What Colorado actually requires: soil pits first, perc tests maybe
A thousand people a month search for “perc test cost” — and most of them are about to order the wrong thing. Under Colorado’s statewide septic rule (Regulation 43, section 43.5 — Site and Soil Evaluation), the baseline requirement is a visual and tactile evaluation of two or more soil profile test pit excavations. The classic percolation test — pouring water in holes and timing it — is the optional supplement, run when the pits warrant it.
What a soil profile test pit is
An excavator digs a trench deep enough to expose the soil horizons — the layers a soil scientist reads like a book: texture, structure, evidence of seasonal high groundwater, depth to bedrock or another limiting layer. A qualified evaluator examines the exposed profile by sight and feel and classifies the soil, which drives the septic design (and the cost of the system your future house needs).
Larimer County publishes the most concrete spec: two pits in the proposed treatment area, roughly 40–50 feet apart, 8 feet deep or down to groundwater/bedrock, with a benched entry cut so a human can safely get in and look.
Where the perc test still fits
Percolation testing didn’t disappear — Regulation 43 allows it in addition to the two pits, to add information about the soil’s long-term acceptance rate. Some designers still run one routinely; some counties’ worksheets reference perc rates for design classes (Larimer’s engineer-design trigger kicks in when rates fall outside 5–60 minutes per inch). But if someone quotes you “a perc test” with no mention of pits, ask what your county’s regulation actually calls for — the pit evaluation is what the permit needs.
Who’s allowed to do it
The state rule and county regulations use a “competent technician” standard — someone able to conduct and interpret pit excavations, perc tests, and site evaluations. In practice that means a professional engineer or a county-recognized tester: El Paso County formally designates competent technicians against listed competencies, and Larimer requires an engineer or work under one’s supervision. Hiring someone the county won’t recognize is how you pay for the same dig twice.
The rules that surprise people
- Pits stay open. Counties can require excavations to remain open — flagged and barricaded — until their staff has looked (Elbert and El Paso both carry this rule). Don’t let an excavator backfill the same afternoon.
- Seasons are real. Summit County generally issues no OWTS permits October–May; groundwater not checked in the seasonally high period can be conservatively assumed shallow — and designed (expensively) accordingly.
- The county’s word varies. “Soil evaluation,” “site evaluation,” “profile pit,” “perc test” — different counties lead with different terms for the same gate. Your county page uses your county’s own language and links its regulation.
If you’d rather hand the whole thing — excavator, evaluator, county paperwork — to someone who does it weekly, that’s what the form below is for.