Perc Tests and Soil Evaluations in Missouri

Called here: Percolation Test or Soil Morphology Evaluation (Registered PT / OSE)

Missouri does not run one statewide permitting desk. Step 1: use DHSS's county jurisdiction selector to find whether your property falls under the DHSS Onsite Program directly, a local health department operating under contract with the state, or a local government operating under its own (possibly stricter) ordinance. Step 2: get your soil evaluated per your permitting authority's process, unless you're exempt (see below). Step 3: apply for the construction permit and pay the fee, which varies by county. Step 4: do not start work before a permit is issued or an exemption confirmed. The Department of Natural Resources, not DHSS, has jurisdiction over larger systems, subdivisions, mobile home parks, and campgrounds -- confirm jurisdiction with DNR if your project fits one of those categories.

Missouri has a statutory acreage exemption from permitting, construction, operation, and repair requirements: a single-family lot of three acres or more is excluded if all system points sit more than ten feet from any adjoining property line and no effluent crosses onto a neighboring property or contaminates water; a broader exemption applies to lots of ten acres or more under similar conditions. The catch, per DHSS's own FAQ: 'This exemption does not apply in some counties; check with the county authority' -- a stricter local ordinance can still require a permit on an otherwise-exempt lot. Confirm the exemption with your county before skipping a step.

One verified Missouri operator (Show Me Soils, serving the area west of St. Louis including Franklin County) advertises $350 to $550 for the evaluation depending on location -- one operator's current price, not a market average; get quotes from two or three registered evaluators serving your county. The permit fee is separate and varies by jurisdiction: three counties checked directly charge $90 (Moniteau), $150 plus a separate $60 building permit (Monroe), and $510 (Boone) for new construction. There is no statewide flat fee.

Missouri's construction rule builds in a real seasonality constraint: installation of an absorption system is not allowed while the soil is wet or moist, since working wet soil smears and destroys its structure (for mound systems specifically, soil may not be plowed until moisture content at 8 inches drops below its plastic limit). The percolation test procedure itself normally requires a minimum four-hour presoak followed by a full 24-hour swelling period before measurement (sandy soils are exempt if a test filling seeps away in under 10 minutes). A test run right after heavy rain risks an unreliable or unusable reading; wetter stretches of the year carry a real risk of delay or a re-test.

Missouri's construction rule (19 CSR 20-3.060) names two site evaluation methods -- a percolation test and a soil morphology evaluation -- and gives the administrative authority with permitting jurisdiction the power to decide which one your site needs. DHSS runs two matching registrations: a Registered Percolation Tester (DHSS-approved training course plus written and practical exams, or existing OSE/PE/geologist qualification) and a Registered On-Site Soil Evaluator, most commonly qualified as a soil scientist (15+ semester credit hours of soils coursework, 3+ specifically in soil morphology, then a written and field exam scored 70% or higher). Both registrations run on a 36-month renewal cycle with continuing education. Conducting either evaluation for OWTS design or installation without the required registration, or misrepresenting yourself as registered, is a class A misdemeanor under RSMo 701.053.

Missouri Department of Health and Senior Services (DHSS), Onsite Wastewater Program · Verify a license