Perc Testing in Southern MD: Why Fall Is the Time to Get in Line

If you've bought, or are thinking about buying, a wooded lot in Southern Maryland, there's one test that decides whether you can build a house on it at all. Most lots in Calvert, Charles and St. Mary's counties aren't on public sewer, which means the home will need a septic system, and a septic system needs soil that passes a percolation test. The catch is that most of these tests can only happen during a short window in late winter and early spring. That's why November, months before anyone can dig, is the right time to start planning.

What a Perc Test Actually Measures

A percolation test (perc test for short) tells the health department two things: how quickly your soil absorbs water, and how high the groundwater rises at its wettest time of year. Both matter. Soil that drains too slowly can't handle the wastewater from a house, and groundwater that sits too close to the surface means septic effluent won't be filtered properly before it reaches the water table.

Because the test is looking for the worst-case water table, Maryland regulations require testing at the time of year when groundwater is highest. In Southern Maryland, that's the "wet season," which usually falls somewhere between February and May, but the exact dates are set each year by each county's health department based on readings from groundwater monitoring wells.

The test itself involves digging. A backhoe or excavator opens a test pit, often several feet deep, so the health department sanitarian can look at the soil layers and the water level. Depending on the system type being evaluated, holes are then prepared and filled with water to time how fast it drains. If the first spot doesn't pass, additional holes may be dug elsewhere on the lot.

Why the Wet Season Isn't a Sure Thing

The wet season isn't a fixed calendar event. It opens only when groundwater levels rise high enough, and it closes as soon as they drop. In a dry year, that window can be painfully short.

The 2026 season is a good example. Charles County didn't begin scheduling wet-season perc tests until March 30 because water tables were below normal, and then suspended testing countywide on April 13 when monitoring wells dropped below wet-season levels. St. Mary's County suspended its wet-season testing on April 17, 2026. In both counties, applications that didn't get tested were carried over to the next wet season, which is expected in 2027.

That means the 2027 season will start with a backlog. Health departments schedule wet-season testing in the order applications are received, so anyone who applies now gets in line behind the carried-over applications but ahead of everyone who waits until spring.

What You Can Do This Fall

You can't run a wet-season perc test in November, but almost everything that has to happen before the test can be done now:

Submit your application. Calvert County accepts wet-season applications year-round. Charles County asks residents who need a perc test to apply as soon as possible. In St. Mary's County, new-construction perc test applications go through Land Use and Growth Management. Applying early secures your place in line.
Get the lot surveyed and marked. The test locations need to be accurately placed relative to property lines, wells and setbacks. Your surveyor can handle this before the season opens.
Clear access to the test area. A wooded lot needs a path wide enough for a backhoe and a cleared spot where the test pits will go. The time scheduled for your test doesn't include clearing, so this has to be done ahead of time. Late fall and winter, when the leaves are down and the ground is firm, is a good time for this kind of light clearing. If you're not sure how much clearing you need, our post on land clearing vs. grubbing explains the difference.
Line up your excavation contractor. The health department doesn't dig the holes. Someone with the right equipment has to be on site when the sanitarian arrives, and in Calvert County wet-season testing is scheduled directly with the licensed septic installer listed on your application. When the season opens, every installer and excavator in the area gets busy at once, so it pays to have your contractor arranged before the calls start.

If You're Buying Land, Make the Perc a Condition

A lot that doesn't perc can't support a conventional septic system, and depending on the soil it may need a more expensive alternative system or may not be buildable at all. If you're shopping for land this fall, talk with your agent about making the purchase contingent on a passing perc test, especially for waterfront or low-lying parcels where high groundwater is more common. With the 2027 backlog, build enough time into that contingency to account for a season that might start late or end early.

Frequently Asked Questions

When is perc test season in Southern Maryland?

Wet-season perc testing typically runs from late winter into spring, somewhere between February and May. Each county's health department opens and closes the season based on groundwater levels, so the exact dates change every year.

Can I apply for a perc test before the wet season starts?

Yes. Applications can be submitted ahead of the season, and wet-season tests are scheduled in the order applications are received. Applying in the fall puts you ahead of spring applicants.

What happens if the wet season ends before my test?

In 2026, both Charles and St. Mary's counties carried untested applications over to the next wet season. St. Mary's applicants could also request a refund, but doing so meant reapplying later and losing their place in line.

Who digs the holes for a perc test?

The property owner is responsible for hiring a contractor with a backhoe or excavator to dig the test pits. The contractor charges separately from the health department's application fee.

Plan Now, Build Sooner

A late or short wet season can push a building project back a full year. Getting your application in, your lot surveyed and your test area cleared this fall gives you the best chance of being tested in 2027 and breaking ground on schedule. Scaggs Excavating has been doing site work in Southern Maryland for years, and we can help you get your lot ready for perc testing and the excavation that follows. Contact us or call 410-257-6763 to talk about your property.

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