Estimate tool
Project cost
calculator
Configure your project below and see a live structural cost range. Based on Evercrest construction rates and 2025–2026 Seattle & King County data.
Finish materials — flooring, kitchen, fixtures, appliances — are not included in this estimate and are quoted separately at consultation.
Project type
Home Extension
Add a new wing, level, or room to your existing home
$350 – $450 / sq ft
ADU — Attached
Self-contained unit attached to or inside your home
$350 – $450 / sq ft
DADU — Detached
Standalone backyard cottage, no shared walls
$380 – $480 / sq ft
Layout
Studio
288–450 sf
1 Bed
450–700 sf
2 Bed
700–1,000 sf
3 Bed
1,000–1,400 sf
Square footage
Bathrooms
1 bath
1.5 bath
2 bath
Additional bathrooms increase plumbing and layout complexity.
Site conditions
Flat lot
Moderate slope
Steep / ECA
Slope and soil conditions heavily affect foundation cost.
Structural add-ons
Covered deck / patio+$18K – $32K
Garage — 1 car+$42K – $68K
Garage — 2 cars+$65K – $105K
Two-story structure+17% to base
In-unit laundry+$5K – $10K
Home office / flex room+$12K – $22K
Financing
HELOC Rate
7.2% APR
Loan Term
20years
Down Payment
0%
Monthly Payment
—/mo
Avg HELOC: 7.18% (Mar 2026)
Your estimate
Seattle ADU owners earn an average of $24,000–$38,000/year in rental income. With 94% occupancy and no owner-occupancy requirement, your ADU starts working for you from day one.
Estimated Construction Cost
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Excludes finish materials · Permit fees shown separately below
City permit fees (paid directly to SDCI): $9,000 – $38,000
Not collected by Evercrest. Shown as an estimate only.
Not collected by Evercrest. Shown as an estimate only.
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Est. Monthly Rent
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Property Value Increase
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Break-even
(with appreciation)
(with appreciation)
Included in estimate
Construction labor & materials
Foundation & site work
Framing & structural
Rough plumbing & electrical
Utility connections
Insulation & drywall
Roofing & siding
Design & engineering
Not included
Flooring (hardwood, LVP, tile)
Cabinets & countertops
Fixtures & faucets
Appliances
Paint & wall finishes
Lighting fixtures
Permit fees (paid to SDCI)
Disclaimer: This calculator is for educational and informational purposes only. It does not constitute financial, investment, legal, or tax advice. All figures shown are estimates based on general market data and actual Evercrest project history. Actual results will vary based on site conditions, material selections, and market changes. Consult with qualified professionals before making financial decisions.
