Accessory Dwelling Units in Washington State: What Homeowners Need to Know Before They Build

I have had more conversations about accessory dwelling units in the last two years than in the two decades before that combined, and there is a very real reason for it. Washington State has fundamentally changed the rules, and homeowners across King and Snohomish County are waking up to just how much opportunity that change actually represents.

Whether you are a homeowner dreaming of rental income, a family wanting a private, connected space for an aging parent, or a seller wondering if adding one could meaningfully increase your home's value, this is a genuinely important topic to understand clearly. I want to walk you through what has changed, what it means practically, and how to think about whether an ADU is the right move for your specific property.

What Is an Accessory Dwelling Unit

An accessory dwelling unit, commonly called an ADU, is a smaller, self-contained residential unit located on the same lot as an existing single family home. It has its own kitchen, its own bathroom, and its own sleeping area, functioning fully independently from the main home. You have probably heard these called backyard cottages, granny flats, in-law suites, or guest houses.

There are three general types. An attached ADU, sometimes called an AADU, is built within or connected to the main house, like a basement apartment or an addition with its own entrance. A detached ADU, commonly called a DADU, is a standalone structure in the backyard, often what people picture when they hear the term backyard cottage. And a conversion ADU takes an existing structure, like a garage or outbuilding, and converts it into a fully livable independent unit. DADUs in particular have become the most requested project type across King and Snohomish County, since they offer the clearest separation of space for renters, aging parents, or adult children.

What Has Genuinely Changed Under Washington Law

For years, many Washington cities made ADUs and DADUs difficult or nearly impossible to build. Some required the property owner to live on site. Others imposed steep fees, restrictive setback rules, or banned detached units outright.

Washington House Bill 1337, signed in 2023, changed that fundamentally for cities within urban growth areas, and its requirements are now fully in effect statewide. Cities and counties that fully plan under the state's Growth Management Act must generally allow at least two ADUs per residential lot, in any combination of attached and detached units, meaning a homeowner can add both an AADU and a DADU on the same lot in many jurisdictions. Owner occupancy is no longer required in most jurisdictions, meaning you are not required to live on the property where your ADU or DADU is located. Cities cannot impose a size limit below 1,000 square feet, giving homeowners meaningful flexibility in design. And in many areas, additional parking is no longer required specifically because of the ADU or DADU, removing a barrier that previously made smaller urban lots impractical for this kind of project.

📌 Statutory source: The two-ADU requirement is written directly into Washington law at RCW 36.70A.681(1)(c), which requires cities and counties to allow at least two accessory dwelling units on all lots in urban growth area zoning districts that permit single-family homes, in configurations that include one attached and one detached unit, or two detached units. The related owner-occupancy prohibition is at RCW 36.70A.681(1)(b), and the impact fee cap (no more than 50 percent of what the principal unit would be charged) is at RCW 36.70A.681(1)(a). The 1,000 square foot minimum size floor and the requirement to allow conversion of existing structures sit in the neighboring section, RCW 36.70A.680. Both are viewable directly at app.leg.wa.gov/RCW.

HB 1337 passed alongside a companion bill, House Bill 1110, in the same 2023 legislative session. HB 1110 is a related but distinct piece of legislation focused on middle housing, requiring many cities to allow duplexes through sixplexes on lots that were previously zoned for single-family homes only. It is not an ADU-specific bill, but it works alongside HB 1337 to meaningfully expand housing density and options across Washington's cities.

For homeowners outside of urban growth areas, a newer bill, House Bill 1345, took effect in mid 2026 and allows certain Washington counties to permit detached ADUs on rural and semi-rural properties for the first time, provided the ADU and county meet specific requirements. This is a genuinely significant expansion for homeowners on larger, unincorporated lots who previously had no path to build a DADU at all.

It is worth noting that specific requirements still vary somewhat by city and county, so confirming your particular jurisdiction's current ordinance before finalizing plans remains an important step, even with these strong statewide protections now in place.

Why Homeowners Across King and Snohomish County Are Building Them

The motivations I hear most often fall into a few clear categories. Rental income is a significant driver, particularly for homeowners looking to offset a mortgage payment or build an additional income stream in a high cost housing market. A well designed DADU in a desirable Snohomish County community can generate meaningful monthly rent, and for some buyers, factoring in that potential income becomes part of the affordability conversation when purchasing a home with ADU or DADU potential in the first place.

