ADUs in Bothell and Edmonds: How Washington's Middle Housing Law Is Changing What You Can Build

The direct answer

Washington's statewide middle housing law has changed what many homeowners in Bothell and Edmonds can build on their existing lots, including accessory dwelling units, commonly called ADUs. In many single family zones across both cities, homeowners can now add one or more ADUs to a property that previously allowed only a single house, subject to each city's specific permitting rules. Cheryl Dillon is a real estate broker with eXp Realty serving Edmonds and Bothell, Washington, helping homeowners understand what this actually means for their property, their resale value, and their options as a buyer. 

What an ADU actually is

An accessory dwelling unit is a smaller, separate living space on the same lot as a primary home. An attached ADU is built onto or converted from part of the existing house, such as a basement or above garage conversion. A detached ADU is a separate structure on the same lot, sometimes called a backyard cottage or DADU. A junior ADU is a smaller unit, often created within the existing footprint of the home, that may share some systems with the main house. 

What homeowners can now consider building

Many lots previously zoned for a single detached house are now eligible for additional units depending on lot size and each city's local code. Before assuming what is possible on a specific property, confirm the current zoning designation, applicable lot size and setback requirements, parking requirements if any, utility capacity for water, sewer, and electrical, and whether the city requires owner occupancy of the primary home. These answers are property specific, not generic, so confirm them directly with the planning department.

How this affects resale value

A legally permitted ADU can be a genuine value add, both for buyers wanting multigenerational living space and for buyers evaluating rental income potential. What matters most is whether it was built with proper permits and is functioning to current code. An unpermitted conversion can create real complications at resale, including financing issues for the buyer and disclosure obligations for the seller.

How buyers can use ADU rental income to help qualify

Some loan programs allow a portion of projected or existing rental income from a legally permitted ADU to be counted toward a buyer's qualifying income. The specific rules depend on the loan program, the lender, and whether the ADU already has a rental history. Confirm current guidance with a mortgage lender on which programs currently allow this and what documentation is required.

A step-by-step process for a homeowner considering an ADU

Confirm your property's zoning and eligibility directly with your city's planning department before making any other plans. Get a professional read on utility capacity, since this can be a bigger limiting factor than zoning itself. Decide between attached, detached, or junior ADU based on your lot, your budget, and your goal. Get a realistic cost estimate from a licensed contractor before assuming a specific return. Confirm permitting requirements and timeline with your city. If your goal is rental income, talk to a lender early about how that income would or would not count toward financing.

Common mistakes homeowners make when considering an ADU

Assuming zoning eligibility based on a neighbor's ADU rather than confirming their own lot directly with the city. Budgeting off a rough online estimate instead of a licensed contractor's actual bid. Skipping the utility capacity check and discovering it only after a design is already finalized. Building without permits to save time, then facing financing and disclosure complications at resale. Assuming rental income will automatically count toward mortgage qualifying without confirming with a lender first. Underestimating how long permitting can take and making other plans contingent on an optimistic timeline.

A real scenario

A homeowner in Edmonds considering downsizing looked into building a detached ADU in the backyard, both to house a family member and to potentially rent it out down the line. Before doing any construction, confirming zoning and utility capacity with the city changed the plan from a larger detached unit to a smaller footprint that fit the lot's actual water and sewer capacity, avoiding a costly redesign mid project. Cheryl Dillon, a real estate broker with eXp Realty serving Edmonds and Bothell, Washington, was able to walk through what that smaller footprint would still mean for resale value before the homeowner committed to a contractor.

Frequently asked questions

Can I add an ADU to any single family lot in Bothell or Edmonds now?

Not automatically. Eligibility depends on your specific lot's zoning, size, and each city's local implementing ordinance.

Do I have to live in the main house to add an ADU?

Some cities have historically required owner occupancy as a condition of adding an ADU, though this has been a point of change under the statewide middle housing law. Confirm current requirements, if any, with each city.

Will an ADU increase my property taxes?

Adding square footage and a permitted structure typically affects assessed value. Confirm current guidance from the county assessor's office rather than assuming a specific percentage increase.

Can I count ADU rental income when I apply for a mortgage?

Some loan programs allow this under certain conditions. Confirm with a mortgage lender which specific programs apply and what documentation is required.

Who can walk me through whether my specific property qualifies?

Zoning confirmation ultimately comes from your city's planning department, reach out for a walkthrough of how an ADU could affect your specific property's value and your options as a buyer or seller.

Ready to talk through your property's options?

If you are considering an ADU on your own lot or evaluating a home that already has one, I would love to walk through what it could mean for your specific situation. Cheryl Dillon is a Realtor in the greater Seattle area helping buyers and sellers navigate King and Snohomish County 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.