Reductions

HB 1337 limits the impact fees that cities can charge for ADUs. Impact fees for ADUs cannot exceed 50% of the impact fees charged for a single-family home. This provision reduces one of the hidden costs that made ADU construction prohibitively expensive in some jurisdictions.

When Did HB 1337 Take Effect?

The law was signed in May 2023 and required cities to update their local regulations in phases:

As of April 2026, all major Washington cities have updated their ADU ordinances to comply with HB 1337. Some cities, like Seattle, already had relatively permissive ADU rules and needed fewer changes. Others, like many suburban cities on the Eastside and in Pierce County, had to make substantial revisions.

Which Cities Does It Apply To?

HB 1337 applies to all cities and counties in Washington that are required to plan under the Growth Management Act (GMA). In practice, this covers virtually every incorporated city in the Puget Sound region and most cities statewide. The key cities where HB 1337 has had the most impact include:

How HB 1337 Works with HB 1110 (Middle Housing)

HB 1337 doesn't exist in isolation. It's part of a broader package of housing legislation that includes HB 1110, the middle housing bill passed in the same 2023 legislative session. While HB 1337 focuses on ADUs, HB 1110 requires cities to allow duplexes, triplexes, fourplexes, and sixplexes on lots previously zoned exclusively for single-family homes.

Together, these two laws dramatically increase the development potential of single-family residential lots in Washington:

For most homeowners, HB 1337 is the more immediately actionable law. You don't need to tear down your house to take advantage of it — you just need to add a unit (or two) to your existing property. HB 1110 is more relevant for developers and investors looking at larger-scale redevelopment.

Impact on Property Values and Investment Potential

The combined effect of HB 1337 and HB 1110 has increased the value of single-family lots across Washington, particularly in high-demand areas. Properties that can support one or two ADUs now have significantly more income potential, which translates to higher market values.

Consider a typical Seattle single-family lot. Before HB 1337, the property supported one household and generated zero rental income (unless you rented rooms). After HB 1337, that same property can support up to three households — the main house, an attached ADU, and a DADU — potentially generating $3,000 to $5,000+ per month in combined rental income from the ADU units alone.

Real estate agents across the Puget Sound report that lots with existing ADUs or clear ADU potential are selling at a premium. And because investors can now build and rent ADUs without living on-site, the buyer pool for single-family homes with ADU potential has expanded significantly.

What Should Homeowners Do Now?

If you own a single-family home in Washington, here's how to take advantage of HB 1337:

  1. Assess your lot: Look at your property's size, zoning, setbacks, and available space. Most lots that are 4,000 square feet or larger can accommodate at least one DADU. Lots over 5,000 square feet may be able to support two ADUs.
  2. Check your city's updated ADU regulations: While HB 1337 sets the statewide baseline, each city still has its own specific design standards for setbacks, height, lot coverage, and maximum unit size. Your city's planning department website will have the current rules.
  3. Talk to a builder: An experienced ADU builder can do a site feasibility assessment and give you a realistic estimate of what you can build and what it will cost. Many builders in our directory offer free initial consultations.
  4. Run the numbers: Factor in construction costs, financing, expected rental income, and the potential increase in property value. Our ADU cost guide can help you build a realistic financial picture.
  5. Consider your timeline: The ADU market in Washington is busier than ever, and top builders are booking out 3 to 6 months in advance. If you're serious about building, start the conversation sooner rather than later.

HB 1337 has created a once-in-a-generation opportunity for Washington homeowners. Whether you want to generate rental income, house a family member, or simply increase your property's long-term value, the legal barriers that once stood in the way are largely gone. The question now is not whether you can build an ADU — it's whether you're ready to get started.

Ready to find a builder? Browse our directory of vetted ADU contractors across the Puget Sound region, or request free quotes to compare options.