ADU & DADU Construction in Seattle & the Puget Sound
Accessory Dwelling Units and Detached ADUs built to Passive House standards in Seattle.
ADU and DADU Planning Snapshot for Seattle Homeowners
ADU construction in Seattle usually starts with a lot feasibility review, then design, permits, utility planning, construction and inspections. Attached ADUs often cost less because they reuse existing structure; detached ADUs usually cost more because they need a separate foundation, roof, utilities and site work.
- Common attached ADU planning range: $85,000 to $280,000 depending on the existing space.
- Common DADU planning range: $175,000 to $550,000 depending on size, access and finishes.
- Typical planning window: 3 to 9 months for design and permits, then 8 to 18 months for construction.
About Our ADU & DADU Construction Services
Seattle’s ADU-friendly zoning makes it one of the best cities in the country for backyard cottages and Detached Accessory Dwelling Units (DADUs). Love Construction builds ADUs to Passive House standards—super-insulated, airtight, and incredibly efficient—so your rental unit, guest house, or aging-in-place suite costs almost nothing to heat and cool. These small, efficient buildings are the perfect canvas for Passive House construction: manageable budgets, incredible performance, and they pay for themselves through rental income.
- Detached ADUs (DADUs) and backyard cottages up to 1,000 sq ft
- Attached ADUs including basement conversions and garage conversions
- Passive House performance with ultra-low energy costs
- Full kitchens, bathrooms, and living spaces designed for independent living
- Aging-in-place design with accessible features and single-floor layouts
- Rental-ready finishes with separate entrances and utilities
Our ADU & DADU Construction Process
Site & Zoning Review
We evaluate your property for ADU feasibility, review zoning requirements, setbacks, lot coverage limits, and utility connections.
Design & Energy Modeling
We design your ADU to maximize livability within the allowed footprint while achieving Passive House energy performance.
Permitting
We handle all City of Seattle permits, including building, mechanical, electrical, and plumbing. We navigate the ADU-specific requirements.
Passive House Construction
Our crew builds your ADU with the same precision and attention to airtightness and insulation as our full-size Passive House projects.
Testing & Handover
Blower door testing, energy performance verification, and complete walkthrough. We hand over a move-in ready space with your 5-year labor warranty.
ADU & DADU Construction Service Areas
We provide adu & dadu construction services throughout Seattle & the Puget Sound. Click on a location to learn more about our services in your area.
Frequently Asked Questions
What is a DADU vs. an ADU?
An ADU (Accessory Dwelling Unit) is a smaller, self-contained living unit on a property with a primary home. A DADU (Detached Accessory Dwelling Unit) is a freestanding structure—a backyard cottage. Seattle allows both types on most single-family lots.
How much does an ADU cost to build in Seattle?
ADU costs in Seattle vary by type. Attached ADUs often range from $85,000-$280,000 depending on the existing basement, garage, or addition scope. New DADUs commonly range from $175,000-$550,000 depending on size, site access, utility work, finishes, and whether Passive House performance is included.
How long does ADU construction take in Seattle or King County?
Attached ADUs commonly take 8-12 months from contract to certificate of occupancy. New DADUs often take 12-18 months because they require separate foundation, utility, structural, permit, and inspection work. City review time varies by Seattle, SeaTac, Bellevue, Kirkland, Renton, and other King County jurisdictions.
What are Seattle’s ADU rules?
Seattle allows both an attached ADU and a detached ADU (DADU) on most single-family lots—meaning up to three dwelling units on one property. DADUs can be up to 1,000 sq ft. We handle all zoning research and permitting.
Can a Passive House ADU really have near-zero energy costs?
Yes. A well-built Passive House ADU in Seattle’s mild climate can have total heating and cooling costs under $20/month. Many of our ADU clients add a small solar array to achieve true Net Zero.
Ready for Your ADU & DADU Construction Project?
Contact Love Construction today for a free consultation.