SepticFixes
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Land and perc tests: before you buy or build

The soil under a lot decides whether it can take a septic system, which kind, and how much of your budget it will eat.

The short answer

Before buying land that will need a septic system, get a perc test or soil evaluation — ideally as a condition of the purchase. It tells you whether a system can be permitted and what type, which is the difference between a roughly $10,000 conventional system and a $25,000 mound or aerobic unit, or land that can't be built on at all.

In this section

Perc testWhat a perc test is, what it costs, and what to do if land fails.Alternative septic systemsMatched to why the soil failed — with costs for each option.
ToolSeptic system cost calculatorA planning range for a new or replacement system from type, bedrooms, soil and region — then check a quote for price and scope red flags.ToolSeptic tank size calculatorThe minimum tank for your bedrooms under the model code, with the garbage disposal allowance, the size you'd actually buy and the design flow.

Land-buying checklist for septic

  1. Ask the county for any existing perc or soil records on the parcel.
  2. Make the purchase contingent on a passing soil evaluation.
  3. Check setbacks from wells, streams, property lines and slopes against the lot's size and shape.
  4. Ask whether a reserve drain field area must be set aside.
  5. Price the system type the results allow before closing.

How perc tests work

Setbacks that shrink the buildable area

A lot can pass a perc test and still not fit a system once the required distances are applied. These are typical minimums; your state and county set the real ones, and they are often larger for drain fields than for tanks.

Distance fromTypical minimum
Private well to drain field50 – 100 ft
Private well to septic tank50 ft
House foundation to tank5 – 10 ft
House foundation to field10 – 20 ft
Property line to field5 – 10 ft
Stream, lake or wetland to field50 – 100 ft or more

Typical ranges only. Check the county's septic rules before you buy.

How long the septic step takes

Budget one to three months from ordering a soil evaluation to having a septic permit: the test itself takes a day or two, but scheduling the tester, the design and the county's review each add weeks, and spring is the busiest season. Installation then takes a few days to two weeks depending on the system.