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Contra Costa County, California

Shrink-swell clay limits houses without basements on 48% of Contra Costa County's mapped land, a higher share than 90% of US counties and the 3rd highest of 54 California counties. A shallow water table limits basements on 11%.

None of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 178 flood losses here since 2016, 12% of them in buildings with basements. 53% of homes were built before 1980, against 50% nationally; the median home dates from 1978.

Data as of October 13, 2026 · sources

48%

expansive clay: shrink-swell limits slab homes

11%

high water table: limits basements

0%

in drought, October 13, 2026

53%

of homes built before 1980

Soil under Contra Costa County

Expansive clay (limits slab homes)48.0%
High water table (limits basements)11.4%
Septic fields very limited74.9%
Clay swell (LEP, by area)6.9%
High-swell land (LEP 6%+)46.8%
Urban land, not rated1.6%
Mapped soil511,635 acres
Soil seriesShare hereSeries nationally: expansive clay
Los Osos9%n/a
Altamont7%n/a
Diablo6%n/a
Millsholm5%n/a
Los Gatos5%n/a
Clear Lake4%n/a
Alameda County: 38%Solano County: 50%San Joaquin County: 30%San Mateo County: 10%Marin County: 20%Napa County: 14%Santa Clara County: 24%Sacramento County: 16%Contra Costa County AlamedaSolanoSan JoaquinSan MateoMarinNapaSanta ClaraSacramentoContra Costa
Contra Costa 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 CA013 (2025-09-02)

What the numbers mean here

Clay swell averages a linear extensibility of 6.9% by land area here, and 47% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 48% of the mapped land.

Rain over the 12 months to August 2026: 15.34 in, 3.97 in below the 1991-2020 average year of 19.31 in.

USDA rates 75% 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%
D1 moderate drought or worse0%
D2 severe or worse0%
D3 extreme or worse0%
D4 exceptional0%

Flood losses since 2016

178 flood losses since 2016, $5.6 million paid. The worst year was 2017, with 60. Foundations of the buildings, where recorded: 12% basements, 45% slabs, 30% crawl spaces, 14% elevated. Median water depth outside the building: 6 in.

YearFlood losses
20168
201760
20183
201912
20200
202118
202238
202330
20245
20253
2026 to date1

Rain

Average year, 1991-202019.31 in
12 months to August 202615.34 in (-3.97)
202514.16 in (-5.15)
Wettest year since 18951983: 38.43 in
Driest year since 18952013: 5.27 in
Days with 1 inch or more, normal at MT DIABLO JUNCTION (3 mi away)6

Homes

Housing units429,260
Median year built1978
Built before 198053%
Built 2000 or later19%
Owner-occupied68%
Median value, owner-occupied$866,800
Single-family detached68%

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
Alameda County, CA 19 mi 38% 8%
Solano County, CA 25 mi 50% 21%
San Joaquin County, CA 37 mi 30% 23%
San Mateo County, CA 41 mi 10% 3%
Marin County, CA 44 mi 20% 6%
Napa County, CA 46 mi 14% 6%
Santa Clara County, CA 50 mi 24% 3%
Sacramento County, CA 50 mi 16% 26%

Sources: USDA SSURGO (September 2, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status