Mercer Island, WA · DADU
DETACHED ADU Builder in Mercer Island, WA
A detached accessory dwelling unit (DADU) is a fully independent residence on your existing single-family lot. We design and build DADUs that meet rental code, hold long-term value, and look like they belong — not like a kit dropped in the yard. Every project starts with a feasibility study and ends with a turn-key handover.
Build timeline
6–8 months
Typical sqft
400–1,000 sqft
Mercer Island cost
$370K – $599K
Permit window
12–16 wk
What detached adu looks like in Mercer Island
DETACHED ADU projects in Mercer Island are shaped by local zoning (R-8.4 / R-9.6 / R-12), a lot minimum of 8,400 sqft, and the kinds of sites we see day-to-day: North End, First Hill, Mid-Island. Below is what every Mercer Island DADU project from Golden State includes — fixed-price, one contract, one warranty.
- ✓Stamped architectural set
- ✓Structural + MEP engineering
- ✓Permit submittal & city liaison
- ✓Foundation, framing, weather envelope
- ✓Finish carpentry, cabinetry, fixtures
- ✓Final inspection + warranty package
Other services in Mercer Island
More from Golden State ADU in Mercer Island
AADU
ATTACHED ADU in Mercer Island
Seamlessly integrated additions — basement, ground-floor, or wing conversions that maintain the architectural integrity of your existing home.
Garage
GARAGE CONV. in Mercer Island
Convert an under-used garage into a high-end studio, office, or rental quarter without surrendering your driveway.
Additions
MODERN ADDITIONS in Mercer Island
Structural expansions for growing families — second stories, primary suites, kitchen extensions. Steel, glass, and Pacific Northwest materials.
GC
GENERAL CONTRACTING in Mercer Island
Full-service general contracting for remodels, renovations, and ground-up custom homes throughout the Puget Sound region.
Feasibility
FEASIBILITY STUDIES in Mercer Island
Before you commit, we analyze zoning, lot coverage, setbacks, utilities, and ROI so you have a real number — not a guess.
DETACHED ADU across the Puget Sound
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Start your Mercer Island DADU feasibility
$1,500 buys lot analysis, Mercer Island zoning verification, and a fixed price you can build a budget on.
Request Mercer Island feasibilityFrequently asked
What is included in DETACHED ADU in Mercer Island?
Our DETACHED ADU in Mercer Island scope includes design through completion: feasibility, schematic design, construction documents, permit submittal and review management, construction with one self-performed framing crew plus L&I-licensed MEP subcontractors, all required WA inspections, certificate of occupancy, and warranty. One contract, one license number, one warranty — not a coordination headache across five vendors.
How is DETACHED ADU in Mercer Island different from a generic remodel?
DETACHED ADU in Mercer Island carries code, permit, and construction details that a one-off remodeler hasn't repeated enough times to optimize. Examples: IECC R406 envelope path selection, fire separation when units are within 5 ft, side-sewer slope requirements, IRC Appendix AQ when permitted as a tiny house, separate utility metering rules. We do this scope every month — repetition is what makes our first-cycle approval rate and our schedule reliability stand out.
What is the typical price range for DETACHED ADU in Mercer Island?
DETACHED ADU in Mercer Island pricing in 2026 lands in the $295–$465/sqft all-in range depending on size, lot conditions, finish tier, and AHJ. The wide range is real — small lots with difficult access, soils requiring pier-and-beam, or service upgrades push the upper end; flat lots with good access and a catalog plan land at the lower end. Our feasibility report gives you a ±15% number tailored to your specific lot in two weeks.