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Marin County, California

Shrink-swell clay limits houses without basements on 20% of Marin County's mapped land, a higher share than 70% of US counties. A shallow water table limits basements on 6%.

None of the county is in drought on the October 13, 2026 Drought Monitor map. FEMA's flood data list 269 flood losses here since 2016, 6% of them in buildings with basements. 75% of homes were built before 1980, against 50% nationally; the median home dates from 1967.

Data as of October 13, 2026 · sources

20%

expansive clay: shrink-swell limits slab homes

6%

high water table: limits basements

0%

in drought, October 13, 2026

75%

of homes built before 1980

Soil under Marin County

Expansive clay (limits slab homes)19.9%
High water table (limits basements)5.9%
Septic fields very limited61.4%
Clay swell (LEP, by area)4.1%
High-swell land (LEP 6%+)20.6%
Urban land, not rated2.5%
Mapped soil391,500 acres
Soil seriesShare hereSeries nationally: expansive clay
Tocaloma9%n/a
Tomales6%n/a
Saurin6%n/a
Bonnydoon5%n/a
McMullin4%n/a
Los Osos4%n/a
San Francisco County: 0%Sonoma County: 22%Napa County: 14%Contra Costa County: 48%Solano County: 50%San Mateo County: 10%Alameda County: 38%Yolo County: 44%Marin County San FranciscoSonomaNapaContra CostaSolanoSan MateoAlamedaYoloMarin
Marin 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 CA041 (2025-08-29)

What the numbers mean here

Clay swell averages a linear extensibility of 4.1% by land area here, and 21% of the land reaches USDA's high class (6% or more). Clay like that swells when wet and shrinks when dry, under 20% of the mapped land.

Rain over the 12 months to August 2026: 30.00 in, 5.79 in below the 1991-2020 average year of 35.79 in.

USDA rates 61% 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

269 flood losses since 2016, $8.3 million paid. The worst year was 2017, with 58. Foundations of the buildings, where recorded: 6% basements, 45% slabs, 33% crawl spaces, 16% elevated. Median water depth outside the building: 10 in.

YearFlood losses
201615
201758
20188
201935
20203
202140
20227
202328
202415
20253
2026 to date57

Rain

Average year, 1991-202035.79 in
12 months to August 202630.00 in (-5.79)
202529.43 in (-6.36)
Wettest year since 18951983: 73.85 in
Driest year since 18952013: 8.40 in
Days with 1 inch or more, normal at KENTFIELD (13 mi away)15.4

Homes

Housing units112,116
Median year built1967
Built before 198075%
Built 2000 or later8%
Owner-occupied65%
Median value, owner-occupied$1.5 million
Single-family detached63%

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
San Francisco County, CA 27 mi 0% 3%
Sonoma County, CA 34 mi 22% 5%
Napa County, CA 39 mi 14% 6%
Contra Costa County, CA 44 mi 48% 11%
Solano County, CA 46 mi 50% 21%
San Mateo County, CA 49 mi 10% 3%
Alameda County, CA 54 mi 38% 8%
Yolo County, CA 63 mi 44% 23%

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