Can I tour a project like "Mercer Island Interior"?

Yes — we typically have 6–10 ADUs in framing or finish at any time across the Puget Sound, so we can usually arrange a site walk on a project at the same stage you're planning. For completed projects, tours depend on the current owner's availability and privacy preferences. Request a tour and we'll match you to the closest active or completed project that resembles "Mercer Island Interior" in size, configuration, and AHJ. Go deeper: Read the Eastside deck cost & permits: cedar, IPE, composite (2026) guide (/guides/eastside-deck-cost-permits).

What did this project actually cost?

Each project page includes the all-in cost range for the size and configuration shown, scaled to 2026 prices. Actual contract value at time of build is confidential to the client, but the range we publish is anchored in real numbers from our bid archive — not aspirational pricing. For a tailored estimate against your lot, run our feasibility study or use the cost calculator. Go deeper: Read the Five hidden ADU costs that wreck Puget Sound budgets guide (/guides/hidden-adu-costs).

How long did this project take?

Most projects of this size and configuration run 9–14 months from contract to certificate of occupancy: 6–8 weeks of design, 8–16 weeks of permit, 18–26 weeks of construction. Schedule variance is driven by AHJ workload, owner selection pace, and field-discovered conditions during demo or excavation. We publish a Gantt at contract and update it weekly. Go deeper: Read the Eastside deck cost & permits: cedar, IPE, composite (2026) guide (/guides/eastside-deck-cost-permits).

Could you build "Mercer Island Interior" again on my lot?

Often yes — most of our projects are anchored on repeatable design DNA (envelope details, structural assemblies, MEP standard layouts) that adapt to different lot conditions with modest configuration changes. We'll review your lot's zoning, soils, and access to confirm fit. Where the original layout doesn't fit, we propose the closest workable adaptation that preserves the look you liked. Go deeper: Read the Eastside deck cost & permits: cedar, IPE, composite (2026) guide (/guides/eastside-deck-cost-permits).

What inspections did "Mercer Island Interior" go through?

Standard Puget Sound ADU inspection sequence applied: footing, underfloor MEP, framing, plumbing rough, electrical rough, mechanical rough, insulation, drywall nailing, shower pan, gas piping, final building, final plumbing, final electrical, final mechanical, and final energy verification (blower-door ≤3 ACH50). All passed with no notable callbacks — typical for our builds because we run internal QA checklists at each milestone. Go deeper: Read the Puget Sound view corridor & height restrictions on ADUs (2026) guide (/guides/puget-sound-view-corridor-restrictions).

REF_4607 · Custom Addition · Mercer Island, WA

Mercer Island Interior

Open-plan kitchen + living addition with polished concrete floors and a 12'x4' fixed skylight — Mercer Island's First Hill.

Type: Custom Addition
Sqft: 850
Beds / Baths: 0 / 1
Completed: May 2024
Duration: 30 wks
Budget: $298,000

Mercer Island Interior — Custom Addition in Mercer Island, WA

01 / BRIEF

What the owner asked for

Owners of a 1978 split-level wanted to convert the cramped dining-into-kitchen sequence into a single great room, with a new master suite stacked above. Mercer Island Code 19.02 caps lot coverage at 40% — the existing footprint was at 38%, leaving 240 sqft of horizontal expansion before triggering a variance.

02 / CHALLENGE

What stood in the way

Removing the existing kitchen's load-bearing wall meant introducing a 26' flush LVL beam, with point loads needing transfer through two stories down to a new pier in the crawl space.

03 / APPROACH

How we solved it

RDH Structural designed a triple 1-3/4" x 14" LVL with custom Simpson HUS columns at each bearing point. We used radiant in-slab heat (Uponor 1/2" PEX-A on 9" centers) in the new polished concrete and matched it to the existing perimeter baseboard with a 4-zone Taco SR504-EXP controller.

04 / OUTCOME

What got delivered

Permit in 11 weeks, build in 19 weeks. Project carried zero shoring change-orders. Appraisal post-completion came in at $284K above project cost.

