Washington ADU Rules Under HB 1337: What King County Homeowners Should Know

Washington ADU rules changed in a major way under HB 1337, especially for homeowners in cities and urban growth areas planning under the Growth Management Act. This article is for homeowners who already understand what an ADU is and want to know how Washington’s newer statewide rules affect planning, permits, and feasibility. It is not a basic definition of accessory dwelling units. It is a practical King County homeowner guide to what HB 1337 changed, what it did not change, and what still needs to be verified locally before planning an ADU or DADU.

That is the part homeowners cannot afford to miss.

The keyword focus for this article is Washington ADU rules HB 1337, not general ADU education. That distinction helps keep this guide separate from broader ADU basics content and makes the article more useful for homeowners who are already considering a project.

HB 1337 created a statewide framework for accessory dwelling units, now codified in RCW 36.70A.680, RCW 36.70A.681, and RCW 36.70A.696. According to the Washington State Department of Commerce, fully planning cities and counties must allow two ADUs on residential lots that allow single-family homes within urban growth areas, with limitations. Source: Washington State Department of Commerce.

For King County homeowners, this opens up meaningful possibilities: extra living space, multigenerational housing, long-term rental potential, aging-in-place flexibility, and future resale appeal. Still, the best first step is not calling a builder. It is confirming what your specific city, lot, utilities, and permitting path will allow.

If you only need a plain-English definition, our related guide, What Is an Accessory Dwelling Unit?, covers the basics. This article goes further by focusing on HB 1337, statewide rule changes, King County implementation issues, and homeowner due diligence.

What Did HB 1337 Change About Washington ADU Rules?

Washington HB 1337 changed ADU rules by creating a statewide baseline that requires many cities and counties to allow more accessory dwelling units on eligible residential lots inside urban growth areas.

The Washington State Department of Commerce explains that House Bill 1337 is codified as RCW 36.70A.680, .681, and .696, and requires fully planning cities and counties to allow two ADUs on residential lots that allow single-family homes within urban growth areas, with limitations. Source: Washington State Department of Commerce.

Seattle’s Department of Construction and Inspections also confirms that HB 1337 applies to Seattle and other cities and counties planning under the Growth Management Act, with requirements codified in RCW 36.70A.680 and RCW 36.70A.681. Seattle is one example, and other King County cities may implement rules differently through their own local code and permit processes. Source: Seattle SDCI ADU Code Updates.

In plain English, the state wanted to reduce local barriers that made ADUs difficult or impossible in many residential areas. The law addresses several common restrictions, including the number of ADUs allowed, owner-occupancy requirements, parking limits, and some design or development standards. That makes this article different from an ADU definition post: the focus here is how the law changed the planning and feasibility conversation for Washington homeowners.

But there is an important distinction: HB 1337 does not erase every local rule. It creates a statewide framework that local governments must implement through their own development regulations. That means homeowners in Seattle, Renton, Bellevue, Kent, Kirkland, Redmond, Issaquah, Burien, Shoreline, Bothell, Auburn, Federal Way, and unincorporated King County still need to verify local code.

For homeowners trying to understand broader housing policy changes, our related post on how new WA policies could impact real estate in King County adds helpful context.

Table: Washington ADU Rules at a Glance

Use this table as a starting point, not a permit decision. State law creates the framework, but your city, lot, utilities, site conditions, and design still determine what is actually feasible.

Can King County Homeowners Build Both an ADU and DADU Under HB 1337?

Many eligible King County homeowners may be able to build both an attached ADU and a detached accessory dwelling unit, but the answer depends on local code and property-specific feasibility.

The Department of Commerce states that fully planning cities and counties must allow two ADUs on residential lots that allow single-family homes within urban growth areas, with limitations.

An attached ADU is usually built within or connected to the primary home, such as a basement apartment, garage conversion, or addition. A DADU is a detached accessory dwelling unit, often described as a backyard cottage, detached in-law unit, or small standalone residence.

