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Will it perc?

Without a sewer, a house needs a septic system, and the local health authority decides whether the soil can take one. That is usually the county, though in some states it is a town board or a state agency. That decision usually starts with a perc test or a soil evaluation. Here is what the test involves and what it costs, what a septic system can cost, how far a free soil map gets you before you pay, and what your options are if the land fails.

A perc test (percolation test) is a measurement of how fast water soaks into the soil where a drain field would go. Failing one does not always end the plan: it usually rules out a conventional drain field in that spot, and the county may still allow a different spot or an engineered system.

What a perc test is

Test holes are dug where the drain field would go, filled with water, and timed as the water drops. Too slow (clay, hardpan, a high water table) and effluent can pool at the surface. Too fast (coarse sand, gravel) and it can reach groundwater untreated. Many counties also set a minimum depth to bedrock and to the seasonal water table. Many states now judge a site by a soil profile in a dug pit, instead of or as well as a timed test.

Who does it	It varies: the county health department, a licensed soil evaluator or an engineer. In Genesee County, Michigan, you schedule the county sanitarian and supply your own backhoe and operator.
Who pays	That is for the contract to say. A buyer often pays to test as a contingency, with the seller's written permission to dig.
How long it lasts	Each jurisdiction sets its own rule. Genesee County's test is valid for 5 years. Ask whether the result stays valid if the lot, the rules or the house size change.
When it can be done	Ask about the wait, and whether testing is limited to certain seasons or wet-weather conditions. Do this before you sign, because a short contingency can run out before a test is possible.

What a perc test costs

There is no national price, but county fee schedules show what the public fee can be. These are examples, not averages:

Genesee County, MI	$220 for a residential perc test. The backhoe and operator are yours to supply.
Mecklenburg County, NC	$250 for a residential soil test and improvement permit only.
Frederick County, MD	$325 to $500 for soil evaluation and percolation testing, set by the size of the reserve area (fees as of September 2025).
Lee County, NC	$550 for a new improvement permit, perc test only (July 2026 to June 2027 schedule).

The public fee may not be the whole bill. Ask about excavation, a private soil scientist or engineer, a site plan, and any later permit to install the system. Get the current fee from the county, because these schedules change.

What the soil survey says

The USDA soil survey (Web Soil Survey) maps most of the country into soil map units. It rates most of them for "septic tank absorption fields": how well a conventional drain field is expected to work and what would limit it. LandCheck shows that rating on every click, in the free Septic suitability row:

Not limited	The soil has no property that would restrict a conventional drain field. Good sign; still test.
Somewhat limited	One or more limitations that design or extra cost can usually overcome, for example slope or moderate permeability.
Very limited	A limitation serious enough that a standard gravity drain field is unlikely to work as built: slow percolation, a high water table, shallow bedrock, flooding, steep slope. It is common on rural and mountain land and is not a "no". Many of these lots are permitted with an engineered or alternative system, or on a better spot, and only the official site evaluation or perc test decides.

LandCheck colours Very limited amber, not red, for that reason: it is a cost and design question to take to the county, not a verdict on the lot. The limiting features are listed with the rating (for example "depth to saturated zone" or "slow water movement"). Those are the things the evaluator will be checking. A blank or "Not rated" row means the survey gives no rating there, not that the soil passes.

Why the map and a real perc test can disagree

The soil survey can say very limited about a lot that already has a septic permit. Both can be right.

The rating is for a conventional system. "Very limited" says a standard gravity drain field in trenches is unlikely to work. It says nothing about the mound, drip and treatment systems many states approve for those soils.
The worst property decides it. The survey rates a soil by its most limiting feature, so one problem is enough: shallow rock, a seasonal water table or slope. That is why so many mountain soils read Very limited.
The map is coarse. A map unit is named for its main soil and contains smaller areas of others. The evaluator is looking for the one good spot on the lot, a bench, a ridge or a pocket of deeper soil, which the survey may not draw.
States test differently. Each state sets its own methods, depths to water and rock, and setbacks.
Ground changes. Grading, fill, compaction and old drain fields change soil that was mapped decades ago.

So read the rating as a question to ask, not a verdict. "Very limited: slope" on a mountain lot may mean a system designed for the slope. "Very limited: ponding" on a flat bottom may mean looking for a different spot.

What each limiting feature means

Depth to saturated zone	Groundwater rises close to the surface for part of the year. Effluent could reach it before it is treated, and the field can drown in a wet season. The usual answers are a raised system such as a mound, or a higher spot on the lot.
Slow water movement	Dense clay or a tight layer that water soaks through too slowly, so effluent backs up or surfaces. Expect a larger field, pressure distribution, drip lines or a treatment unit.
Seepage, bottom layer	The opposite problem: sand or gravel under the trench passes water too fast to clean it. Lined trenches, a sand filter or a treatment unit ahead of the field are common fixes.
Filtering capacity	Coarse soil that filters effluent poorly; handled much like seepage.
Depth to bedrock, depth to cemented pan	Not enough soil above rock or a hard layer to treat effluent. Shallow trenches, a mound or drip may fit; on some lots nothing does.
Slope	Steep ground makes level trenches hard and lets effluent surface downhill. Drip lines laid along the contour and stepped trenches are designed for it, and some states cap the slope for a field.
Flooding, ponding	Water runs over or stands on the soil. Expect the county to keep the field out of that ground or require it to be raised.
Large stones	Rocks big enough to make trenching hard and to cut the volume of soil that treats effluent.
Subsidence	Organic soil, muck or peat, that settles as it dries. Usually unsuitable.

These are the usual responses. Which system is allowed on a given lot is the evaluator's and the health department's call.

