An accessory dwelling unit (ADU) is one of the smartest investments a Washington homeowner can make in 2026. Whether you call it a backyard cottage, mother-in-law suite, garage conversion, or detached accessory dwelling unit (DADU), this second home on your lot can generate steady rental income, house family, and add 15–35% to your property value. At NW ADU Builders, we design, permit, and build ADUs and DADUs across Seattle, Tacoma, Bellevue, Everett, Issaquah, and the wider Puget Sound region — all under one fixed-price contract.
What Is the Difference Between an ADU and a DADU?
An ADU is a self-contained living space attached to or inside your existing home — a finished basement, a garage conversion, or a small addition. A DADU is a standalone backyard cottage, fully separate from the main house. DADUs command higher rent and more privacy; attached ADUs cost less and build faster. Both are now permitted by right on most single-family lots thanks to Washington State law.
How Has HB 1337 Changed the Rules in 2026?
Washington's House Bill 1337 took full effect statewide and reshaped what homeowners can build. Today you can place up to two ADUs per lot, you are no longer required to live on-site, and cities cannot require off-street parking. Lots backing onto an alley can build with a zero rear setback. In many jurisdictions, a DADU can even be subdivided and sold separately with its own title — turning your backyard into an independent property.
How Much Does an ADU Cost in Seattle, Tacoma & Bellevue?
Costs vary by city, site, and finish level. As a 2026 benchmark: garage and basement conversions run roughly $100,000–$200,000, while detached backyard cottages range from $250,000–$650,000. Bellevue and Issaquah trend higher due to sloped lots and design-review standards; Tacoma and Everett are among the most affordable, with Tacoma offering four pre-approved DADU plans that cut review time to just weeks.
What Rental Income and ROI Can You Expect?
In Puget Sound's tight rental market, a well-built DADU can rent for $2,000–$2,500+ per month, while an attached ADU typically earns $1,500–$1,800. After vacancy and maintenance, many Seattle-area owners net around $27,000 annually. Combined with strong appreciation and the value boost ADUs add at resale, the long-term return is hard to beat.
Why Build With NW ADU Builders?
We are a licensed, bonded, and insured design-build team with 93+ ADUs completed across the region. We handle everything — feasibility, design, permitting, and construction — on a transparent fixed-price contract, so there are no surprises. From a Tacoma backyard cottage to a Bellevue in-law suite or an Everett garage conversion, we make the process simple, fast, and fully compliant.
Ready to unlock your property's hidden income? Book your free consultation and we'll tell you exactly what you can build on your lot.