What Is a Perc Test and Do You Need One in Ontario?

A perc test is the cheapest way to find out your dream cottage lot can’t actually support a septic system.

We see it every spring around Bobcaygeon and Coboconk. Someone buys a beautiful lakefront lot. The realtor says “septic-ready.” The buyer plans the build. Then the soil engineer drills four holes, runs a percolation test, and the numbers come back at 70 minutes per inch. Conventional septic is off the table. The path forward is a $30,000 to $40,000 raised bed, or a tertiary treatment system, or in rare cases, no buildable septic at all.

Here’s the short version. A perc test (short for percolation test) measures how fast water drains through your soil. In Ontario, you need one before you can build, replace, or significantly modify a septic system. The test costs $500 to $1,500 in most cases, takes a day or two of fieldwork, and the results decide what kind of septic system you’re allowed to install, or whether you can install one at all.

Below: what the test actually is, when Ontario requires it, who’s qualified to run it, what passing and failing look like, and what to do if your soil says no.

The Quick Answer: Who Needs a Perc Test in Ontario

If you’re not sure whether your project triggers a perc test, the safe answer is: ask the Kawartha Lakes building department before you commit to a design or order materials. They’ll tell you within one phone call.

What a Perc Test Actually Is (and What It Measures)

A septic system doesn’t just hold sewage in a tank. The real work happens in the leaching bed, where partially treated effluent leaves the tank and slowly filters down through the soil. The soil is the treatment plant. If it’s too fast, contaminants reach groundwater before the soil can filter them. If it’s too slow, the effluent has nowhere to go and surfaces, ponds, or backs up.

A perc test answers a single question: how fast does water move through this specific patch of soil at the depth a leaching bed would sit?

The technician drills three to four holes (sometimes more for larger systems) at the proposed bed location. Each hole is about 4 to 12 inches across and reaches the depth of the planned soakaway. The holes are filled with water and allowed to pre-soak for at least four hours, sometimes overnight, so the soil reaches a saturated state similar to real-world operation. Then the technician refills each hole to a known level and times how long it takes the water level to drop one inch.

The result is the percolation rate or T-time, expressed in minutes per inch.

A few examples:

• Sandy lake-edge soil might run 2 to 5 minutes per inch, drains fast.
• Sandy loam (a common Kawartha Lakes mix) might run 10 to 20 minutes per inch, ideal.
• Heavy clay near the Lindsay area might run 60 to 90 minutes per inch, too slow for a conventional bed.

That single number drives the entire system design that follows.

When Ontario Requires a Perc Test

Ontario regulates on-site sewage systems through Part 8 of the Ontario Building Code. The code requires a percolation test (and often deeper soil profiling) any time a system is being designed, redesigned, or significantly altered. In practice, that means:

• New construction on a vacant lot.
• Replacement of a failed system, even if the old footprint is still there.
• Additions that add bedrooms (more bedrooms = bigger system requirement = potential redesign).
• Lot severance, splitting one parcel into two means the new lot must prove it can support its own system.
• Some accessory structures with plumbing fixtures, depending on use.

Routine maintenance, pumping, cleaning, replacing a baffle, swapping out an effluent filter, does not trigger a perc test. Neither does buying a property where the existing system is functional and within its expected service life.

Mark and Allison closed on a 1990s rural property near Lindsay last fall. The previous owner’s septic records were a mess. They worried they’d need a perc test before they could move in. We did an inspection instead, found the system in good shape, gave them a baseline, and got them on a maintenance schedule. No perc test needed because nothing was being changed.

The trigger is change, not ownership.

Who’s Allowed to Perform a Perc Test in Ontario

Not just anyone. Ontario requires perc testing to be performed (or at minimum supervised and signed off) by someone qualified under Part 8 of the Building Code. In practice, that’s one of:

• A licensed sewage system designer holding a current BCIN qualification in the relevant category.
• A professional engineer with experience in on-site wastewater systems.
• A licensed sewage system installer with appropriate qualifications.

