Piatt County's SSURGO septic rating
Very limited
USDA puts Piatt County, IL at "Very limited" for septic absorption fields, the dominant reading across 97.7% of the county's soil. Practically, that rules out a simple gravity system for most of the county - 97.7% of it sits in USDA's most-constrained bucket. The county spans 439.188 square miles, home to 16,628 people.
What this number does and doesn't tell you
Two lots a mile apart in Piatt County can carry different soil, even though they share one countywide rating. Budget $750-$1,850 for a real perc test on any specific parcel - that's the only figure a permit application accepts.
Piatt County: conventional vs. engineered cost
97.7% of Piatt 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.
How Piatt County, IL compares
Among the 3,058 rated counties on this site, Piatt County, IL ranks 292 most-constrained nationally, and 14 of 101 within Illinois.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems