What Is a Perc Test and What Does It Tell You About Your ADU Septic Cost?

By Buz Artiano, BuildX | September 2026

Watch: MA ADUs, Title V & Why Septic Might Be the Wildcard w/ Kevin Grady | BuildX Podcast #01

You called three builders about an ADU and they all said the same thing about the septic. You need to test the soil first. It doesn't seem like the right answer, because everything else in the project already has a cost to it. The septic system just looks like a tank and a hole in the yard so how important can it be?

My name is Buz Artiano, and I am the founder of BuildX. We've built dozens of ADU projects across Massachusetts, mostly around the South Shore and Plymouth County areas. We're the only team you need because we do the design, permitting, and construction. So we see every phase of a project from the first site visit up until you move in. We check septic problems, zoning issues, utility connections and the requirements for a permit from the city. Massachusetts allows one accessory dwelling unit by right in single family zoning areas, although municipal building code, health code, and dimensional requirements still apply to every project. You can review the state requirements at mass.gov.

Here's what we think about the answer you're getting: it's not meant to stall anything but it isn't complete. The test that produces your septic number has a name, a procedure written into state regulation, and a result you can read yourself once you know what the two numbers mean. That changes what you do next, because now you don't have to wait for a quote and you can pay for something that will give you one.

What Is a Perc Test, and What Is the Engineer Actually Measuring?

A perc test, short for percolation test, tells you how porous your soil is. The engineer wants to know how fast water will go down through the ground in the same area where your leaching field would be. Your leaching field is the buried bed of stone and pipe where treated wastewater comes out of the septic tank and soaks through the ground. Soil that moves water quickly can work with a small field but soil that holds water needs a big one.

The procedure isn't made up. Under the state's Title 5 regulations, an excavator digs a deep hole at least ten feet below existing grade, and someone who evaluates the soil looks at it layer by layer: surface A layer, the brown B layer underneath and then the soil under those. A separate hole then is filled with water and the drop is timed to produce a rate in minutes per inch. Both the deep hole test and the perc test are performed in the presence of the local approving authority, which in Massachusetts towns is your Board of Health. You can review the siting and testing requirements at mass.gov.

Two numbers come out of that. The percolation rate, in minutes per inch. And the estimated seasonal high groundwater elevation, read off the soil profile in the deep hole. All of the cost for your septic system is because of those two numbers.

Why Does the Perc Rate Change the Size of My Septic System?

Because the soil is doing the treatment work, and slow soil needs more surface area to do the same amount of work.

Pour water into a hole in dense soil carrying a lot of fines and it sits there. Our engineer has watched an excavator struggle to pull that material out of the ground, and that struggle is the preview of the perc result. When water won't go through the soil quickly, then the leaching field has to be spread out over a much larger area. And a larger area means there has to be more excavating and more stone and pipe will be used. Your yard will be dug up as well.

From Kingston south through the South Shore the picture often flips. That ground runs to sand, and the results are predictable enough that our engineer knows roughly what is coming before the excavator arrives. If you pour water into the sand it'll go straight through it. A fast percolation rate means that you will need just a small leaching field.

What the Deep Hole Tells You That the Perc Test Cannot

The perc test gives you a rate. While the perc hole is being timed, the person who evaluates the soil is looking at the ten foot hole to see if the water table has come up into the soil profile in the past. If you dig a hole in April you'll see water coming out at one elevation. But if you dig that same hole in August the water level will be lower. The soil shows signs of the wet season even if the hole is dry. It shows discoloration which are called mottles. Sometimes it looks like just a clean line across the profile or more often it might be splotchy. If so, the tester checks one wall of the hole against the opposite wall to see if the same elevation shows up twice.

It's part science and part craft. A good evaluator knows that if you design a septic system into the water table the system fails. The test is there to prevent that outcome.

The elevation the evaluator marks will be the bottom line that your design has to be above. Under Title 5, the bottom of the stone underneath your leaching field has to be at least five feet above high groundwater in soils that perc at two minutes per inch or faster, and at least four feet above it in slower soils. That's why those two tests are done on the same visit, because each result depends on the other one.

How Do I Read a Fast or Slow Perc Result as a Cost Signal?

Once you know what those two numbers are, you can read your own result. Below are the signals about direction instead of prices, because your final septic cost depends on the number of bedrooms in the ADU, the condition of any current existing system on the property, whether or not usable data on the soil is already in your town file, and the Board of Health requirements that are put on top of the state code.

The five-foot requirement on fast soil takes people by surprise. Sand moves wastewater through sand so quickly that the state requires more space to the water table. On a high, dry lot that separation won't cost you anything, since the water table is already far below where your system will be. On a low lot near wet ground it can be a problem that can decide if a system can be built in the ground at all.

Can I Use the Perc Data Already on File for My Property?

Sometimes, and it's something you should ask about before you spend any money.

Soil testing that was done on your lot before doesn't necessarily expire. Under Title 5, percolation data that was obtained using the proper procedures can be used for an indefinite period as long as the soil has stayed undisturbed and unchanged. What boards of health look for is that the work was done by a licensed soil evaluator and witnessed by the board, and our engineer looks for testing done after the 1995 Title 5 rewrite. If that is all in place, then the boards have let us move ahead with the existing data.

That's what we did on a recent Plymouth ADU. There was already usable soil testing on the property, so we didn't have to pay to do all the testing again. Before you budget for testing, ask your Board of Health whether a soil evaluation is already in your property file. That single phone call is free, and sometimes it will remove the need to budget for a new septic system.

Why Can't Anyone Quote My ADU Septic System Before the Hole Is Dug?

This is the part homeowners find hardest to accept, and it's the honest answer to the issue you brought to us.

We've dug on one side of a lot and found perfect material, then dug on the other side of the same lot and it was solid clay. The very same property just a hundred feet apart. That's the reason that Title 5 requires the testing to be done in the area where the system is actually going to be built, and the slowest result will control the design. You are not allowed to average perc rates taken from different places across the site, and that tells you how much difference the state expects to find on a single piece of property.

Our engineer also checks the neighborhood. When a test hole shows the water table is at eight feet and the property next to that one has a septic system sitting up above grade, those two facts contradict each other. The solution is to check the neighbor's records and find out which reading is wrong, before the excavator comes back. That kind of cross-check doesn't show up on any invoice but it's the difference between a design that will pass and a design that has to be redone.

So the reason we won't give you a price for the septic system before the work happens isn't just being cautious. It's because that cost would be fiction. Our engineer does the survey and the perc testing on the same day, and the result of that test is a plan based on conditions that are existing at that time, and that becomes the design plan. It's the drawing that decides every other decision. Until that plan exists, the septic line in your budget is useless.

When Is Paying for a Perc Test Early the Wrong Move?

We tell our customers to fund the survey and soil evaluation early for most ADU projects. You'd expect a company that is design-build to say that, because that plan you get is what lets us do our work. But here's the limit on that advice.

If you haven't made up your mind if you want an ADU at all, then you don't need to test yet. It means an excavator will come to your property to answer questions you aren't ready to ask yet. If your lot already has soil evaluation documents that meet the conditions above, testing again is money you don't need to spend. And if you're thinking about two different locations for the ADU on the property, understand that testing one spot doesn't work for another spot, because the state requires the work to happen where the system is going.

There's also a structural point worth mentioning. Some families want to hold the engineering contract themselves and bid the septic work separately instead of using a design-build company to handle it. That's a workable way to manage a project and you'll get the test results either way.

How to Turn a Hole in the Ground Into a Real Septic Number

The decision here is simple and worth making now. Pay for the survey and the soil evaluation before you ask anyone for a price for the septic system, and ask your Board of Health first whether that work already exists in your file.

The two numbers that come back, your percolation rate and your groundwater elevation, are the difference between a septic budget and a guess. Everything after that in your ADU budget will depend on which one you are using.