This matters for two reasons. First, the inspector reviewing your permit application will reject test results from an unqualified person. Second, the test design itself, hole locations, depths, pre-soak time, number of replicate holes, has to follow the code. A bad test can produce a misleading number that gets your design approved on paper but fails in the ground.

Always ask for the BCIN number or P. Eng. designation before hiring.

What the Test Looks Like in the Field

Day of testing, expect:

1. Site walk and hole layout. The technician identifies the best location for the proposed leaching bed based on setbacks (from wells, lake, property lines, slope) and digs or drills the test holes.
2. Pre-soak. Each hole gets filled with water and topped up over a 4-hour-plus period (sometimes left overnight) so the soil is saturated.
3. Measurement. Holes are refilled to a reference depth. The technician times how long the water level takes to drop a fixed amount, usually one inch.
4. Multiple readings. Tests are repeated until consistent. The slowest acceptable rate across the holes is typically what gets used for sizing.
5. Soil profiling. While they’re there, a deep test pit is often dug to identify soil layers, bedrock depth, and the seasonal high water table. This data feeds directly into the system design.

Total field time: half a day to a full day, depending on access and soil. Total elapsed time including pre-soak: often two days.

Then the engineer or designer takes those numbers, runs them against the OBC tables, and produces a sized leaching bed design, that drawing is what you submit with your permit application.

Pass, Fail, and What Counts as “Good” Soil

There’s no single pass/fail line in the Ontario Building Code. Instead, the percolation rate determines what kind of system the soil can support and how big the leaching bed has to be.

A “passing” test in Ontario typically means the rate falls within a range that supports a Class 4 leaching bed (the most common conventional septic) under reasonable site conditions. Anything outside that range doesn’t necessarily mean no system, it means a different, usually more expensive, system.

The other key data point is seasonal high water table depth. Even with a good perc rate, if your water table comes within a metre of the proposed bed bottom in spring, the design has to compensate (usually by raising the bed). Lakefront properties around Bobcaygeon and Fenelon Falls hit this often.

What a Perc Test Costs in Ontario

Most homeowners pay for the test bundled with the system design, since a standalone perc test without a follow-up design is rarely useful. Expect the total cost of design plus permit plus installation for a new conventional septic in Ontario to land somewhere between $25,000 and $45,000, see our deeper breakdown in the septic system replacement cost guide.

If your test fails, the cost of moving to an alternative system can add $10,000 to $25,000+ on top of the conventional base. That’s why getting the perc test done before committing to a lot, or before signing on a property where the existing system has failed, is the cheapest insurance available.

Kawartha Lakes Soils: What You’re Probably Dealing With

The Kawartha Lakes region has wildly mixed geology because of how the glaciers laid down material. In a single 30-minute drive you can pass through:

• Sandy lake-edge ridges along Sturgeon Lake, Pigeon Lake, Balsam Lake, usually fast perc, septic-friendly, but sometimes too fast and requiring filter media.
• Glacial till in much of the agricultural belt around Lindsay and Ops Township, variable, often workable.
• Heavy clay in pockets, slow perc, often requiring raised beds.
• Shallow bedrock in the northern Coboconk area and along parts of the Canadian Shield edge, limits bed depth and often forces raised or mound systems.
• High seasonal water tables on lower-lying lots, especially near the Trent-Severn Waterway shorelines.

Tony bought a wooded acre near Coboconk five years ago, planning to build a four-bedroom cottage. Initial perc test at the planned bed location came back fast, looked great. Then the deep test pit hit bedrock at 80 cm. The whole bed had to be redesigned as a raised system using imported fill, and the build cost ran $22,000 over the original septic budget. The perc rate was fine; the soil profile killed the conventional design.

The lesson: a perc test alone isn’t the full picture. The deep test pit and soil profile that come with a proper Ontario sewage system design matter just as much, especially in cottage country.