For King County homeowners, the possibility of two ADUs can be meaningful. A homeowner might consider one unit for extended household needs and another for long-term rental flexibility. Another homeowner might think about a detached unit for privacy while using an internal ADU for future aging-in-place options.

However, two ADUs are not automatically realistic on every lot. Even when the zoning framework allows them, the site still needs to work. A property may be limited by sewer or septic capacity, setbacks, tree protection, critical areas, slope conditions, building code, utility upgrades, stormwater requirements, or access.

RCW 36.70A.680 makes this caution clear. State law does not require or authorize ADUs where development is restricted by other laws, rules, or ordinances because of proximity to on-site sewage system infrastructure, critical areas, or other unsuitable physical property characteristics. Source: RCW 36.70A.680.

That is why homeowners should avoid assuming that a statewide ADU rule equals a ready-to-build property. The better approach is to ask: what does the state allow, what does the city code say, and what does this specific property physically support?

What Are the HB 1337 Parking, Size, and Owner-Occupancy Rules?

HB 1337 limits how local governments can regulate ADU parking, size, and owner occupancy, but homeowners still need to confirm the current local code before designing or budgeting a project.

Under the statewide ADU framework, Washington has moved toward fewer local barriers. The law addresses restrictions that previously made ADUs less practical for many homeowners, including parking mandates and owner-occupancy rules.

Parking is one of the most common questions. Many homeowners wonder whether adding an ADU means they must add a new driveway, garage space, or off-street parking stall. HB 1337 limits certain local parking requirements, especially in places near major transit, but the details can depend on how local code is implemented and whether site-specific conditions apply.

Owner occupancy is another major change. Historically, some cities required the property owner to live in either the main home or the ADU. HB 1337 restricts local governments from imposing certain owner-occupancy requirements, but homeowners should still verify the current rules in their jurisdiction before making rental or ownership plans.

Size also matters. ADUs are usually smaller than the main residence, but maximum size, height, lot coverage, design, and placement can vary by jurisdiction and lot. Seattle’s ADU permitting information, for example, notes that ADUs must meet residential, building, mechanical, electrical, energy, land use, environmentally critical areas, and shoreline code standards. Seattle is used here as an example of local implementation, not as a substitute for checking rules in Renton, Bellevue, Kent, Kirkland, Redmond, Issaquah, or unincorporated King County. Source: Seattle SDCI ADU Permits.

Here is the key reframe: the state may reduce barriers, but the building still has to be legal, safe, properly permitted, and physically workable.

For homeowners who are considering renovations or additions, our guide to the risks of buying a home with unpermitted renovation work is also relevant. Unpermitted work can create resale, financing, insurance, and safety concerns, especially when a property has a converted living space.

Why Do City-Specific ADU Rules Still Matter After HB 1337?

City-specific ADU rules still matter because Washington law sets the baseline, but local governments administer zoning, permits, inspections, utilities, and property-specific development standards.

This is especially important in King County, where homeowners may be under different jurisdictions even if they live only a few miles apart. Seattle, Bellevue, Renton, Kent, Kirkland, Redmond, Issaquah, Auburn, Federal Way, Shoreline, Bothell, Burien, Des Moines, Tukwila, Newcastle, Sammamish, and unincorporated King County may each have different permitting processes and implementation timelines.

King County’s residential ADU permit materials explain that an ADU is a separate complete dwelling unit that may be attached to an existing single-family residence, contained within a basement or attic, or located in a separate stand-alone structure. King County also directs applicants to submit online through MyBuildingPermit.com. Source: King County Residential ADU Information.

MRSC, a trusted Washington local government resource, notes that GMA-planning local governments, including counties in unincorporated urban growth areas, must revise ADU regulations as needed to conform with RCW 36.70A.680 and RCW 36.70A.681. Source: MRSC Accessory Dwelling Units.

