Septic suitability in Custer County, MT
Very limited
Custer County, MT: USDA's soil survey calls 73.9% of the county "Very limited" for a septic leach field. This is the reading that pushes a project toward an engineered system rather than a conventional one, and it covers 73.9% of Custer County's land. The county spans 3,783.309 square miles, home to 11,964 people.
What this number does and doesn't tell you
USDA's survey covers the whole of Custer County, MT 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.
Custer County: conventional vs. engineered cost
73.9% of Custer 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, Custer County, MT ranks 2,335 most-constrained nationally, and 39 of 54 within Montana.
Further reading
How this rating is built, and its real limits · Conventional vs. engineered septic systems