What Happens If Your Lot Fails

Failing a perc test isn’t always the end of the project, but it changes everything about budget and timeline. The options, roughly in order of cost:

1. Re-test in a different location. If part of the lot percs and another part doesn’t, you may be able to relocate the bed. This is the cheapest path forward, adds maybe $500 to $1,500 in additional testing.
2. Raised or mound bed. Imported fill creates an above-grade bed when native soil isn’t workable. Adds $10,000 to $25,000+ to the install.
3. Tertiary treatment system. Advanced units like Ecoflo, Waterloo Biofilter, or aerobic treatment units treat effluent to a higher standard before it reaches a smaller bed. Capital cost can be $25,000 to $40,000+, plus higher ongoing maintenance.
4. Holding tank. A sealed tank with no leaching bed at all. Sewage gets pumped out periodically by truck. Practical only on very small lots with very specific conditions, and ongoing pumping costs are significant.
5. Lot is unbuildable for residential. Rare, but it happens. Most often on tiny shoreline lots where setbacks plus poor soil leave nowhere viable for any system.

If you’re staring at a bad perc result on a property you’re trying to buy, that’s almost always a renegotiation moment, not a “walk away” moment, unless options 4 or 5 are the only path. Talk to the seller, get the design redone with the alternative system in mind, and use the cost difference to drive price.

Perc Test FAQ

How long does a perc test take? The fieldwork is typically a half day to a full day. With pre-soak time and the lab/design work afterward, expect 1 to 2 weeks before you have a full report and design.

Can I do a perc test myself? For Ontario permit purposes, no. The test has to be performed (or at minimum supervised and signed) by a qualified BCIN-licensed designer or P. Eng. You can dig your own holes for curiosity, but those numbers don’t go on a permit application.

Do perc test results expire? There’s no formal expiry, but municipalities will often want a fresh test if the design is more than a few years old, or if conditions have changed (drainage, regrading, nearby construction). Your designer can advise.

My neighbour’s lot perced fine, does mine? Maybe, maybe not. Soil in Kawartha Lakes can change dramatically over a hundred metres because of how glacial till was laid down. One lot can be sandy loam and the next over can be clay with shallow bedrock. Always test your own site.

Does winter affect a perc test? Yes. Frozen ground gives unreliable readings. Most testing happens between late spring and fall. Some designers will not test in deep winter at all.

Is the perc test the same as a septic inspection? No. A septic inspection evaluates an existing system to see how it’s running. A perc test evaluates the soil, usually before a system is built or replaced. Different purposes.

Will my Conservation Authority be involved? Possibly. Properties near a Conservation Authority regulated area (which includes parts of the Kawartha Conservation watershed) may need additional review on top of the standard OBC Part 8 process. Your designer will flag this early. For more on local regulatory specifics, see our septic system rules in Kawartha Lakes guide.

Don’t Buy the Lot Until It Percs

If there’s one principle to take away: the perc test should happen before the offer goes firm, not after. A few hundred dollars of testing during due diligence can prevent a buyer from inheriting a $30,000 septic surprise, or, worse, an unbuildable lot. (More on the full septic-side checklist for a cottage purchase in what to expect when buying a cottage with a septic system.)

If you’re already past that point and dealing with the results, the path forward depends on the numbers. Slow soil isn’t always a deal-killer. Shallow bedrock often is. The right move is to get the perc test results, the soil profile, and the design options all in front of you before you decide.

We service the Kawartha Lakes region, Lindsay, Bobcaygeon, Fenelon Falls, Coboconk, and surrounding rural and waterfront properties. We don’t perform perc tests ourselves (that’s the licensed designer’s job), but we can refer you to qualified local designers, inspect existing systems before you spend money on a redesign, and pump or service whatever you end up installing.

Have an existing septic to inspect or maintain first? Call (705) 242-0330 or book online. Use the cost calculator for a 60-second estimate on routine pumping.