Story County's SSURGO septic rating
Very limited
Story County, IA's SSURGO dominant class comes back "Very limited", covering 70.2% of the county. That reading means a conventional system is a poor bet here - 70.2% of Story County lands in the strictest "very limited" bucket USDA uses. The county spans 572.545 square miles, home to 102,498 people.
What this number does and doesn't tell you
USDA's survey covers the whole of Story County, IA 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.
Story County: conventional vs. engineered cost
70.2% of Story 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.
Story County's rank among Iowa counties
Among the 3,058 rated counties on this site, Story County, IA ranks 2,454 most-constrained nationally, and 92 of 99 within Iowa.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems