San Diego County, CA: the USDA septic-suitability picture
Very limited
San Diego County, CA's SSURGO dominant class comes back "Very limited", covering 89.8% of the county. This is the reading that pushes a project toward an engineered system rather than a conventional one, and it covers 89.8% of San Diego County's land. The county spans 4,210.884 square miles, home to 3,298,799 people.
What this number does and doesn't tell you
San Diego County's rating is an average across the whole county, not a promise about any one lot inside it. Budget $750-$1,850 for a real perc test on any specific parcel - that's the only figure a permit application accepts.
San Diego County: conventional vs. engineered cost
89.8% of San Diego County carrying a "Very limited" reading changes the realistic budget here. Conventional systems run $3,000-$8,000 where they pass; where they don't, engineered alternatives (mound or aerobic treatment, whichever the health department requires) run $10,000-$40,000 - budget toward the higher figure until a perc test says otherwise.
San Diego County's rank among California counties
Among the 3,058 rated counties on this site, San Diego County, CA ranks 1,284 most-constrained nationally, and 34 of 57 within California.
Related guides
How this rating is built, and its real limits · Conventional vs. engineered septic systems