Selling a Home With an ADU in Snohomish or King County (2026)

Short answer: An accessory dwelling unit can help a sale, but only if a buyer can see it’s legal, and their lender can work with it. Before you list a home with a backyard cottage, garage apartment or basement unit, gather the permits and final inspections, find out whether a buyer’s loan can count it, and know where any tenant stands. Washington’s 2023 ADU law opened the door to two ADUs on most lots in urban growth areas, but mortgage rules haven’t all caught up, and that gap is where ADU sales tend to get stuck.

What Washington law now allows

RCW 36.70A.681 sets limits on how cities and counties can regulate ADUs. The Department of Commerce sums it up this way: inside urban growth areas, most cities and counties must

• allow two ADUs per residential lot, attached, detached, a mix of both, or converted from an existing structure such as a detached garage;
• allow an ADU of at least 1,000 square feet;
• charge no more than 50% of the impact fees charged for the main house;
• not require the owner to live on the property (Commerce notes an exception for short-term rentals); and
• allow ADUs to be sold as independent units. The statute’s wording is narrower: a city or county can’t prohibit selling a condominium unit separately just because it was originally built as an ADU.

The rules don’t apply to lots with designated critical areas or their buffers, or to certain watersheds. Local codes also differ in how they’ve put the law into effect, which is why Commerce tells owners to check with their local planning department for site-specific requirements. For a seller, the practical point is simple: what your city allows today may be more than what it allowed when your unit was built. That can help with an older unit, but it doesn’t replace a permit that was never pulled.

Where buyers’ loans draw the line

Most buyers will finance the purchase, and their lender’s rules decide what your ADU is worth to them. Fannie Mae’s Selling Guide is a useful benchmark for conventional loans sold to Fannie Mae:

Sources: Fannie Mae Selling Guide B2-3-04, B4-1.3-05 and B3-3.8-02, checked October 30, 2026. FHA, VA and portfolio loans have their own rules; buyers should ask their lender.

Fannie Mae also requires the appraisal to describe the ADU and analyze its effect on value and marketability, and to show the improvements are acceptable for the market. An older closed sale with an ADU can be used as a comparable, and a current listing or pending sale can be included to show demand. If your area has few ADU sales, give your agent anything you know about nearby ones.

Paperwork to gather before you list

• Building permits and final inspections for the ADU and any conversion. Washington’s seller disclosure form asks directly whether there have been conversions, additions or remodeling, and whether all building permits and final inspections were obtained.
• Utility setup: whether the unit has its own meters and its own mailing address. Appraisers look at both.
• Any lease or rental agreement. The disclosure form asks whether title is subject to a lease or rental agreement.
• Any recorded documents, such as a covenant or condominium declaration, if the unit was set up for separate ownership.
• The disclosure form version. RCW 64.06.020 currently has two versions on the Legislature’s site: one marked effective until January 1, 2027, and one effective January 1, 2027. The 2027 version adds Environmental question 7L, which asks whether the property includes a family burial ground, mausoleum or columbarium. If your listing runs across the new year, confirm which form you’re using when the disclosure is delivered.

If something is missing, it’s better to find out now than in the middle of a buyer’s inspection. Our guide to selling as-is in Washington explains what you still have to disclose even if you won’t fix anything, and our article on what changes for sellers in January 2027 covers the new disclosure form.

If someone is renting the ADU

Commerce reminds owners that if you rent out an ADU, Washington’s landlord-tenant laws apply. Those laws limit when you can end a tenancy. RCW 59.18.650 lists the allowed causes. One of them covers an owner who elects to sell a single-family residence and gives at least 90 days’ written notice; the statute defines “elects to sell” as listing the home at a reasonable price, with a brokerage or on the MLS, within 30 days after the tenant leaves. Whether that cause fits a home with an ADU, and how it interacts with a lease, depends on your facts, so get legal advice before you give any notice. Some buyers will want the tenant to stay; others won’t. Decide which buyer you’re marketing to before you list.

If you’re still deciding whether to sell at all, our article on whether to sell or rent out your home walks through that choice.

