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San Francisco County, California
USDA's soil survey flags shrink-swell clay on 0% of San Francisco 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 18 flood losses here since 2016. 77% of homes were built before 1980, against 50% nationally; the median home dates from 1946.
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 Francisco County
| Expansive clay (limits slab homes) | 0.0% |
| High water table (limits basements) | 3.4% |
| Septic fields very limited | 33.3% |
| Clay swell (LEP, by area) | 3.2% |
| High-swell land (LEP 6%+) | 3.4% |
| Urban land, not rated | 31.0% |
| Mapped soil | 45,705 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Sirdrak | 9% | n/a |
| Candlestick | 1% | n/a |
| Kron | under 1% | n/a |
| Barnabe | under 1% | n/a |
| Buriburi | under 1% | n/a |
How we count · survey areas CA689 (2025-08-29)
What the numbers mean here
Rain over the 12 months to August 2026: 22.83 in, 2.64 in below the 1991-2020 average year of 25.47 in.
With shrink-swell clay on 0% and a shallow water table on 3% of the land, the soil survey points to neither problem across most of San Francisco 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
18 flood losses since 2016, $1.1 million paid. The worst year was 2019, with 5.
| Year | Flood losses |
|---|---|
| 2016 | 0 |
| 2017 | 1 |
| 2018 | 0 |
| 2019 | 5 |
| 2020 | 0 |
| 2021 | 0 |
| 2022 | 4 |
| 2023 | 3 |
| 2024 | 4 |
| 2025 | 1 |
| 2026 to date | 0 |
Rain
| Average year, 1991-2020 | 25.47 in |
| 12 months to August 2026 | 22.83 in (-2.64) |
| 2025 | 20.78 in (-4.69) |
| Wettest year since 1895 | 1983: 49.30 in |
| Driest year since 1895 | 2013: 6.21 in |
| Days with 1 inch or more, normal at MUIR WOODS (28 mi away) | 11.5 |
Homes
| Housing units | 414,602 |
| Median year built | 1946 |
| Built before 1980 | 77% |
| Built 2000 or later | 14% |
| Owner-occupied | 38% |
| Median value, owner-occupied | $1.4 million |
| Single-family detached | 18% |
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 |
|---|---|---|---|
| Marin County, CA | 27 mi | 20% | 6% |
| San Mateo County, CA | 42 mi | 10% | 3% |
| Sonoma County, CA | 55 mi | 22% | 5% |
| Contra Costa County, CA | 60 mi | 48% | 11% |
| Alameda County, CA | 62 mi | 38% | 8% |
| Napa County, CA | 66 mi | 14% | 6% |
| Solano County, CA | 70 mi | 50% | 21% |
| Santa Cruz County, CA | 75 mi | 10% | 1% |
Sources: USDA SSURGO (August 29, 2025) · US Drought Monitor (October 13, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status