How to Prepare for a Perc Test

A percolation test measures how fast water drains through your soil: the number that decides whether a lot can support a septic system, what kind, and how big the drainfield must be. If you are building on unsewered land, everything downstream of this test depends on it, which is why it happens first.

When you need one

• Before buying raw land you intend to build on (make the purchase contingent on it).
• Before designing a new septic system or a replacement.
• Often before a lender will close on a vacant-land loan.

Who runs it

In most states, a licensed site or soil evaluator, a separate license class from installer or designer. Many states also require the local health department to schedule or witness the test. Our perc test cost guide covers pricing ($250 to $1,000 for a perc test; $500 to $1,500 for a full soil evaluation), and the site's city pages list evaluators by the license class that covers the work.

How to prepare the site

• Clear access. The evaluator may bring a backhoe for deep observation pits. Know where they can drive in without crossing the area being tested.
• Do not compact the test area. Keep vehicles and equipment off the likely drainfield location entirely; compacted soil percs worse than the same soil undisturbed.
• Know your boundaries. Have property corners, the intended house footprint, and any well locations (yours and neighbors') marked or mapped. Setback requirements between wells, tanks, fields, and property lines shape where the system can go.
• Bring the paper. Prior test results, old permits, soil-survey printouts, and plot plans all help, and can save you from paying to relearn what the county already knows.
• Ask about season. Some jurisdictions require testing during the wet season to measure worst-case conditions. A dry-August pass that ignores a February water table can come back to bite the design.

If the soil fails

A failed perc is not always the end. Options include testing another area of the parcel, an engineered or alternative system designed for poor soils (mound, aerobic, drip dispersal, at higher cost; the numbers are in the installation cost guide), or in rare cases soil replacement. What you should not do is buy land without the contingency, or size your budget assuming a conventional system on untested ground.

Need an evaluator? Request free quotes and we will route you to licensed site evaluators near the parcel.

Common questions