A perc test (percolation test) measures how fast water drains through the soil on a parcel, and the county uses the result to decide whether the land can have a septic system. No sewer line and no passing soil test usually means no septic permit, and no septic permit usually means no house.
A survey tells you where the lines are. A perc test tells you whether anyone can live there. The money side is covered in what a perc test costs.
What a perc test is
A septic tank settles out solids. The liquid flows to a drain field, where the soil finishes the treatment. So the soil is part of the system. Too slow and the field floods. Too fast and effluent reaches groundwater before the soil has cleaned it.
A perc test measures that speed. The result is stated in minutes per inch: how many minutes it takes the water level in a test hole to drop one inch.
Why the county requires one
States write septic rules and the county health department usually issues the permit. Before it does, it wants proof the soil on that spot can treat wastewater.
In many states the timed water test is only part of that proof. The other part is a soil evaluation, a deep pit where an evaluator reads the soil layers, depth to rock, and signs of a seasonal high water table. Texas grades soil by texture class instead of by stopwatch (30 TAC § 285.30). People still call the whole thing a perc test. Ask the county what it actually requires.
How is a perc test done
Details vary by state, but Pennsylvania's rule (25 Pa. Code § 73.15) spells out a procedure most states follow in some form:
- Apply and schedule. You file with the health department and pay a fee before anyone digs.
- Dig the deep hole (where required). A backhoe opens a pit so the evaluator can log the soil profile and look for mottling, the streaks that show water sits there part of the year.
- Dig the perc holes. Pennsylvania calls for six or more holes, 6 to 10 inches across, spaced evenly over the proposed drain field at the depth the field would sit. Virginia requires at least three, or five if the results spread more than 30 minutes per inch (12VAC5-610-490).
- Presoak. The holes get at least 12 inches of water and sit 8 to 24 hours, so the test reflects wet-season ground. Right before the test, water is brought back to 6 inches and topped off every 30 minutes for an hour.
- Measure. The drop in water level is read at 10 or 30 minute intervals until eight readings are taken or the rate stabilizes.
- Report. The final-period drops are averaged into one minutes-per-inch figure, and the health department approves a system type or denies the site.
How long does a perc test take
Plan on two days. The presoak alone is 8 to 24 hours under the Pennsylvania rule, and the timed readings the next day run until eight readings are in, about four hours at 30 minute intervals. Add backhoe time and the write-up.
The slow part is waiting for an inspector or licensed evaluator to have an open slot. Tests cannot be run in frozen ground, and some counties have a testing season. Ask before you book a backhoe.
How long is a perc test good for
It depends entirely on the state. The test is usually tied to a permit, and the permit is what expires. Six states, from the rules:
| State | How long the approval lasts | Rule |
|---|---|---|
| North Carolina | Improvement permit: five years with a site plan, no expiration with a recorded plat; survives a change of owner if the site is unchanged | N.C. Gen. Stat. § 130A-336(a); rules at 15A NCAC 18E |
| Texas | Authorization to construct valid one calendar year | 30 TAC § 285.3(d)(1) |
| Georgia | Construction permit valid not more than twelve months | Ga. Comp. R. & Regs. 511-3-1 |
| Virginia | Construction permit null and void after 18 months; revalidated if site conditions have not changed | 12VAC5-610-300 |
| Tennessee | Construction permit expires three years from issuance | Tenn. Comp. R. & Regs. 0400-48-01-.06(4) |
| Michigan | No state code; each county decides. Saginaw County permits last two years; Kent County permits and site evaluations last two years and do not transfer | Saginaw County Environmental Health Code; Kent County Health Department |
If you have an old perc result in a drawer, do not assume it is dead or good. Call the health department with the parcel number and ask. A valid result on file is a real asset when you sell.
