Perc tests: what they are, and what to do if the land fails

The short version

If the land has no sewer line, you will need a septic system to build a house, and you can’t get a septic permit until the soil is tested. That test, a percolation (perc) test or, in many states now, a soil evaluation, shows whether the ground can absorb and treat wastewater. Land that fails may still be buildable with an engineered system that costs much more, and some land can’t get a permit at all. That is why the test belongs before you close, written into the contract.

What the test actually is

• Perc test: holes are dug where the drain field would go, soaked, filled with water, and timed as the water level drops. The rate (minutes per inch) tells the designer how big a drain field the soil needs, or whether it can take one at all.
• Soil evaluation (soil profile test): a deeper pit or boring where a licensed evaluator reads the soil layers: texture, depth to rock, and signs of a seasonal high water table. Many states now rely on this instead of, or alongside, a perc test.
• Who does it: depending on the state, the county or state health department, a licensed soil scientist or evaluator, or an engineer. Call the county health department or local septic permitting office first: it will tell you which test it requires and who may do it.
• Timing: some places only accept tests in the wet season, when the water table is highest (Anne Arundel County, Maryland, for example, evaluates some sites only from February to April), so a summer contract can stall for months.

Check before you pay for a test

• USDA Web Soil Survey (free): draw the parcel on the map and look up its soil ratings for septic absorption fields. A “very limited” rating is a warning, not a verdict.
• Ask the county whether earlier tests or permits exist for the parcel or the neighbours, and whether they are still valid and fit the house you plan (in North Carolina, for example, a permit issued with a site plan is good for five years).
• Look at the land: standing water, wetland plants, steep slopes and exposed rock all point to trouble.

If the land fails

• Test another spot. A failed hole is about one location; a different part of the parcel may pass.
• An engineered system: a mound, a sand filter, an aerobic treatment unit or drip dispersal can work where a conventional drain field can’t. They cost considerably more to install and usually need ongoing maintenance or a service contract, and not every county allows every type.
• Connect to a sewer if one is nearby; the cost depends on distance and the utility.
• Walk away if your contract lets you. Without a septic permit, a rural lot is usually unbuildable for a house, and its value reflects that.

Protect yourself in the contract

Make the purchase contingent on a passing perc test or soil evaluation and a septic permit for the house you plan, with enough days to schedule the test (including any wet-season wait) and the right to cancel and get your deposit back if it fails. Our due diligence checklist covers the other conditions to put in the same contract.

Quick answers

What is a perc test?

A test of how fast water soaks into the soil where a septic drain field would go. Permitting agencies (usually the health department) use it, or a soil evaluation, to decide whether a septic system can be permitted and how large the drain field must be.

What happens if land fails a perc test?

You can test another spot, use an engineered system such as a mound or aerobic treatment unit (more expensive, and not allowed everywhere), connect to a nearby sewer, or walk away if the contract was contingent on the test.

Who does a perc test?

It depends on the state: the county or state health department, a licensed soil evaluator, or an engineer. The county health department or local septic permitting office can tell you which test it needs and who may do it.

Should I buy land before a perc test?

Not if you plan to build a home without sewer. Make the purchase contingent on a passing test and a septic permit.

General information, not legal, tax or financial advice. Land rules differ by state and county; confirm with the county health or planning department, a real estate attorney or a licensed professional before you buy.