How Much Value Does an ADU Add to Your Home in Seattle & King County? (2026)

The short answer: 20–35% for most King County homes

Homes with a permitted ADU in strong rental markets like Seattle and the Eastside typically appraise 20–35% higher than comparable homes without one. On an $900,000 Eastside home, that's roughly $180,000–$315,000 of added value — often more than the ADU cost to build. The spread depends on three things: whether the unit is detached or attached, whether it's fully permitted, and how strong ADU comparable sales are in your neighborhood. A detached DADU with its own entrance, kitchen, and laundry adds the most; an unpermitted conversion can add nothing at all, because lenders can't count it.

How appraisers actually value an ADU

Appraisers in King County use two methods. The sales comparison approach looks for recent sales of homes with ADUs nearby — increasingly easy in Seattle, where thousands of ADUs have been permitted since the 2019 reforms. The income approach capitalizes the unit's rental income, which matters most for buyers using the rent to qualify: since 2023, lenders on many loan programs can count ADU rental income toward the buyer's qualifying income, which widens your pool of buyers when you sell. Keep every permit record and final inspection sign-off — that paper trail is what lets the value count.

What ADUs rent for in 2026

Long-term ADU rents near Seattle and Bellevue commonly run $1,800–$3,000+ per month depending on size, finish, and location — studios and one-bedroom garage conversions at the lower end, new two-bedroom DADUs in Bellevue, Kirkland, and North Seattle at the top. Against a typical build cost of $150,000–$300,000 for an attached ADU or $250,000–$450,000 for a detached one (see our 2026 ADU cost guide), gross rental yields of 7–12% on construction cost are realistic — stronger than most remodel ROI in Washington.

The rules are on your side

Washington's HB 1337 requires cities in urban growth areas to allow at least two ADUs per lot, caps ADU-specific parking mandates, and bars owner-occupancy requirements for most new ADUs. Seattle allows up to two accessory units per lot (one attached, one detached), and Bellevue, Kirkland, Redmond, and Renton have all brought their codes into line. That regulatory floor is one reason ADU value has held up even as the broader market has cooled and heated — the buyer of your home is also buying a legal income stream.

What to watch before you build

Three honest caveats. Property taxes rise — the ADU is new assessed value, so budget for the county's reassessment. Over-improvement risk is real on lower-value lots: a $400,000 DADU behind a $600,000 house rarely returns its full cost at resale, which is why we price the project against your neighborhood, not a brochure. And quality of permits matters more than quality of countertops — a modest, fully-permitted unit beats a lavish unpermitted one every time.

Run the numbers on your lot

Blue Lake Construction designs and builds ADUs and DADUs across Seattle, Bellevue, Kirkland, Redmond, Sammamish, Issaquah, Renton, Mercer Island, and Kent — zoning check, design, permits, and construction in one fixed-price contract. Licensed WA GC #BLUELLC760C3. Call (206) 786-5988 or book a free consultation.