How Much Does It Cost to Finish a Basement in Mercer Island, WA?
Updated July 2026 · Sourced from the King County Price Index (updated July 2026) · Serving Mercer Island
Finishing a basement across King County — including Mercer Island — typically runs $60–$150 per square foot, averaging about $95/sf, per the King County Price Index. For an example 800 sq ft basement, that works out to roughly $48,000–$120,000, averaging around $76,000 — your exact size and finish level will change the number.
What projects cost, by scope
| Project scope | What to expect | Notes |
|---|---|---|
| Small basement ~500 sq ft, example calculation | Roughly $30,000–$75,000 | Avg. ~$47,500 |
| Medium basement ~800 sq ft, example calculation | Roughly $48,000–$120,000 | Avg. ~$76,000 |
| Large basement ~1,200 sq ft, example calculation | Roughly $72,000–$180,000 | Avg. ~$114,000 |
Methodology: The $60–$150/sf rate comes from the King County Price Index (job-band data as of July 2026, confidence: medium). Size-based totals above are illustrative calculations (rate × an example square footage) — not separately-sourced figures — since actual basements vary widely. Estimates, not quotes; an on-site walkthrough locks your price.
What drives where your project lands
- Finish level — basic drywall-and-paint vs. custom built-ins, wet bar, or theater finishes.
- Adding a bathroom — plumbing a new basement bathroom is one of the biggest cost adders.
- Egress requirements — code-required egress windows for bedrooms add cost.
- Moisture control — waterproofing and drainage work in older King County homes.
- Ceiling height & permits — low ceilings and permit complexity affect scope.
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See my range →Frequently asked questions
How much does it cost to finish a basement in Mercer Island?
Finishing a basement in Mercer Island and across King County typically runs $60 to $150 per square foot, averaging about $95/sf, per the King County Price Index (July 2026). For an example 800 sq ft basement, that works out to roughly $48,000–$120,000, averaging around $76,000.
What is the average cost per square foot to finish a basement?
About $95 per square foot on average, with most basement finishing projects in King County falling between $60 and $150 per square foot depending on finish level and scope.
How much does a typical 800 sq ft basement cost to finish?
Using the $60–$150/sf King County Price Index range, an example 800 sq ft basement works out to roughly $48,000–$120,000, averaging around $76,000. This is an illustrative calculation — your actual size and finishes will change the number.
What affects basement finishing cost the most?
Finish level (basic vs. custom), whether a bathroom is added, egress window requirements, ceiling height and permit complexity, and moisture control all move the number.
Does Prolific serve Mercer Island?
Yes — Mercer Island is one of our core King County service areas, dispatched from our Issaquah headquarters.
How do I get an exact price for my basement?
Use our free 90-second price-range tool for a private estimate with no contact info required, then book a walkthrough to lock an exact price.
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Figures are estimates based on Prolific project data and the The King County Price Index as of July 2026 — not quotes. An on-site walkthrough locks your price.