Alternative and engineered systems, in plain words

When a conventional field will not work, many states allow other designs. The EPA's overview of septic system types covers most of these:

Mound: a raised bed of sand fed by a pump, for shallow soil, high groundwater or shallow bedrock
Pressure distribution: a pump doses effluent through small pipes so it spreads over the whole field instead of soaking in at the first few feet
Drip dispersal: thin tubing in the top 6 to 12 inches of soil, dosed from a tank and needing power
Sand filter: effluent is treated in a lined or concrete box of sand before it reaches a field
Aerobic treatment unit: a small treatment plant in a tank that adds air; it cleans effluent further and needs power and regular service
Chambers: open-bottom plastic chambers in place of gravel trenches

Open-bottom plastic chambers, which replace gravel, are different: the EPA groups them with conventional systems, so do not count on them to fix soil that fails. Most of the designs above cost more to build than a conventional system, and many need power, a pump and regular servicing, which the county may require by contract. Ask the health department which ones it permits on that soil and who designs and installs them locally.

What a septic system costs

Costs swing widely with soil, access, system type and local rules, so no single national figure fits. These published examples show the spread. Neither is an average for your area:

Arizona homeowner manual	The Septic System 101 manual on Mohave County's site says a new conventional system can cost $4,000 or more, and an alternative system $10,000 to $20,000 or more. The manual shows no date, so treat these as rough floors.
Provincetown, MA (2022)	The town's public works fact sheet lists $8,000 to $12,000 for the simplest single-family installation and $15,000 to $40,000 for more complex work. A system needing a variance, a pump or alternative technology runs $30,000 to $70,000. It describes one coastal Massachusetts town under that state's rules.

Get written quotes for the system the county says the lot needs, and include the designer, permit, tank, field, any pump, power to the pump, and restoring the ground. Then budget for upkeep: the EPA says a household system should be inspected at least every three years and is typically pumped every three to five, and alternative systems generally once a year. Provincetown's fact sheet puts service contracts for alternative systems there at about $1,200 to $3,000 a year.

If the land fails

A failed test tells you about that spot and that kind of system. It may not be the end of the lot, but start with the clock if you are under contract. What is allowed varies by state and county.

If you are under contract, check your deadline first. Know the date your contingency ends and how to exercise it. The steps below can outlast it, so ask for a written extension before it expires, or use your right to cancel.
Find out exactly why it failed. Get the written result. Slow soil, a high water table and shallow rock each point to different answers.
Test another spot. Ask whether a different part of the lot could work, and whether a retest is allowed and in which season.
Price an engineered system. Ask which designs the county would permit for that limitation, and get a designer's opinion and a quote before you decide.
Ask about other wastewater options. A public sewer connection, a shared system or a variance may exist in some places. Holding tanks and composting or waterless toilets are restricted in many places. Do not buy assuming any of them, and get the answer in writing.
If you already own the lot, ask what uses remain possible without a septic system before you spend more.

Records of past permits

The county or district health department, usually its environmental health office, is the first place to ask for septic permits, site evaluations and repair records on a parcel. In some states the records sit with a town board or a state agency. A listing that says "septic approved" or "perc tested" should come with the document. Ask for:

The permit or site evaluation, its date, and the system type and number of bedrooms it allows
Whether it is still valid: some approvals expire, or lapse when the rules or the lot change
The soil evaluation report and where the test pits were dug
Repair permits on the lot and its neighbors: a run of failures on the same soil tells you something
Well permits and well logs nearby, which can set the drain field's setbacks

The Who to call card in LandCheck's panel links the county's own website and the state's septic program page, the places to start looking for the office that holds these records.

What the soil map cannot tell you

Map units are drawn at survey scale; a 5-acre parcel may straddle two or contain unmapped pockets
The rating is for a conventional system; mound, drip and aerobic systems have different rules
County setback rules (wells, streams, property lines) decide where the field can even go
Minimum lot sizes for septic come from state or local rules, and the soil can affect them; the rating does not tell you the minimum
A previous failed test on the lot is a public health record, not a soil map

How to use it

Turn on the Soil layer and click the parcel. Read the map unit, the Septic suitability row and its limiting features. The row is the survey's septic rating for the main soil in the map unit under your click; go by the row, not the map colour. The Other soils row names other soils mapped inside the unit, without ratings. One may be a better pocket, or a worse one.
Click a few spots across the parcel; the rating can change at a soil boundary. What matters is whether there is a good spot for the drain field with room for setbacks, not how the soil rates where the house would go.
Read the limiting features, not just the rating. Slope and depth to rock may point to a designed system; flooding and ponding may point to a different spot or a different lot.
Turn on Slope to shade the ground under 15%, 15 to 25% and over 25%, and click the likely drain field for the slope at that spot. Flatter ground is often where the evaluator looks first.
Check the flood zone and wetlands layers on the same area. Wetlands and floodplains can limit or rule out where a field goes, so ask the county. The Wetlands row names any wetland the national inventory maps at the spot you click, in plain words beside the inventory's own type, or says none is mapped there.
Ask the seller for any prior perc test or soil evaluation and check with the county health department for records on the parcel.
Make the contract contingent on septic approval, not only on a perc test: many places use a soil evaluation instead, and a lot may be approved only for a system you cannot afford. Allow time for the county's wait and any testing season, include the seller's permission to dig, and have a local real estate attorney write the clause.

Open the map with the soil layer on →

Sources

U.S. EPA: septic systems, types of septic systems and how to care for your septic system
USDA NRCS Web Soil Survey, the source of the septic tank absorption field rating
The county fee schedules and cost examples linked above, as each one publishes them

Related guides

What the parcel source shows in each county LandCheck covers: parcel maps by county.

The soil survey is a screening tool. Only a site evaluation by the county or a licensed evaluator determines whether a lot can be permitted for septic.