HomeCaliforniaAlameda County
Alameda County, California
Shrink-swell clay limits houses without basements on 38% of Alameda County's mapped land, a higher share than 86% of US counties and the 6th highest of 54 California counties. A shallow water table limits basements on 8%.
None of the county is in drought on the September 22, 2026 Drought Monitor map. FEMA's flood data list 123 flood losses here since 2016, 20% of them in buildings with basements. 65% of homes were built before 1980, against 50% nationally; the median home dates from 1969.
Data as of September 22, 2026 · sources
expansive clay: shrink-swell limits slab homes
high water table: limits basements
in drought, September 22, 2026
of homes built before 1980
Soil under Alameda County
| Expansive clay (limits slab homes) | 38.4% |
| High water table (limits basements) | 8.1% |
| Septic fields very limited | 67.2% |
| Clay swell (LEP, by area) | 5.8% |
| High-swell land (LEP 6%+) | 35.4% |
| Urban land, not rated | 3.6% |
| Mapped soil | 530,244 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Vallecitos | 8% | n/a |
| Los Osos | 7% | n/a |
| Millsholm | 6% | n/a |
| Altamont | 5% | n/a |
| Diablo | 5% | n/a |
| Los Gatos | 4% | n/a |
How we count · survey areas CA609 (2025-09-02), CA610 (2025-08-29)
What the numbers mean here
Clay swell averages a linear extensibility of 5.8% by land area here, and 35% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 38% of the mapped land.
Rain over the 12 months to August 2026: 18.28 in, 0.82 in below the 1991-2020 average year of 19.10 in.
USDA rates 67% of the land very limited for septic absorption fields.
Drought
0% of the county is in drought (D1 or worse) on the September 22, 2026 map. No part of the county was in drought in the last 52 weeks.
| Category, September 22, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 0% |
| D1 moderate drought or worse | 0% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
123 flood losses since 2016, $2.3 million paid. The worst year was 2022, with 43. Foundations of the buildings, where recorded: 20% basements, 27% slabs, 33% crawl spaces, 19% elevated. Median water depth outside the building: 7 in.
| Year | Flood losses |
|---|---|
| 2016 | 9 |
| 2017 | 14 |
| 2018 | 1 |
| 2019 | 4 |
| 2020 | 1 |
| 2021 | 8 |
| 2022 | 43 |
| 2023 | 35 |
| 2024 | 5 |
| 2025 | 2 |
| 2026 to date | 1 |
Rain
| Average year, 1991-2020 | 19.10 in |
| 12 months to August 2026 | 18.28 in (-0.82) |
| 2025 | 15.58 in (-3.52) |
| Wettest year since 1895 | 1983: 40.14 in |
| Driest year since 1895 | 2013: 5.08 in |
| Days with 1 inch or more, normal at LIVERMORE (6 mi away) | 1.8 |
Homes
| Housing units | 637,212 |
| Median year built | 1969 |
| Built before 1980 | 65% |
| Built 2000 or later | 15% |
| Owner-occupied | 54% |
| Median value, owner-occupied | $1.1 million |
| Single-family detached | 51% |
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 |
|---|---|---|---|
| Contra Costa County, CA | 19 mi | 48% | 11% |
| San Mateo County, CA | 30 mi | 10% | 3% |
| Santa Clara County, CA | 31 mi | 24% | 3% |
| San Joaquin County, CA | 40 mi | 30% | 23% |
| Solano County, CA | 43 mi | 50% | 21% |
| Santa Cruz County, CA | 44 mi | 10% | 1% |
| Stanislaus County, CA | 50 mi | 24% | 7% |
| Marin County, CA | 54 mi | 20% | 6% |
Sources: USDA SSURGO (September 2, 2025) · US Drought Monitor (September 22, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status