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Sonoma County, California

Shrink-swell clay limits houses without basements on 22% of Sonoma County's mapped land, a higher share than 72% of US counties. A shallow water table limits basements on 5%.

None of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 974 flood losses here since 2016, 1% of them in buildings with basements. 52% of homes were built before 1980, against 50% nationally; the median home dates from 1979.

Data as of October 13, 2026 · sources

22%

expansive clay: shrink-swell limits slab homes

5%

high water table: limits basements

0%

in drought, October 13, 2026

52%

of homes built before 1980

Soil under Sonoma County

Expansive clay (limits slab homes)21.6%
High water table (limits basements)4.7%
Septic fields very limited82.6%
Clay swell (LEP, by area)5.0%
High-swell land (LEP 6%+)24.1%
Mapped soil1,021,332 acres
Soil seriesShare hereSeries nationally: expansive clay
Hugo14%n/a
Laughlin6%n/a
Josephine6%n/a
Goulding5%n/a
Goldridge3%n/a
Los Gatos3%n/a
Napa County: 14%Marin County: 20%Lake County: 8%San Francisco County: 0%Solano County: 50%Yolo County: 44%Colusa County: 55%Contra Costa County: 48%Sonoma County NapaMarinLakeSan FranciscoSolanoYoloColusaContra CostaSonoma
Sonoma County (heavy outline) and nearby counties, shaded by expansive clay share.under 10%10-25%25-40%40-60%60%+

How we count · survey areas CA097 (2025-09-08)

What the numbers mean here

Clay swell averages a linear extensibility of 5.0% by land area here, and 24% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 22% of the mapped land.

Rain over the 12 months to August 2026: 36.89 in, 5.04 in below the 1991-2020 average year of 41.93 in.

USDA rates 83% of the land very limited for septic absorption fields.

Drought

0% of the county is in drought (D1 or worse) on the October 13, 2026 map. No part of the county was in drought in the last 52 weeks.

October 21, 2025Share of area in drought, weeklyOctober 13, 2026
Category, October 13, 2026Share of county
D0 abnormally dry or worse0.7%
D1 moderate drought or worse0%
D2 severe or worse0%
D3 extreme or worse0%
D4 exceptional0%

Flood losses since 2016

974 flood losses since 2016, $33.2 million paid. The worst year was 2019, with 735. Foundations of the buildings, where recorded: 1% basements, 18% slabs, 12% crawl spaces, 69% elevated. Median water depth outside the building: 53.5 in.

YearFlood losses
20165
2017121
20184
2019735
20200
20215
20221
202338
202433
202529
2026 to date3

Rain

Average year, 1991-202041.93 in
12 months to August 202636.89 in (-5.04)
202536.58 in (-5.35)
Wettest year since 18951983: 88.81 in
Driest year since 18952013: 10.12 in
Days with 1 inch or more, normal at GRATON (7 mi away)13.4

Homes

Housing units207,699
Median year built1979
Built before 198052%
Built 2000 or later18%
Owner-occupied63%
Median value, owner-occupied$815,500
Single-family detached67%

What repairs cost

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

WorkRangeSource
Foundation repair, US $2,200 to $8,200 Fixr, January 27, 2026
Pier-and-beam repair, US $2,000 to $15,000 Fixr, January 27, 2026
Foundation inspection, US $300 to $600 Fixr, January 31, 2025

Nearby counties

PlaceDistanceExpansive clayHigh water table
Napa County, CA 32 mi 14% 6%
Marin County, CA 34 mi 20% 6%
Lake County, CA 42 mi 8% 1%
San Francisco County, CA 55 mi 0% 3%
Solano County, CA 56 mi 50% 21%
Yolo County, CA 56 mi 44% 23%
Colusa County, CA 58 mi 55% 37%
Contra Costa County, CA 67 mi 48% 11%

Sources: USDA SSURGO (September 8, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status