FAQ

Can I sell my ADU separately from my house in Washington?

Only if it has been legally set up as a separate unit, typically by creating a condominium. State law (RCW 36.70A.681) bars cities and counties from prohibiting the sale of a condominium unit just because it was originally built as an ADU. Creating a condominium is its own legal process, so talk to a real estate attorney and your planning department before you market the ADU on its own.

Does an ADU count toward my home's square footage in an appraisal?

Usually not. For loans sold to Fannie Mae, a detached ADU's living area isn't added to the main home's above-grade square footage. The appraiser reports it on a separate line and has to analyze its effect on value and marketability. An ADU inside the main house with interior access and above grade can be the exception.

Can a buyer use ADU rent to qualify for a mortgage?

Under Fannie Mae's rules, rental income can count from one existing ADU only, on a purchase or limited cash-out refinance, and it's capped at 30% of the buyer's total qualifying income. Other loan programs have their own rules, so buyers should ask their lender early.

My property has two ADUs. Is that a problem when I sell?

State law requires most cities and counties to allow two ADUs per lot inside urban growth areas, but Fannie Mae permits only one ADU on a one-unit property. A buyer using a loan sold to Fannie Mae may not be able to treat both units as ADUs. Ask a lender how your property would be classified before you list.

What do I have to disclose about an ADU?

Washington's seller disclosure form (RCW 64.06.020) asks whether there have been conversions, additions or remodeling and whether all building permits and final inspections were obtained. It also asks whether title is subject to a lease or rental agreement. Answer from your records, and attach permit documents where you can.

Talk it through with a local agent

Peter Kim and Odigo Real Estate Club can review your permits, help you describe the unit accurately in the listing, and price it against real closed sales, with 25+ years combined experience, 593+ closings, and 122 five-star Zillow reviews. See our home selling services or request a property valuation.

Odigo Real Estate Club · REAL Brokerage, WA #112064 · 16108 Ash Way, Suite 201, Lynnwood, WA 98087 · (425) 409-3823 · Mon–Sat 9 AM–6 PM, closed Sunday

Sources

• Washington State Legislature, RCW 36.70A.681 (accessory dwelling units; limits on local regulation, including two ADUs per lot inside urban growth areas and separate condominium sale), https://app.leg.wa.gov/RCW/default.aspx?cite=36.70A.681 , accessed Oct 30, 2026.
• Washington State Department of Commerce, Accessory Dwelling Units (ADUs) (what most cities and counties must allow inside urban growth areas; check with your local planning department; landlord-tenant law applies if you rent), https://www.commerce.wa.gov/growth-management/housing-planning/adus/ , accessed Oct 30, 2026.
• Fannie Mae Selling Guide, B2-3-04, Special Property Eligibility Considerations (one ADU per one-unit property; required separate features; zoning), https://selling-guide.fanniemae.com/sel/b2-3-04/special-property-eligibility-considerations , accessed Oct 30, 2026.
• Fannie Mae Selling Guide, B4-1.3-05, Improvements Section of the Appraisal Report (ADU living area reported on a separate line; one-unit-with-ADU versus two- to four-unit), https://selling-guide.fanniemae.com/sel/b4-1.3-05/improvements-section-appraisal-report , accessed Oct 30, 2026.
• Fannie Mae Selling Guide, B3-3.8-02, Rental Income: Subject Property (ADU rental income limits), https://selling-guide.fanniemae.com/sel/b3-3.8-02/rental-income-subject-property , accessed Oct 30, 2026.
• Washington State Legislature, RCW 64.06.020 (seller disclosure statement form; version effective until Jan 1, 2027 and version effective Jan 1, 2027), https://app.leg.wa.gov/RCW/default.aspx?cite=64.06.020 , accessed Oct 30, 2026.
• Washington State Legislature, RCW 59.18.650 (causes for ending a residential tenancy, including an owner’s election to sell a single-family residence with at least 90 days’ written notice), https://app.leg.wa.gov/RCW/default.aspx?cite=59.18.650 , accessed Oct 30, 2026.

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