A grey water pit is a real, engineered piece of infrastructure with numbers attached to it, and almost nothing written about it online gives you those numbers. You get health unit PDFs written for a different region, US blogs describing a gravel hole, and a lot of forum advice.
So this is the mechanical version: what a Class 2 grey water pit is, how big yours would have to be, how deep it can go, how far it has to sit from your well, and why a fully plumbed cottage often cannot legally have one at all. If what you actually want to know is whether you are allowed one in the first place, that is a different question and it is covered in our grey water systems at Ontario cottages guide.
What the Thing Actually Is
Under the Ontario Building Code, a grey water system is a Class 2 sewage system — one of the five classes, sitting between the Class 1 privy and the Class 4 septic bed. Our Ontario septic classes guide covers the whole taxonomy.
Physically, a grey water pit is an excavation with open-jointed sidewalls, a cover, and a mound of earth around it. Water from your sinks, shower, tub, and washing machine runs into it and leaches out through the sides into the surrounding soil.
That last detail is the one that governs everything else on this page. A grey water pit is sized on its sidewall area. The bottom of the pit does not count toward capacity at all, because it silts up and seals. A pit that is wide and shallow is a worse pit than one that is narrow and deep, for the same volume of digging.
The Cap That Rules Everything: 1,000 Litres a Day
The Building Code sets a hard ceiling. A Class 2 system cannot be built where the daily design grey water flow exceeds 1,000 litres per day. Not “should not.” Cannot.
The flow is not measured. It is calculated from your fixtures, using fixture units:
- 200 litres per fixture unit where there is a supply of pressurized water
- 125 litres per fixture unit where there is not
Fixture unit values come from the Code’s own table. The common household ones:
| Fixture | Fixture units |
|---|---|
| Kitchen sink | 1.5 |
| Bathtub, with or without shower | 1.5 |
| Shower drain, one head | 1.5 |
| Laundry tray | 1.5 |
| Clothes washer | 1.5 |
| Lavatory (bathroom sink) | 1 to 1.5 depending on trap size |
| Dishwasher | 1, and no load at all if it drains through the kitchen sink |
Now do the arithmetic on an ordinary cottage with pressurized water — kitchen sink, bathroom sink, tub with shower, and a washing machine:
1.5 + 1 + 1.5 + 1.5 = 5.5 fixture units × 200 L = 1,100 litres per day.
That is over the cap. That cottage cannot have a Class 2 grey water pit, and no amount of designing around it changes the arithmetic.
Take the pressurized water away — a hand pump, gravity feed, water hauled in — and the same four fixtures produce 5.5 × 125 = 687.5 litres per day, which passes comfortably.
This is the single most useful thing to understand about grey water pits. They are not a system for a fully serviced house. They are a system for modestly plumbed buildings, and the moment a seasonal cottage gets a pressure system installed, its design flow jumps 60% with no new fixtures added. That alone has pushed plenty of cottages over the line.
Where a use genuinely needs more than 1,000 litres a day, the answer is not a bigger pit. It is multiple systems, or a different class of system entirely.
How Big the Pit Has to Be
Once you know the daily flow, the Code gives you a loading rate:
LR = 400 / T
where T is the percolation time of the soil in minutes per centimetre, and LR comes out in litres per day per square metre of sidewall. You get T from a perc test.
Required sidewall area is then flow divided by loading rate. A worked example, using the kind of numbers a real Kawartha Lakes site produces:
- Four and a half fixture units on pressurized water: 900 litres per day
- Native sand with a percolation time of 10 min/cm: LR = 400 / 10 = 40 L/day per m²
- Required sidewall area: 900 / 40 = 22.5 m²
- A pit 1.0 m deep, 5.5 m by 6.0 m, gives about 23.0 m² of sidewall — just enough
Two things fall out of that example. First, a compliant grey water pit is not a small hole; that one is roughly the footprint of a single-car garage. Second, soil quality moves the answer enormously. Slower soil means a higher T, which means a lower loading rate, which means more sidewall — and on genuinely poor soil the volume of imported material starts to approach what a full septic bed would need. That is the point where grey water stops being the cheap option, which we get into in the cost guide.