Who performs a perc test
This changes most from state to state. A test done by someone the county does not recognize is just a hole in the ground.
| State | Who is allowed to evaluate the site |
|---|---|
| North Carolina | Local health department, licensed soil scientist, or licensed geologist (G.S. 130A-335(a2)); a PE or Authorized On-Site Wastewater Evaluator for construction authorizations (G.S. 130A-335(a5)) |
| Texas | A licensed site evaluator or a PE (30 TAC § 285.30(a)); site evaluators also hold a PE, sanitarian, geoscientist or soil scientist credential (§ 285.60) |
| Georgia | Soil classifiers certified by the Department of Public Health (rule 511-3-1-.16) |
| Virginia | A licensed onsite soil evaluator, or a PE working with one (Va. Code § 32.1-163.5; license required by 18VAC160-40-230) |
| Tennessee | Division of Water Resources staff, or a soil consultant approved under rules 0400-48-01-.03 and .18 |
| Michigan | The county health department. Saginaw County requires a Department of Public Health representative to evaluate every site |
The owner usually hires the backhoe separately, even when a county employee does the reading. Ask the county who may perform the test and whether an official must witness it.
What the perc test results mean
There is no national pass mark. Pennsylvania publishes one of the clearest tables (25 Pa. Code § 73.16), and it is a fair guide to how other states think:
| Result (minutes per inch) | What it means in Pennsylvania |
|---|---|
| Under 3 | Too fast. Unsuitable for any system; effluent would reach groundwater untreated |
| 3 to 90 | An in-ground bed or trench can be approved, with the drain field sized larger as the number climbs |
| 91 to 180 | Too slow for an in-ground system. An elevated sand mound or sand filter can still be approved |
| Over 181 | Unsuitable even for a mound |
Georgia draws its conventional line at 120 minutes per inch on tracts of three acres or more (511-3-1). Virginia's slowest soil group starts at 91 (12VAC5-610-490). Your county's number is the only one that counts.
A parcel can also fail before the stopwatch starts: shallow bedrock, a high water table, steep slope, or no room for setbacks from wells, streams and property lines. And a pass only covers the tested spot and a system of a certain size, usually tied to bedroom count, not the whole parcel.
What happens if land fails a perc test
A failed perc test does not make land worthless. It changes who the buyer is.
A buyer who wants to build needs septic approval, and lenders are wary of land that cannot support a home. The pool shrinks to neighbors, hunters, farmers and investors, who pay less and take longer. The same goes for parcels with no power or water nearby; see selling land with no utilities. How much less depends on the parcel and the market; see what your land is worth.
Your options, in rough order of cost:
- Test a different spot. Soil changes across a parcel. On acreage, a failed corner does not condemn the rest.
- Ask about a retest. Virginia lets a hole slower than 240 minutes per inch be retested once (12VAC5-610-490). Other states allow a retest in a different season.
- Ask about an engineered system. The EPA lists mound systems for shallow soil, high groundwater or near-surface bedrock; aerobic treatment units for small lots and poor soil; drip distribution; and recirculating sand filters for high water tables. All cost more to build and maintain than a conventional system, and not every county permits every type.
- Check for sewer. If a public line is nearby or planned, the perc result may stop mattering.
- Sell to an adjoining owner. A neighbor does not need a septic permit to want more land.
- Sell it as it is. Disclose the result and price for buyers who do not need to build. That is how most raw land with soil problems changes hands.
Do not hide a failed test. The result is usually on file with the health department, and disclosure rules vary by state.
Who is allowed to run the test changes by state; our who does a perc test guide lists the rule for eight of them.
The bottom line
The county decides whether land percs, not the person who dug the hole. Who performs the test, what counts as a pass, and how long it lasts are state and county rules, so the first call is always the county health department. If the test passes, keep the paperwork. It is one of the most useful documents a land seller can hand a buyer.
If it fails and you would rather not pay for engineers and retests on land you do not plan to build on, we buy land that will not perc as it is. Get a cash offer within 24 hours, with no fees and no commissions. If the offer is not right, you have lost nothing.
This is general information, not legal advice. A real estate attorney in your state can confirm how it applies to you.