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San Diego County, California
Shrink-swell clay limits houses without basements on 17% of San Diego County's mapped land, a higher share than 66% of US counties. A shallow water table limits basements on 1%.
The soil survey maps 59% of the county's land area; these shares describe the mapped part. None of the county is in drought on the September 22, 2026 Drought Monitor map. FEMA's flood data list 661 flood losses here since 2016, 8% of them in buildings with basements. 50% of homes were built before 1980, against 50% nationally; the median home dates from 1980.
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 San Diego County
| Expansive clay (limits slab homes) | 17.2% |
| High water table (limits basements) | 1.0% |
| Septic fields very limited | 79.2% |
| Clay swell (LEP, by area) | 3.6% |
| High-swell land (LEP 6%+) | 17.1% |
| Urban land, not rated | 2.4% |
| Mapped soil | 1,580,932 acres |
| Soil series | Share here | Series nationally: expansive clay |
|---|---|---|
| Fallbrook | 10% | n/a |
| Cieneba | 6% | n/a |
| La Posta | 6% | n/a |
| Vista | 4% | n/a |
| Redding | 3% | n/a |
| Huerhuero | 3% | n/a |
How we count · survey areas CA638 (2025-09-08)
What the numbers mean here
Clay swell averages a linear extensibility of 3.6% by land area here, and 17% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 17% of the mapped land.
Rain over the 12 months to August 2026: 11.37 in, 1.63 in below the 1991-2020 average year of 13.00 in.
USDA rates 79% 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. Part of the county was in drought in 8 of the last 52 weeks; the most was 100% of its area, in 8 weeks, most recently on the November 18, 2025 map.
| Category, September 22, 2026 | Share of county |
|---|---|
| D0 abnormally dry or worse | 58.3% |
| D1 moderate drought or worse | 0% |
| D2 severe or worse | 0% |
| D3 extreme or worse | 0% |
| D4 exceptional | 0% |
Flood losses since 2016
661 flood losses since 2016, $32.3 million paid. The worst year was 2024, with 213. Foundations of the buildings, where recorded: 8% basements, 71% slabs, 17% crawl spaces, 3% elevated. Median water depth outside the building: 8 in.
| Year | Flood losses |
|---|---|
| 2016 | 98 |
| 2017 | 101 |
| 2018 | 53 |
| 2019 | 45 |
| 2020 | 38 |
| 2021 | 9 |
| 2022 | 12 |
| 2023 | 59 |
| 2024 | 213 |
| 2025 | 9 |
| 2026 to date | 24 |
Rain
| Average year, 1991-2020 | 13.00 in |
| 12 months to August 2026 | 11.37 in (-1.63) |
| 2025 | 11.80 in (-1.20) |
| Wettest year since 1895 | 1978: 29.05 in |
| Driest year since 1895 | 1989: 4.90 in |
| Days with 1 inch or more, normal at CUYAMACA (11 mi away) | 9.3 |
Homes
| Housing units | 1,250,641 |
| Median year built | 1980 |
| Built before 1980 | 50% |
| Built 2000 or later | 21% |
| Owner-occupied | 55% |
| Median value, owner-occupied | $854,700 |
| Single-family detached | 50% |
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 |
|---|---|---|---|
| Riverside County, CA | 72 mi | 3% | 5% |
| Orange County, CA | 74 mi | 14% | 3% |
| Imperial County, CA | 82 mi | n/a | n/a |
| Los Angeles County, CA | 118 mi | 4% | under 1% |
| San Bernardino County, CA | 131 mi | 1% | under 1% |
| Ventura County, CA | 164 mi | 12% | 4% |
| La Paz County, AZ | 165 mi | 2% | under 1% |
| Yuma County, AZ | 167 mi | 1% | under 1% |
Sources: USDA SSURGO (September 8, 2025) · US Drought Monitor (September 22, 2026) · FEMA NFIP v3 (September 9, 2026) · NOAA (August 2026) · Census ACS 2020-2024. Methods · Data status