Ontario · Design worked example · 2026
The 3-bedroom septic design: every number, start to finish
Three bedrooms is the standard Ontario system — the one most tables, most quotes and most examples silently assume. Here is the complete design walked through in order, with nothing skipped, so you can follow every number to its source in the Code.
The headline numbers: a 3-bedroom house designs at Q = 1,600 L/day, needs a 3,600 L two-compartment tank, and on typical T=18 soil a 144 m leaching bed in 5 runs. Change the soil and only the bed changes — the logic never does.
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1 Something’s wrong2 What it costs3 The permit4 Who designs it5 The paperwork6 Approval
The part nobody tells you
Designers charge $800–$2,000 just for the paperwork in these quotes.
Ontario law lets you do that part yourself, on your own property. OntarioSepticDesigner.ca turns your answers into the same 26-page package the township clerk sees every day — calculations, drawings, forms, all of it.
Step 1 — Daily design flow: Q = 1,600 L/day
The Code assigns residential flow by bedroom count: 3 bedrooms = 1,600 L/day base. Then two multipliers can raise it: more than 200 m² of finished floor area, or a high fixture count, push Q up in steps. A typical 3-bedroom bungalow with normal fixtures stays at 1,600 — but count honestly: a “den” with a closet and a window is a bedroom to a reviewer, and the difference cascades through every number below.
Step 2 — The tank: 3,600 L minimum
Working capacity must be at least twice Q — and the Code table for 1,600 L/day sets 3,600 L minimum, two compartments, with an effluent filter on the outlet. In practice you order the standard 3,600 L (800 gal) concrete tank; going one size up (4,500 L) adds a few hundred dollars and buys longer pump-out intervals — worth it on rental or high-occupancy homes.
Step 3 — The bed: soil decides everything
| Your soil | T-time | Pipe: L = 1,600 × T ÷ 200 | Layout |
|---|---|---|---|
| Coarse sand | 6 | 48 m | 2 runs × 24 m |
| Fine sand | 10 | 80 m | 3 runs × 26.7 m |
| Sandy loam | 18 | 144 m | 5 runs × 28.8 m |
| Silty loam | 30 | 240 m | 8 runs × 30 m |
Runs cap at 30 m, centres at 1.6 m. Your T-time comes from your test holes — the full T-time explanation is here. The T=18 case needs a bed roughly 28.8 m × 7 m plus the working margin: the reason bed placement leads house placement on tight lots.
Step 4 — Vertical check: does it fit DOWN?
The bed bottom needs 900 mm of unsaturated soil above the seasonal high water table, bedrock or impermeable layer. Clear it and the bed sits in-ground (the prices above). Miss it and the bed rises on imported sand — same math, plus a raised-bed premium of $10,000–$20,000. This single measurement is the biggest fork in the whole design.
The $18,950 line that never appears on a quote
A real worked example: 3-bedroom bungalow, conventional bed. Tank supplied and set, pipe, stone, fabric, one day of machine and labour, permit allowance — $13,049 with HST. The same job quoted at $32,000. The gap is design fees, markup, and labour you may not need to buy.
Two minutes on the free checker shows the numbers your own lot generates — daily flow, tank size, risk level — before anyone quotes you.
Step 5 — Paper: where each number lands
Q and the tank go on the application form and Schedule; the bed math fills the design worksheet; the layout draws onto the site plan with every setback dimensioned; the vertical numbers draw onto the cross-section. One number, several documents, zero tolerance for disagreement — the full package list shows all of it. Material quantities fall straight out of the same figures: stone and sand here, the complete order sheet here.
3-bedroom questions, answered straight
What size septic tank for a 3-bedroom house in Ontario?
3,600 L (800 gal) minimum, two compartments, effluent filter. One size up is cheap insurance for heavy occupancy.
How big is a 3-bedroom leaching bed?
Depends entirely on soil: from 48 m of pipe on coarse sand to 240 m on silty loam. On typical T=18 ground: 144 m in 5 runs, roughly a 29 m × 7 m footprint.
Does a finished basement change the design?
Extra floor area and fixtures can push Q up a step, and a basement bedroom counts as a bedroom. Design to the house you will actually live in.
What does a 3-bedroom system cost?
Conventional in-ground: roughly $20,000–$30,000 installed new, or about $13,000 in materials and machine time owner-built. Raised: add $10,000–$20,000. Full breakdown on the cost page.
Your lot’s version of these numbers, in 2 minutes
The free check runs this exact logic on your soil and water answers. The $99.99 builder prints it as the complete township-ready package.