Cook County, IL: the USDA septic-suitability picture

Very limited

USDA puts Cook County, IL at "Very limited" for septic absorption fields, the dominant reading across 65.4% of the county's soil. This is the reading that pushes a project toward an engineered system rather than a conventional one, and it covers 65.4% of Cook County's land. The county spans 944.93 square miles, home to 5,182,617 people.

What this number does and doesn't tell you

USDA's survey covers the whole of Cook County, IL at once, not any single address. Budget $750-$1,850 for a real perc test on any specific parcel - that's the only figure a permit application accepts.

Cook County: conventional vs. engineered cost

65.4% of Cook 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.

Where this sits nationally

Among the 3,058 rated counties on this site, Cook County, IL ranks 2,599 most-constrained nationally, and 86 of 101 within Illinois.

Before you act on this

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