The source

Every county's rating comes from USDA's Soil Data Access, specifically the "ENG - Septic Tank Absorption Fields" component interpretation within the SSURGO soil survey. That's the same interpretation a local health department's own soil scientist would pull, aggregated here by mapunit acreage x component percent into one dominant rating class per county.

Why it's a county figure, not a parcel figure

SSURGO's map units are drawn at roughly 1:12,000-1:63,360 scale - useful for comparing places, not for one address. PercCheck reports the AREA-WEIGHTED DOMINANT condition across every map unit in a county's survey area(s). A real building permit still requires a site-specific percolation test; nothing here substitutes for one.

How survey areas get matched to counties

SSURGO's survey areas don't reliably encode a county FIPS code in their ID - large or complex counties are often split into named sub-areas (for example, "Los Angeles County, California, West San Fernando Valley Area"). Each survey area's free-text name is matched against every county name in its state to make that join. A survey area naming multiple counties credits its weight to every county it names, which makes multi-county areas a modest approximation rather than an exact split. 59 survey areas had no county name findable in their text at all; those counties are reported as unrated rather than assigned a guess.

"Not rated" is a real answer, not a gap

USDA's own data distinguishes "Not rated" (the dominant component - often already-developed urban land, water, or similar - simply carries no absorption-field interpretation) from a county this site's matching process never found at all. Both show up here as honestly unrated, since guessing a class for either would be worse than saying nothing.

The cost figures

Conventional system costs ($3,000-$8,000) and engineered system costs ($10,000-$40,000, covering both mound and aerobic-treatment designs) are sourced from Angi's and HomeGuide's 2026 septic-system cost guides. Perc test cost ($750-$1,850) is sourced from Bob Vila's 2026 percolation-test cost guide. These are national contractor-pricing ranges, not county-specific quotes.

Where to go next

Real counties this plays out on: Sutton County, TX, Stewart County, TN, Sheridan County, KS.

How a percolation test actually works · Conventional vs. engineered septic systems