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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%.
14.5% of the county is in drought on the November 3, 2026 Drought Monitor map. 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 November 3, 2026 · sources
expansive clay: shrink-swell limits slab homes
high water table: limits basements
in drought, November 3, 2026
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 limited | 91.8% |
| Clay swell (LEP, by area) | 1.4% |
| High-swell land (LEP 6%+) | 0.3% |
| Urban land, not rated | 5.1% |
| Mapped soil | 1,265,362 acres |
| Soil series | Share here | Series nationally: high water table |
|---|---|---|
| Alderwood | 15% | n/a |
| Playco | 6% | n/a |
| Everett | 5% | n/a |
| Pitcher | 4% | n/a |
| Nimue | 4% | n/a |
| Tokul | 4% | n/a |
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
14.5% of the county is in drought (D1 or worse) on the November 3, 2026 map. Part of the county was in drought in 24 of the last 52 weeks; the most was 100% of its area, in 5 weeks, most recently on the December 9, 2025 map.
| Category, November 3, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 100% |
| D1 moderate drought or worse | 14.5% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
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.
| Year | Flood losses |
|---|---|
| 2016 | 15 |
| 2017 | 23 |
| 2018 | 4 |
| 2019 | 18 |
| 2020 | 108 |
| 2021 | 18 |
| 2022 | 49 |
| 2023 | 17 |
| 2024 | 6 |
| 2025 | 212 |
| 2026 to date | 1 |
Rain
| Average year, 1991-2020 | 75.42 in |
| 12 months to August 2026 | 77.53 in (+2.11) |
| 2025 | 70.83 in (-4.59) |
| Wettest year since 1895 | 1933: 100.66 in |
| Driest year since 1895 | 1952: 41.43 in |
| Days with 1 inch or more, normal at SNOQUALMIE FALLS (5 mi away) | 13.4 |
Homes
| Housing units | 1,003,284 |
| Median year built | 1983 |
| Built before 1980 | 46% |
| Built 2000 or later | 30% |
| Owner-occupied | 55% |
| Median value, owner-occupied | $859,900 |
| Single-family detached | 50% |
What repairs cost
Published ranges from cost guides, not quotes. More sources, and where they disagree.
Nearby counties
| Place | Distance | Expansive clay | High 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 (November 3, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status