Overview
A Detached Accessory Dwelling Unit (DADU) is a self-contained living space built as a separate structure on your property. In Seattle and King County, DADUs are the most popular ADU type for homeowners looking to maximize rental income, create multigenerational living arrangements, or build long-term property value.
Under Washington's HB 1337 and Seattle's SMC 23.44.041, most residential lots in the city can now support up to two ADUs — meaning you could potentially build a DADU in your backyard alongside a basement conversion in your main home.
Our team handles every phase: site analysis, architectural design, permitting, and construction. Our permit specialist navigates Seattle DCI or King County DPER, and our build team coordinates construction — delivering your project through design, permitting, and build with clear communication at every stage.
What's Included
- Custom architectural design optimized for your lot and setbacks
- Full permitting through Seattle DCI or King County DPER
- Foundation, framing, plumbing, electrical, and finish work
- Kitchen, bathroom, and laundry hookups included
- Energy-efficient construction (qualifies for 30% federal tax credit)
- Separate utility connections where required
- Final inspection coordination and certificate of occupancy
Explore your options: Not sure if detached is right for you? Read our DADU vs ADU comparison or compare with an attached ADU for potentially lower costs. Also see backyard cottage options in Seattle.
See the full cost breakdown for King County, our cost per square foot guide, or visit our pricing page.
Planning your project? Review the construction timeline, explore financing options, and calculate your potential rental income.
Popular neighborhoods for DADUs: Ballard, Capitol Hill, Columbia City, and Columbia City.
Frequently Asked Questions
How much does a detached ADU cost in Seattle?
Detached ADU construction in Seattle and King County typically ranges from $450,000 to $800,000+, depending on size (up to 1,000 sq ft standard under SMC 23.44.041), finishes, site complexity, and utility connections. Seattle permit and impact fees add $20,000–$50,000, and SPU utility connections add $15,000–$40,000. Our feasibility study includes a preliminary cost estimate specific to your property.
What is the maximum size for a detached ADU in Seattle?
Under SMC 23.44.041, the standard maximum is 1,000 sq ft for a DADU in SF5000 zones. Projects exceeding this threshold may be possible through SEPA review or design review processes. The state minimum floor of 1,000 sq ft per HB 1337 means Seattle cannot restrict below this size.
What are the setback requirements for a DADU?
In Seattle SF5000 zones, DADUs require a minimum 5-foot setback from side and rear property lines per SMC 23.44.041. Front setbacks match the primary structure requirements. Setbacks can vary by zone and lot configuration — our team analyzes your specific parcel during the feasibility study.
Do I need a separate driveway for my detached ADU?
No. The ADU and primary residence can share one access (driveway). However, the main entrance to the ADU must have direct access to a street via a lighted pedestrian path, driveway, or alley.
Can I build near the Shoreline Management Act area?
Yes, but properties near shorelines — including Lake Washington, Lake Union, and Green Lake — require Shoreline Management Act (SMA) review in addition to standard DCI permits. SMA review typically adds 4–8 weeks to the permitting timeline. Our team identifies SMA jurisdiction during the initial feasibility study.
