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Snohomish County, Washington
A shallow water table limits basements on 72% of Snohomish County's mapped land, a higher share than 83% of US counties and the highest of 39 Washington counties. Shrink-swell clay limits houses without basements on 1%.
The soil survey maps 47% of the county's land area; these shares describe the mapped part. 63.7% of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 175 flood losses here since 2016, 18% of them in buildings with basements. 35% of homes were built before 1980, against 50% nationally; the median home dates from 1990.
Data as of October 13, 2026 · sources
expansive clay: shrink-swell limits slab homes
high water table: limits basements
in drought, October 13, 2026
of homes built before 1980
Soil under Snohomish County
| Expansive clay (limits slab homes) | 0.9% |
| High water table (limits basements) | 71.7% |
| Septic fields very limited | 89.7% |
| Clay swell (LEP, by area) | 1.7% |
| High-swell land (LEP 6%+) | 0.9% |
| Urban land, not rated | 2.3% |
| Mapped soil | 624,730 acres |
| Soil series | Share here | Series nationally: high water table |
|---|---|---|
| Tokul | 20% | n/a |
| Alderwood | 14% | n/a |
| Elwell | 6% | n/a |
| Pastik | 4% | n/a |
| Puget | 4% | n/a |
| Everett | 4% | n/a |
How we count · survey areas WA661 (2025-08-28)
What the numbers mean here
On 72% of Snohomish 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: 86.67 in, 1.58 in above the 1991-2020 average year of 85.09 in.
USDA rates 90% of the land very limited for septic absorption fields.
Drought
63.7% of the county is in drought (D1 or worse) on the October 13, 2026 map. Part of the county was in drought in 24 of the last 52 weeks; the most was 100% of its area, in 7 weeks, most recently on the December 2, 2025 map.
| Category, October 13, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 100% |
| D1 moderate drought or worse | 63.7% |
| D2 severe or worse | 3.1% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
175 flood losses since 2016, $5.8 million paid. The worst year was 2025, with 73. Foundations of the buildings, where recorded: 18% basements, 20% slabs, 29% crawl spaces, 33% elevated. Median water depth outside the building: 21 in.
| Year | Flood losses |
|---|---|
| 2016 | 20 |
| 2017 | 3 |
| 2018 | 9 |
| 2019 | 6 |
| 2020 | 18 |
| 2021 | 5 |
| 2022 | 8 |
| 2023 | 30 |
| 2024 | 0 |
| 2025 | 73 |
| 2026 to date | 3 |
Rain
| Average year, 1991-2020 | 85.09 in |
| 12 months to August 2026 | 86.67 in (+1.58) |
| 2025 | 78.48 in (-6.61) |
| Wettest year since 1895 | 1990: 106.80 in |
| Driest year since 1895 | 1952: 47.50 in |
| Days with 1 inch or more, normal at STARTUP 1 E (13 mi away) | 14.8 |
Homes
| Housing units | 331,530 |
| Median year built | 1990 |
| Built before 1980 | 35% |
| Built 2000 or later | 32% |
| Owner-occupied | 68% |
| Median value, owner-occupied | $696,000 |
| Single-family detached | 64% |
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 |
|---|---|---|---|
| Skagit County, WA | 28 mi | under 1% | 46% |
| King County, WA | 40 mi | under 1% | 38% |
| Island County, WA | 42 mi | 0% | 65% |
| Kitsap County, WA | 50 mi | under 1% | 65% |
| Whatcom County, WA | 55 mi | 1% | 25% |
| Chelan County, WA | 55 mi | under 1% | 2% |
| San Juan County, WA | 69 mi | 0% | 47% |
| Pierce County, WA | 72 mi | 2% | 34% |
Sources: USDA SSURGO (August 28, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status