The perc test and soil evaluation

Before anything else about a septic system is decided — the type, the size, the position, the price — the soil is tested. What comes back sets the boundaries everything else has to fit inside.

It is the least understood step and the most consequential. A soil result can turn a conventional system into an engineered one, move a house on a lot, or establish that a parcel cannot be built on at all.

Written for: Anyone buying land, adding a bedroom, replacing a failed system, or holding a quote that depends on a soil result they have not seen.
Next step: What the result changes about the cost
Last reviewed: 30 July 2026
About this site: SepticOwner is an independently operated septic guidance publication. We do not perform septic work, diagnose systems, or publish price figures. Homeowners pay nothing. Where a provider covers the area, that provider pays for a qualified request. What we do and do not doHow a request is handled

What is actually being measured

Two different things get called "the perc test", and the distinction matters because they answer different questions.

A percolation test measures how quickly water drains out of a hole in the ground, usually expressed as minutes per inch. A soil evaluation — sometimes a soil profile, a pit test or a deep hole test — is somebody reading the soil itself: its texture, its structure, its horizons, and how far down the restrictive layer or the seasonal water table sits.

Many jurisdictions now rely on the soil evaluation and treat the percolation rate as secondary or optional, because a rate alone does not describe what the soil will do over twenty years. Which one applies to your parcel is set locally.

What the result decides

Whether a conventional gravity drain field is permitted at all, or whether the site needs an engineered or alternative system.
How large the drain field has to be — slower soil needs more of it, which is area you may or may not have.
Where on the parcel the system can sit, which sometimes moves the house, the driveway or the well.
Whether a reserve or replacement area can be shown, which many jurisdictions require before approving the first one.
Whether the parcel supports the number of bedrooms being planned, since sizing is normally driven by bedroom count rather than by occupancy.

Who performs it, and who does not

This is the part most often got wrong, and it is the part that costs a wasted visit.

Who	What they do here	When to call them
Licensed soil evaluator or soil scientist	Reads and classifies the soil profile and produces the report the permit rests on	First. In most places nothing proceeds without this
County or state health department	Sets which test applies, may witness or perform it, and issues the permit	Before anything is dug, to find out what your jurisdiction requires
Septic system designer or engineer	Designs a system that fits the soil result and the parcel	After the soil result exists, not before
Septic installer	Builds what the design specifies	After the design and the permit
Pumping company	Services existing tanks	Not for this — a pumper is the wrong first call for a new system

When a test is required

A new system on undeveloped land, in essentially every jurisdiction.
A replacement system, in many jurisdictions, because the original approval may predate current standards.
Adding bedrooms or living space, where sizing is driven by bedroom count.
Some property transfers, where a point-of-sale requirement applies.
A repair that changes the size or position of the drain field, as opposed to one that restores what is there.

Describe the property and what you are planning — A person reads it. Whether a test is required where you are is a question for your county, and knowing what to ask them is most of the value.

What a homeowner can reasonably establish first

Whether the parcel has an existing soil report — the county, the previous owner, the original permit file or the listing agent may hold one.
How old any existing report is, and whether your jurisdiction accepts one of that age.
What season the test was performed in, since a seasonal water table is the finding most affected by timing.
Whether the report describes the area where the system is actually proposed, rather than another part of the parcel.
Whether a reserve area was identified as well as the primary one.

What this page cannot settle

Whether your parcel will pass. Nobody can say that before the soil is read, and any figure or opinion offered in advance of the test is a guess dressed up as experience.

What the test costs. That varies by jurisdiction, by how many holes are required, and by who is permitted to perform it locally.

Which test your jurisdiction requires, or who it accepts a report from. Only the authority for your address can state that, and it is worth one phone call before spending anything.

Common questions

Does a failed perc test mean the land cannot be built on?

Not necessarily, and this is the most common misreading of a result. It usually means a conventional gravity system is not permitted, which moves the question to whether an engineered or alternative system is allowed on that parcel and whether it is affordable. Some parcels genuinely cannot support any system, and that is a determination your jurisdiction makes rather than the test alone.

How long is a soil evaluation good for?

It depends entirely on the jurisdiction — some accept a report indefinitely, some set a limit in years, and some require a fresh evaluation when standards change. If you are relying on an older report, confirm with the authority that they will accept it before you build a budget on it.

Can I do the test myself?

Digging a hole and timing water is something anyone can do, and it can be a useful private sanity check before spending money on a parcel. It is not the test the permit rests on. That has to be performed or witnessed by whoever your jurisdiction recognises, which is normally a licensed evaluator, the health department, or both.

Does the time of year matter?

It can matter a great deal. The finding most sensitive to timing is the seasonal high water table, and a test performed in a dry period may not show what the ground does in a wet one. Some jurisdictions restrict testing to particular months for exactly this reason.