The 2024 Code also allows for pits built in a horizontal or linear orientation rather than a classic deep pit, sized using the same loading rate. That matters on shallow sites where you cannot go down far.
The Construction Rules
These come straight from the Code and a designer will hold you to all of them:
- The bottom of the pit must be at least 900 mm above the high ground water table
- The sidewalls must be built so they cannot collapse
- Sidewall material must be open jointed, so water can actually leach out through it
- The pit needs a tight, strong cover, kept on except for maintenance
- The earth around the perimeter must be mounded at least 150 mm above ground level
- Surface drainage must be graded away from the pit
- The pit must be surrounded, on all sides and on the bottom, by at least 600 mm of soil with a percolation time under 50 min/cm
That last one is doing quiet work. On a site with shallow soil over bedrock, the 600 mm requirement is what stops the project, even though the Code does not state a bedrock clearance for a Class 2 the way it does for other systems. If you have read that a grey water pit needs 900 mm above bedrock, that is a septic bed rule being applied to the wrong system — the real constraint is the soil envelope.
The 900 mm above groundwater figure is also why spring conditions matter. High water table in April is the number that counts, not what you see in August when the designer visits.
Setbacks, and Why They Are Not the Septic Numbers
This is where most people go wrong, because they have read about septic bed setbacks and assume the same distances apply. They do not. A Class 2 gets easier well setbacks than a Class 4, because the pathogen load is lower.
| From | Class 2 grey water system | Class 4 leaching bed |
|---|---|---|
| Well with a watertight casing at least 6 m deep | 10 m | 15 m |
| Dug well, or any other well or spring used for drinking water | 15 m | 30 m |
| Lake, river, pond, stream, reservoir | 15 m | 15 m |
| Property line | 3 m | 3 m |
Note what is missing: there is no clearance to a building listed for a Class 2. A Class 4 distribution pipe needs 5 m from a structure. The Class 2 table simply does not have that column. (Our well distance guide covers the Class 4 side in detail, and those are the numbers that apply if you have a septic bed.)
One counterintuitive rule: if the percolation time is under 10 minutes, the well and surface water clearances have to be increased. Very fast sand does not earn you a shorter setback — it earns you a longer one, by an amount the inspector determines. Fast-draining soil moves water away before it is treated, which is exactly the risk near a well.
And a local trap worth flagging: the 30 metre water setback people quote around Kawartha Lakes comes from the City’s rural zoning by-law, not the Building Code. The Code says 15 m. Both instruments can apply to the same project. Whether the zoning setback captures a grey water pit specifically is not obvious from the by-law’s own wording, so confirm it with the City rather than assuming — local rules layer on top of the provincial ones more than people expect.
Why Pits Fail
A grey water pit has a shorter design life than a septic bed, typically 15 to 25 years, and it fails in a specific way: the sidewalls blind off and stop accepting water.
Two things drive that.
Kitchen grease. Fats, oils, and food solids coat the sidewall and seal it. Kitchen sink water is legally grey water in Ontario, but it is the dirtiest grey water there is, and it is the usual reason a pit dies early. The Code does not require a settling or trash tank ahead of a Class 2, though a designer may specify one to catch solids. If you are running kitchen water into a pit, what you put down the drain matters more than it would on a full septic system.
Continuous occupancy. This is the interesting one, and it comes from the province’s own guidance. A pit works partly because the wetted sidewall gets a chance to dry out and recover between uses. On a cottage occupied on weekends, that recovery happens naturally. On a building occupied continuously, a fixed portion of the sidewall stays permanently wet and clogs — which is precisely why leaching pit failures cluster on properties that are lived in full time.
That has a direct consequence: a grey water pit that has served a seasonal cottage for twenty years may not survive that cottage becoming a year-round home. If you are converting, the pit is on the list of things to reassess, alongside everything in our seasonal use guide.
