Contra Costa County, CA Shrink-Swell Risk
What Contra Costa County, CA's soil means for a foundation
High
USDA's SSURGO data puts Contra Costa County, CA's soil at "High" shrink-swell potential, an area-weighted linear extensibility of 6.17% across 38.9% of the county's surveyed land. Ground like this needs active moisture control to keep a conventional foundation from cracking - it's not automatic either way.
This is a county figure, not a lot figure
USDA's survey covers the whole of Contra Costa County, CA at once, not any single address. The real answer for one lot is still a site-specific geotechnical report, running about $1,000-$5,000 (typically near $2,700).
Report cost vs. repair cost
Given Contra Costa County's rating, spending a few thousand now is the way to avoid spending tens of thousands later. The report ($1,000-$5,000) is what tells you whether Contra Costa County's county-level reading applies to your lot; the repair ($5,000-$15,000) is what it costs if you skip that and find out from the cracks instead.
Where this sits nationally
Nationally, Contra Costa County, CA is the 108-most-constrained of 3,085 rated counties for shrink-swell soil. Within California alone it ranks 1 of 58. Contra Costa County, CA is 717 square miles with a population of 1,172,607.
See how another county compares
Orleans Parish, LA carries this site's most-constrained reading. Passaic County, NJ and Muscatine County, IA are worth a look for comparison.
How this reading is built, and its real limits · The warning signs of expansive-clay damage, explained
More of California
Every county in California is on this site, including San Francisco County, CA and San Luis Obispo County, CA. The full underlying dataset is downloadable directly, one row per county.