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San Mateo County, California
USDA's soil survey flags shrink-swell clay on 10% of San Mateo County's mapped land and a shallow water table under basements on 3%, both below the national shares of 15% and 29%. None of the county is in drought on the October 13, 2026 Drought Monitor map.
FEMA's flood data list 240 flood losses here since 2016, 12% of them in buildings with basements. 73% of homes were built before 1980, against 50% nationally; the median home dates from 1966.
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 San Mateo County
| Expansive clay (limits slab homes) | 9.7% |
| High water table (limits basements) | 2.8% |
| Septic fields very limited | 56.6% |
| Clay swell (LEP, by area) | 4.2% |
| High-swell land (LEP 6%+) | 11.8% |
| Urban land, not rated | 9.3% |
| Mapped soil | 375,002 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Hugo | 5% | n/a |
| Lobitos | 4% | n/a |
| Josephine | 4% | n/a |
| Gazos | 4% | n/a |
| Santa Lucia | 3% | n/a |
| Butano | 3% | n/a |
How we count · survey areas CA637 (2025-09-02), CA689 (2025-08-29)
What the numbers mean here
Rain over the 12 months to August 2026: 26.27 in, 2.42 in below the 1991-2020 average year of 28.69 in.
USDA rates 57% of the land very limited for septic absorption fields.
With shrink-swell clay on 10% and a shallow water table on 3% of the land, the soil survey points to neither problem across most of San Mateo County; conditions still vary lot by lot.
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.
| Category, October 13, 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
240 flood losses since 2016, $10.3 million paid. The worst year was 2022, with 97. Foundations of the buildings, where recorded: 12% basements, 46% slabs, 35% crawl spaces, 8% elevated. Median water depth outside the building: 8.5 in.
| Year | Flood losses |
|---|---|
| 2016 | 20 |
| 2017 | 28 |
| 2018 | 6 |
| 2019 | 4 |
| 2020 | 7 |
| 2021 | 44 |
| 2022 | 97 |
| 2023 | 27 |
| 2024 | 2 |
| 2025 | 4 |
| 2026 to date | 1 |
Rain
| Average year, 1991-2020 | 28.69 in |
| 12 months to August 2026 | 26.27 in (-2.42) |
| 2025 | 23.20 in (-5.49) |
| Wettest year since 1895 | 1983: 60.24 in |
| Driest year since 1895 | 2013: 6.19 in |
| Days with 1 inch or more, normal at HALF MOON BAY (6 mi away) | 5.5 |
Homes
| Housing units | 287,184 |
| Median year built | 1966 |
| Built before 1980 | 73% |
| Built 2000 or later | 12% |
| Owner-occupied | 59% |
| Median value, owner-occupied | $1.6 million |
| Single-family detached | 55% |
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 |
|---|---|---|---|
| Alameda County, CA | 30 mi | 38% | 8% |
| Santa Cruz County, CA | 34 mi | 10% | 1% |
| Santa Clara County, CA | 40 mi | 24% | 3% |
| Contra Costa County, CA | 41 mi | 48% | 11% |
| San Francisco County, CA | 42 mi | 0% | 3% |
| Marin County, CA | 49 mi | 20% | 6% |
| Solano County, CA | 63 mi | 50% | 21% |
| San Joaquin County, CA | 70 mi | 30% | 23% |
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