Ogle County, IL's soil, for a septic absorption field
Very limited
57% of Ogle County, IL's surveyed soil comes back "Very limited" in USDA's own SSURGO data, the county's dominant condition for a septic absorption field. Expect the ground itself to work against a plain gravity system; USDA's strictest bucket alone covers 57% of the county. Ogle County covers 758.654 square miles and counts 51,345 residents.
This is a county figure, not a parcel figure
USDA's survey covers the whole of Ogle County, IL at once, not any single address. A site-specific perc test, usually $750-$1,850, is what a health department will actually require before permitting a lot in Ogle County.
System cost, conventional vs. engineered
Given Ogle 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.
How Ogle County, IL compares
Nationally, Ogle County, IL is the 2,781-most-constrained of 3,058 rated counties for septic soil. Within Illinois alone it ranks 97 of 101.
Before you act on this
How this rating is built, and its real limits · Conventional vs. engineered septic systems