SPEC_05 / FACTS

Project specification

New footprint: 240 sqft (horizontal) + 600 sqft (vertical)
Total addition: 850 sqft
Ceiling height: 10'-6" living, 9'-0" master

SCHEDULE_06 / PHASES

30-week delivery

01
Feasibility & design
4–6 wks
Site survey, zoning memo, schematic plans, owner review.
02
Construction docs
3–4 wks
Structural, MEP, energy compliance, permit-ready set.
03
Permit review
11 wks
City corrections, utility sign-offs, building permit issued.
04
Site work + foundation
3–4 wks
Erosion control, excavation, footings, slab or stem wall.
05
Framing + dry-in
5–6 wks
Floor, walls, roof, windows, weather-resistive barrier.
06
MEP rough-in + insulation
4–5 wks
Plumbing, electrical, HVAC, blower-door prep.
07
Finish + cabinetry
6–8 wks
Drywall, paint, flooring, cabinets, tile, fixtures.
08
Inspections + handover
1–2 wks
Final inspections, punch-list, 19-week build closeout.

MATERIAL_07

Materials

Floor
Polished concrete with hydronic radiant, 4" slab
Beam
Triple 1-3/4" x 14" LVL, Simpson HUS columns
Windows
Marvin Essential, ultrex, U-0.28

SYSTEM_08

Systems

Heating / cooling
Existing forced air + new hydronic radiant
Skylight
Velux FCM 4622, 12'-0" x 4'-0" fixed laminated
Kitchen
Wolf 36" DF range, Sub-Zero 36" column refrigerator

PERMIT_09 / RECORD

Permits & compliance

Permit number
Mercer Island BLD24-0319
Structural review
Engineer-stamped per IBC §1604

METRIC_10 / OUTCOME

By the numbers

+$284,000
Reassessment delta: Ff 35 / Fl 25
Slab flatness: 39.4%
Lot coverage post-build

LESSON_11 / FIELD NOTES

What we'd repeat

On Mercer Island, getting the lot coverage calc letter-perfect with the city before CDs saves a guaranteed correction cycle.
Hydronic radiant in polished slab costs roughly $11/sqft more than baseboard but it's the only system that doesn't conflict with a great-room ceiling plan.

Code · permits · cost

START YOURS

Talk to us →

(425) 642-4142

ARCHIVE_∞ / MORE

Other field reports

FAQ

Frequently asked

Can I tour a project like "Mercer Island Interior"?
Yes — we typically have 6–10 ADUs in framing or finish at any time across the Puget Sound, so we can usually arrange a site walk on a project at the same stage you're planning. For completed projects, tours depend on the current owner's availability and privacy preferences. Request a tour and we'll match you to the closest active or completed project that resembles "Mercer Island Interior" in size, configuration, and AHJ.
Go deeper: Read the Eastside deck cost & permits: cedar, IPE, composite (2026) guide
What did this project actually cost?
Each project page includes the all-in cost range for the size and configuration shown, scaled to 2026 prices. Actual contract value at time of build is confidential to the client, but the range we publish is anchored in real numbers from our bid archive — not aspirational pricing. For a tailored estimate against your lot, run our feasibility study or use the cost calculator.
Go deeper: Read the Five hidden ADU costs that wreck Puget Sound budgets guide
How long did this project take?
Most projects of this size and configuration run 9–14 months from contract to certificate of occupancy: 6–8 weeks of design, 8–16 weeks of permit, 18–26 weeks of construction. Schedule variance is driven by AHJ workload, owner selection pace, and field-discovered conditions during demo or excavation. We publish a Gantt at contract and update it weekly.
Go deeper: Read the Eastside deck cost & permits: cedar, IPE, composite (2026) guide
Could you build "Mercer Island Interior" again on my lot?
Often yes — most of our projects are anchored on repeatable design DNA (envelope details, structural assemblies, MEP standard layouts) that adapt to different lot conditions with modest configuration changes. We'll review your lot's zoning, soils, and access to confirm fit. Where the original layout doesn't fit, we propose the closest workable adaptation that preserves the look you liked.
Go deeper: Read the Eastside deck cost & permits: cedar, IPE, composite (2026) guide
What inspections did "Mercer Island Interior" go through?
Standard Puget Sound ADU inspection sequence applied: footing, underfloor MEP, framing, plumbing rough, electrical rough, mechanical rough, insulation, drywall nailing, shower pan, gas piping, final building, final plumbing, final electrical, final mechanical, and final energy verification (blower-door ≤3 ACH50). All passed with no notable callbacks — typical for our builds because we run internal QA checklists at each milestone.
Go deeper: Read the Puget Sound view corridor & height restrictions on ADUs (2026) guide

INTERNAL_LINKS / Deep Map