What Contra Costa County, CA's soil rates for a septic system

Very limited

84.4% of Contra Costa County, CA's surveyed soil comes back "Very limited" in USDA's own SSURGO data, the county's dominant condition for a septic absorption field. A standard gravity system routinely fails on ground like this; the same share of the county falls in USDA's worst absorption-field bucket outright. Contra Costa County covers 716.901 square miles and counts 1,172,607 residents.

This is a county figure, not a parcel figure

Nothing about Contra Costa County, CA's SSURGO figure applies to a specific address - it's a county-wide average. A site-specific perc test, usually $750-$1,850, is what a health department will actually require before permitting a lot in Contra Costa County.

System cost, conventional vs. engineered

Given Contra Costa County's rating, plan for the engineered-system range rather than the conventional one. The engineered-system range, $10,000-$40,000 (mound or aerobic treatment), is the more realistic number to plan around, against $3,000-$8,000 for a conventional system on the rare lot that passes.

Where this sits nationally

Nationally, Contra Costa County, CA is the 1,740-most-constrained of 3,058 rated counties for septic soil. Within California alone it ranks 44 of 57.

Further reading

How this rating is built, and its real limits · Conventional vs. engineered septic systems