That means a homeowner in Seattle may have a different permit process than a homeowner in Renton. A Bellevue homeowner may have different design or lot constraints than someone in Kent. A homeowner in unincorporated King County may need to work through county permitting rather than a city department.

Publishing note: Local implementation can change. Before publishing this article or advising a homeowner, verify the latest ADU code, permit requirements, and city guidance for the exact jurisdiction being discussed.

This is where ADU planning can get confusing. A headline may say “Washington allows two ADUs,” but your property still needs a local feasibility check.

A good local review should answer:

• Which jurisdiction controls the property?
  
  
• Is the lot inside an urban growth area?
  
  
• What zoning district applies?
  
  
• Are there critical areas, slopes, shorelines, easements, or tree rules?
  
  
• Is the property connected to sewer, or does it use septic?
  
  
• Are utilities adequate for another dwelling unit?
  
  
• What permits, impact fees, utility charges, or inspections may apply?

For countywide housing context, our post on how Washington’s housing shortage is affecting King County explains why ADUs are part of a broader conversation about housing supply and flexibility.

How Could HB 1337 ADU Flexibility Affect Resale, Rental Income, or Multigenerational Living?

An ADU can affect resale, rental income, and multigenerational living by adding flexibility, but the actual value depends on legality, design quality, location, rental demand, and buyer priorities.

Most homeowners think about ADUs for one of three reasons: family flexibility, income potential, or future resale appeal. All three can be valid, but each needs a different lens.

For multigenerational living, an ADU may create privacy while keeping household members close. This can be useful for aging parents, adult children, guests, caregivers, or changing household needs. The value is not only financial. Sometimes the biggest benefit is daily flexibility.

For rental income, an ADU may create a long-term rental opportunity, but homeowners should not treat projected rent as guaranteed. Rents vary by city, unit size, finish level, parking, privacy, transit access, utilities, and market conditions. Homeowners should also verify local rental rules, licensing requirements, insurance implications, taxes, and financing impacts before depending on rental income.

For resale, a legal and well-designed ADU may broaden the buyer pool, especially among buyers who value flexible space. But a poorly planned or unpermitted unit can do the opposite. Buyers, appraisers, lenders, and insurers may ask whether the unit is legal, permitted, safe, and properly documented.

That is why documentation matters. Keep permits, inspections, plans, utility records, and final approvals organized. If you ever sell, that paper trail may help buyers feel more confident about the property.

Homeowners thinking about long-term value may also want to review our guide to home improvements and repairs that add the most value. An ADU can be a major investment, so it should be evaluated alongside other projects that may improve livability and marketability.

What Should Homeowners Verify Before Starting an ADU Project?

Before starting an ADU project, homeowners should verify jurisdiction, zoning, utilities, site constraints, permit requirements, budget, rental rules, taxes, insurance, and long-term resale goals.

An ADU can be one of the most useful property improvements a homeowner considers, but it is not a casual weekend project. It is a second dwelling unit, which means it touches land use, construction, safety, utilities, financing, and long-term ownership planning.

ADU Planning Checklist for King County Homeowners

Use this checklist before spending heavily on design, construction bids, or financing. It can help you organize the right questions early.

The order matters. A homeowner should not begin with finishes, colors, or rental income projections. Start with feasibility.

Once feasibility is clear, then design can become more strategic. Is the ADU for family? Long-term rental use? Future resale appeal? Aging in place? A private office or guest space? The intended use can shape layout, privacy, parking, entry placement, storage, and finish level.

Homeowners should also think about how an ADU fits the broader property. A well-designed ADU should feel intentional, not squeezed onto a lot as an afterthought. Privacy, outdoor space, noise, light, access, and maintenance all matter.

For tax-related planning, our local guide to real estate taxes in King County, WA may be a useful starting point, though homeowners should always confirm their specific situation with a qualified tax professional.