Permits, and the Pit Nobody Has Paperwork For
A Class 2 needs a sewage system permit. In the City of Kawartha Lakes the 2026 fee is $433, and it comes from the City’s Building and Septic Division — not the health unit, which stopped handling onsite sewage systems. Only Class 1 systems, meaning privies, are exempt from needing a permit, and even those still have standards to meet. The broader process is in our septic permit guide.
You will also need a permit application for a detached building that adds fixtures. A bunkie with a sink produces grey water, and grey water is sewage — the City charges a $244 review for any additional building on the property specifically to assess the impact on the existing system. Our bunkie and sleeping cabin guide has that in detail.
Here is the part that catches people out. The City holds sewage system records going back to 1974. So when a search turns up nothing for a pit that was clearly dug in the 1990s, that is not a lost record. That is a pit that was never permitted. And bringing unpermitted work into compliance costs the applicable permit fee plus an increase of 100% — the fee doubles.
If you are buying a property with a drain you cannot account for, pulling the septic records before you waive conditions is a cheap hour.
Frequently Asked Questions
How big does a grey water pit need to be in Ontario?
It depends on your daily design flow and your soil. Required sidewall area equals daily flow divided by a loading rate of 400 divided by the percolation time. For 900 litres a day in soil with a percolation time of 10 min/cm, that works out to about 22.5 square metres of sidewall — roughly a pit one metre deep and five and a half by six metres across. Only sidewall area counts toward capacity; the bottom of the pit does not.
How deep does a grey water pit have to be?
The Code does not set a fixed depth. It sets constraints that determine one: the bottom must sit at least 900 mm above the high ground water table, and the pit must be surrounded on all sides and its bottom by at least 600 mm of soil with a percolation time under 50 min/cm. Between the water table above and the soil envelope below, most sites end up with a fairly narrow workable range.
How far does a grey water pit have to be from a well?
Ten metres from a well with a watertight casing at least six metres deep, and fifteen metres from a dug well or any other well or spring used for drinking water. These are smaller than the equivalent septic bed setbacks. If the soil percolation time is under ten minutes, those distances have to be increased.
Can I have a grey water pit if my cottage has pressurized water?
Often not. With pressurized water each fixture unit counts as 200 litres a day rather than 125, and the Class 2 cap is 1,000 litres a day. A kitchen sink, bathroom sink, tub with shower, and washing machine come to 5.5 fixture units, which is 1,100 litres a day on a pressure system — over the limit. The same fixtures without pressurized water come to 687.5 litres a day and pass.
Does kitchen sink water count as grey water in Ontario?
Yes. The Building Code defines grey water as sewage from fixtures other than sanitary units, and a sanitary unit is a water closet, urinal, bidet, or bedpan washer. A kitchen sink is none of those. Kitchen water is legally grey water, but it is also the main cause of early pit failure because of grease and food solids.
How long does a grey water pit last?
Typically 15 to 25 years before it needs rehabilitation or replacement, against 30 to 50 years for a settling chamber. Pits serving continuously occupied buildings fail sooner than those serving seasonal cottages, because the sidewall never gets a chance to dry out and recover.
Do I need a permit for a grey water pit in Ontario?
Yes. A Class 2 grey water system requires a sewage system permit, which in the City of Kawartha Lakes costs $433 as of 2026 and is issued by the City’s Building and Septic Division. Work started without a permit is charged the applicable fee plus an additional 100%.
If You Have One, or Think You Might
Plenty of properties around the Kawarthas have something pit-shaped taking grey water, and the owner has no drawings, no permit, and no idea how big it is or how close it sits to the well. That is worth resolving on your own schedule rather than during a closing.
We pump, inspect, and assess grey water systems, including the informal arrangements that came with older cottages. On new work, the design is stamped by a licensed sewage system designer — we coordinate that design and the permit, subcontract the excavation, and book the inspections.
We serve Kawartha Lakes — Lindsay, Bobcaygeon, Fenelon Falls, Coboconk, and the rural and waterfront properties between them — with no travel surcharge anywhere in the city.
Call (705) 702-5526 or book online.