Multigenerational living is another major driver, and one close to my heart. A dedicated ADU or DADU with its own entrance, bathroom, and small kitchenette offers real privacy and independence for an aging parent or an adult child, while keeping family genuinely close. Across King and Snohomish County, a growing number of listings now specifically highlight ADU or DADU potential, or existing mother in law suites, reflecting real, growing demand for exactly this kind of flexible living arrangement.

And for some homeowners, the motivation is simply flexibility. A home office, a guest space for visiting family, or a future rental unit once life circumstances change all become genuinely achievable in ways they were not just a few years ago.

Does an ADU Add Value to Your Home

This is one of the questions I get asked most often, and the honest answer is that it depends on execution and market. A well built, code compliant ADU in a desirable community can meaningfully increase both your home's market value and its appeal to a wider pool of buyers, particularly buyers interested in rental income or multigenerational living. Appraisers increasingly recognize ADUs as legitimate value adding features, especially when the unit is permitted properly and reflects quality construction consistent with the main home.

That said, an ADU built purely as an afterthought, without attention to design cohesion or quality finishes, will add less value than one that is thoughtfully integrated into the overall property. If you are building with resale value specifically in mind, working with a designer and contractor experienced in ADU construction, and confirming your specific city's permitting requirements early, protects both your budget and your eventual return.

What It Actually Costs to Build

Costs vary significantly depending on whether you are pursuing an attached AADU conversion or new detached DADU construction. Attached ADU conversions, such as a basement or garage conversion, typically run lower than new detached construction, since they take advantage of existing structure, foundation, and in some cases existing utility connections. New DADU construction carries a higher price tag, reflecting a fully new build, but it also offers the most design flexibility and the clearest separation of space for renters or family members.

I always encourage homeowners considering this investment to get multiple contractor bids and to factor in permitting timelines honestly, since these can vary meaningfully by city even under the more homeowner friendly state framework now in place.

Frequently Asked Questions

What is an accessory dwelling unit in Washington State?

An ADU is a smaller, self-contained residential unit located on the same lot as an existing single family home, with its own kitchen, bathroom, and sleeping area. It can be attached, detached, or created through the conversion of an existing structure.

What is the difference between an ADU and a DADU?

ADU is the general term for any accessory dwelling unit. A DADU, or detached accessory dwelling unit, refers specifically to a standalone structure separate from the main house, commonly called a backyard cottage. An attached unit within or connected to the main house is sometimes called an AADU.

Do I need to live on my property to build an ADU or DADU?

No, in most Washington jurisdictions, owner occupancy is no longer required under state law, meaning you can build an ADU or DADU and rent out either unit, or the main home, without living on site.

How many ADUs can I build on my property in Washington?

Under House Bill 1337, most fully planning cities and counties within urban growth areas must allow at least two ADUs per residential lot, in various combinations of attached and detached units, subject to lot size and local requirements. House Bill 1345, effective in 2026, extends limited DADU rights to certain counties for properties outside urban growth areas as well.

Does Washington require extra parking for an ADU?

In many jurisdictions, additional parking is no longer required specifically because of an ADU, though specific requirements can still vary somewhat by city, so it is worth confirming with your local planning department.

Can I sell my ADU separately from my main home?

In certain jurisdictions, separate sale is permitted depending on local zoning and subdivision laws, though this varies significantly by city and is worth confirming directly with your local planning department.

Can I build a DADU on rural property outside city limits in Washington?

In some counties, yes, as of House Bill 1345 taking effect in 2026. This is a newer and more limited right than the urban growth area requirements under HB 1337, so confirming your specific county's participation and requirements is an important first step.

Does building an ADU add value to my home?

Yes, generally, particularly when the unit is well built, code compliant, and thoughtfully integrated into the property. Appraisers increasingly recognize ADUs as legitimate value adding features, especially in markets where rental income potential matters to buyers.

Ready to Talk?

Whether you already own a home with ADU potential or are searching for a property that could support this kind of flexibility, I would love to help you think through the possibilities. No pressure. No pitch. Just honest guidance from someone who has spent over 40 years living and working in this part of the world.

Cheryl Dillon is a Realtor in the greater Seattle area helping buyers and sellers navigate life transitions with clarity, strategy, and a genuinely personalized approach.

📞 425-954-5622 📧 Cheryl@CherylDillonRealEstate.com 🌐 CherylDillonRealEstate.com 📍 1455 Leary Way #400, Seattle, WA 98107

Cheryl Dillon is a licensed REALTOR® in the state of Washington with EXP Realty.