Expert Insight: What This Means for King County Homeowners

For King County homeowners, the new Washington ADU rules should be treated as an opportunity to investigate, not a guarantee to build.

The strongest ADU projects usually start with clear goals. A homeowner who wants rental income should analyze likely rent, vacancy, management, taxes, insurance, utilities, and maintenance. A homeowner planning for multigenerational living should focus on privacy, accessibility, comfort, and long-term household needs. A homeowner thinking about resale should prioritize legality, quality, documentation, and broad buyer appeal.

This is also where local real estate guidance can help. An ADU may look great on paper, but its market impact depends on how buyers in that specific area value flexible living space. A DADU near transit in Seattle may be viewed differently than an internal ADU in suburban Renton, a backyard cottage in Shoreline, or a converted lower-level unit in Kent.

Our team’s practical advice is to think in layers:

1. What does state law generally allow?
   
   
2. What does your city or county currently permit?
   
   
3. What does your specific property physically support?
   
   
4. What would the project cost?
   
   
5. How would the ADU support your life, income goals, or future sale?

That approach keeps the conversation grounded. It also helps homeowners avoid overbuilding, under-documenting, or relying on assumptions that may not hold up during permitting or resale.

FAQ

How Many ADUs Can I Build in Washington?

Many eligible residential lots in Washington urban growth areas must be allowed two ADUs if they allow single-family homes, with limitations. This comes from HB 1337 and the related RCW sections governing local ADU regulation. Homeowners still need to verify whether their specific lot, jurisdiction, utilities, and site conditions support one or two ADUs.

Do I Have to Live on the Property to Build an ADU?

In many cases, HB 1337 restricts local governments from requiring owner occupancy as a condition for ADU development. However, homeowners should still verify how their city or county has implemented the state rules. Rental plans may also be affected by local rental rules, HOA restrictions, financing, and insurance requirements.

Can I Rent Out an ADU in King County?

Many ADUs can be used as rental housing, but the rules depend on the local jurisdiction, rental type, and property-specific restrictions. Long-term rental rules may differ from short-term rental rules. Homeowners should confirm local licensing, lease rules, tax implications, insurance coverage, and HOA restrictions before relying on rental income.

Do ADUs Require Parking?

Washington’s ADU framework limits certain local parking requirements, but parking questions still need local review. Your city may have rules tied to transit proximity, street conditions, access, or specific site constraints. Before designing an ADU, verify current parking standards with the city or county permit office.

Does an ADU Increase Home Value?

A legal, well-designed ADU may increase a home’s flexibility and market appeal, but it does not guarantee a specific increase in value. Buyers may value an ADU differently depending on location, layout, privacy, permits, rental potential, and construction quality. If resale value is the goal, keep the project legal, well documented, and aligned with buyer demand in your area.

What is the Difference Between an ADU and a DADU?

An ADU is an accessory dwelling unit located on the same lot as a primary home. It may be attached to the home, built inside the home, or detached. A DADU is a detached accessory dwelling unit, meaning it is a separate standalone structure, often called a backyard cottage or detached in-law unit.

Helpful Resources

1. Washington State Department of Commerce, Accessory Dwelling Units

Explains Washington’s ADU framework under HB 1337 and its related RCW sections.

2. RCW 36.70A.680, Accessory Dwelling Units, Local Regulation

Provides the statutory language for local ADU regulation and property-specific limitations.

3. RCW 36.70A.681

Provides related statutory requirements for accessory dwelling units.

4. Seattle SDCI, Accessory Dwelling Units Code Updates

Confirms that HB 1337 applies to Seattle and other GMA-planning cities and counties.

5. Seattle SDCI, Accessory Dwelling Unit Permits

Useful example of how one King County city handles ADU permitting and code standards.

6. King County Residential Accessory Dwelling Units Information

Explains King County ADU permitting basics for properties under county jurisdiction.

7. MRSC, Accessory Dwelling Units

Provides local government context on ADU regulation and GMA-planning requirements.

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