CAP_15 / HOW-TO / MERCER ISLAND
How to build an ADU in Mercer Island, WA
The full sequence in 12 honest steps — feasibility, design, 12–16 wk of permitting, foundation, framing, finish, and Certificate of Occupancy. Roughly 10 months end-to-end at $484k all-in for a typical Mercer Island 800 sqft DADU.
The 12 steps, in order
STEP_01
1wk
Feasibility & site analysis
We pull the Mercer Island parcel report, GIS overlays, and R-8.4 / R-9.6 / R-12 code to confirm setbacks, lot coverage, height limits, and any ECA/critical-area triggers. Outcome: a 1-page go/no-go with maximum buildable envelope on your Mercer Island lot.
STEP_02
3wk
Schematic design & client review
Two to three floor-plan options sized to your goals (rental income, multi-gen, home office). We pair each with an exterior massing and a target budget anchored to Mercer Island's current trade-bid environment.
STEP_03
4wk
Construction documents & engineering
Structural, MEP, energy-code (WSEC 2021), and any geotech reports required for Mercer Island (micc tree code is the strictest in the state — landmark trees ≥10" dbh are protected). Stamped CDs are what gets submitted, not preliminary plans.
STEP_04
14wk
Mercer Island permit submittal & review
Intake with Mercer Island permitting, response to correction cycles, and any required side-sewer / right-of-way / tree-protection sub-permits. Mercer Island currently averages 12–16 wk on ADU review.
STEP_05
2wk
Mobilization & site work
Erosion control, tree-protection fencing, demolition (if any), excavation, utility trenching. 8,400 sqft lot minimum applies for new DADUs.
STEP_06
2wk
Foundation
Forms, rebar, inspection, pour, cure. Stem-wall or slab-on-grade per the soils report. Standard spread footings work on most lots.
STEP_07
4wk
Framing, roof & weather-in
Floor, walls, roof, sheathing, windows, and dried-in envelope. From here the structure is protected from Puget Sound weather and trades can sequence in parallel.
STEP_08
3wk
Rough MEP & inspections
Plumbing, electrical, HVAC (typically heat-pump mini-split for Mercer Island), low-voltage. Each scope passes its rough-in inspection before insulation.
STEP_09
4wk
Insulation, drywall & exterior
WSEC-compliant insulation, vapor control, drywall hang/finish, siding (Hardie/wood/metal), and roofing. Exterior finish locks down the envelope's long-term durability.
STEP_10
4wk
Interior finish, fixtures & cabinets
Flooring, paint, doors, trim, cabinets, counters, plumbing fixtures, light fixtures, appliances. Where most of the "luxury feel" budget actually shows up.
STEP_11
1wk
Final inspections & Certificate of Occupancy
Mercer Island final building, electrical, mechanical, plumbing inspections; energy-code verification; address assignment; CofO issued. Once issued, the unit can legally be occupied or rented.
STEP_12
2wk
Punch list, warranty & lease-up
30/60/90-day touch-ups. We hand over a warranty binder (manuals, model numbers, Golden State ADU Builders Inc 2-year workmanship, 10-year structural). If renting, we provide Mercer Island comparable rent data for the listing.
Building an ADU in Mercer Island — straight answers
How long does it take to build an ADU in Mercer Island?
Roughly 10 months end-to-end on a typical Mercer Island lot — about 14 weeks of permitting (Mercer Island currently averages 12–16 wk) plus 30 weeks of design, site work, framing, and finish. Lots with ECA, geotech, or shoreline overlays add 4–8 weeks.
What does an 800 sqft DADU cost in Mercer Island?
Around $484k all-in for a mid-spec 800 sqft detached ADU in Mercer Island — design, permits, construction, utility connections. Garage conversions run 35–45% lower, attached ADUs (AADU) about 10–15% lower.
Do I need an architect, a designer, or just a contractor in Mercer Island?
Most Mercer Island ADUs under 1,000 sqft can be permitted with a stamped designer set; full architectural service is optional but valuable on tight or sloped lots. We carry both in-house, so the client doesn't pay for a coordination layer they don't need.
Go deeper: Read the Eastside deck cost & permits: cedar, IPE, composite (2026) guide
Can I live in the main house while the ADU is being built in Mercer Island?
Yes for detached ADUs (DADUs) — the main house stays habitable the entire build. Attached ADUs and garage conversions involve 2–6 weeks of louder/dustier work near the main house, but the main house remains occupiable throughout.
What's the riskiest part of building an ADU in Mercer Island?
MICC tree code is the strictest in the state — landmark trees ≥10" DBH are protected — that's where Mercer Island projects most often slip schedule or pop the contingency. We sequence geotech, arborist, and side-sewer scope before permit submittal so surprises surface during design (cheap) rather than after foundation (expensive).
What permits do I need to build an ADU in Mercer Island?
Master Use Permit (where applicable), building permit, mechanical, plumbing, electrical, side-sewer connection, and any tree/right-of-way/shoreline sub-permits. Mercer Island permitting packages most of these under a single ADU intake.
Go deeper: Read the Puget Sound view corridor & height restrictions on ADUs (2026) guide
Keep going on Mercer Island
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