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King County, Washington

A shallow water table limits basements on 38% of King County's mapped land, a higher share than 57% of US counties and the 9th highest of 39 Washington counties. Shrink-swell clay limits houses without basements on under 1%.

14.8% of the county is in drought on the September 22, 2026 Drought Monitor map, down from 43.9% a week earlier. FEMA's flood data list 471 flood losses here since 2016, 17% of them in buildings with basements. 46% of homes were built before 1980, against 50% nationally; the median home dates from 1983.

Data as of September 22, 2026 · sources

<1%

expansive clay: shrink-swell limits slab homes

38%

high water table: limits basements

15%

in drought, September 22, 2026

46%

of homes built before 1980

Soil under King County

Expansive clay (limits slab homes)0.3%
High water table (limits basements)38.0%
Septic fields very limited91.8%
Clay swell (LEP, by area)1.4%
High-swell land (LEP 6%+)0.3%
Urban land, not rated5.1%
Mapped soil1,265,362 acres
Soil seriesShare hereSeries nationally: high water table
Alderwood15%n/a
Playco6%n/a
Everett5%n/a
Pitcher4%n/a
Nimue4%n/a
Tokul4%n/a
Pierce County: 34%Snohomish County: 72%Kitsap County: 65%Kittitas County: 14%Thurston County: 34%Chelan County: 2%Island County: 65%Mason County: 45%King County PierceSnohomishKitsapKittitasThurstonChelanIslandMasonKing
King County (heavy outline) and nearby counties, shaded by high water table share.under 10%10-25%25-40%40-60%60%+

How we count · survey areas WA633 (2025-08-28), WA634 (2025-08-28), WA775 (2025-08-28)

What the numbers mean here

On 38% of King County's mapped land, soil stays saturated within about 6 feet of the surface for part of the year, which is basement depth.

Rain over the 12 months to August 2026: 77.53 in, 2.11 in above the 1991-2020 average year of 75.42 in.

USDA rates 92% of the land very limited for septic absorption fields.

Drought

14.8% of the county is in drought (D1 or worse) on the September 22, 2026 map. Part of the county was in drought in 24 of the last 52 weeks; the most was 100% of its area, in 11 weeks, most recently on the December 9, 2025 map.

September 30, 2025Share of area in drought, weeklySeptember 22, 2026
Category, September 22, 2026Share of county
D0 abnormally dry or worse100%
D1 moderate drought or worse14.8%
D2 severe or worse0%
D3 extreme or worse0%
D4 exceptional0%

Flood losses since 2016

471 flood losses since 2016, $16.6 million paid. The worst year was 2025, with 212. Foundations of the buildings, where recorded: 17% basements, 20% slabs, 38% crawl spaces, 24% elevated. Median water depth outside the building: 16 in.

YearFlood losses
201615
201723
20184
201918
2020108
202118
202249
202317
20246
2025212
2026 to date1

Rain

Average year, 1991-202075.42 in
12 months to August 202677.53 in (+2.11)
202570.83 in (-4.59)
Wettest year since 18951933: 100.66 in
Driest year since 18951952: 41.43 in
Days with 1 inch or more, normal at SNOQUALMIE FALLS (5 mi away)13.4

Homes

Housing units1,003,284
Median year built1983
Built before 198046%
Built 2000 or later30%
Owner-occupied55%
Median value, owner-occupied$859,900
Single-family detached50%

What repairs cost

Published ranges from cost guides, not quotes. More sources, and where they disagree.

WorkRangeSource
Basement waterproofing, US $3,000 to $10,000 Fixr, February 6, 2026
Sump pump installation, US $800 to $2,000 Fixr, January 31, 2025
Crawl space encapsulation, US $2,000 to $8,000 Fixr, August 29, 2025

Nearby counties

PlaceDistanceExpansive clayHigh water table
Pierce County, WA 33 mi 2% 34%
Snohomish County, WA 40 mi 1% 72%
Kitsap County, WA 41 mi under 1% 65%
Kittitas County, WA 59 mi 5% 14%
Thurston County, WA 61 mi 3% 34%
Chelan County, WA 62 mi under 1% 2%
Island County, WA 62 mi 0% 65%
Mason County, WA 64 mi under 1% 45%

Sources: USDA SSURGO (August 28, 2025) · US Drought Monitor (September 22, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status