When does Part 8 (septic systems) of the Ontario Building Code apply?
Part 8 governs on-site sewage systems with a design capacity of 10,000 litres per day or less serving a single lot. Installing, altering or repairing one requires a permit from the local principal authority.
If a property is not connected to a municipal sewer, its septic system is a regulated building governed by Part 8 — larger systems (over 10,000 L/day) fall instead under the Ontario Water Resources Act and MECP approval. Part 8 classifies systems from Class 1 (privies) through Class 4 (the familiar septic tank plus leaching bed) and Class 5 (holding tanks), and requires a permit for new systems, replacements, and alterations. Critically for renovators: adding bedrooms or plumbing fixtures to a house increases its design sewage flow, which can push an existing septic system beyond its approved capacity — a common reason building departments ask for septic assessments on addition permits.
No sewer connection means your septic system is part of the building code, and it needs its own permit — not just when you build it, but when your renovation adds bedrooms or bathrooms that make the house produce more wastewater than the system was sized for.
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This page is provided for general information. Building code requirements depend on your specific project, occupancy and municipality, and code editions change. Always verify requirements against the current Ontario Building Code and confirm with your local building department.