What plumbing code requirements does the Ontario Building Code have for a new basement bathroom?

A new basement bathroom in Ontario must meet all Ontario Building Code (OBC) plumbing requirements including proper drain sizing, venting, backwater valve protection, minimum fixture clearances, and a plumbing permit with inspection from the City of Toronto Building Division.

Basement bathrooms are one of the most common renovation projects across the GTA — particularly in post-war homes in Scarborough, North York, Etobicoke, Mississauga, and Brampton where many homes were built with rough-in plumbing in the basement slab specifically for a future bathroom. Whether you're connecting to an existing rough-in or starting from scratch, the code requirements are detailed and non-negotiable.

Drain and Waste Requirements

Every fixture in a basement bathroom requires a properly sized drain connected to the building's main drain system. The OBC specifies minimum drain pipe sizes: 4-inch for the toilet (or 3-inch if connecting to a 4-inch main), 2-inch for a shower or bathtub, and 1.5-inch for a vanity sink. All horizontal drain runs must maintain a minimum slope of 1/4 inch per foot (1:48 ratio) to ensure proper drainage by gravity.

If your basement floor is concrete — as it is in virtually all GTA homes — installing new drains means cutting through the concrete slab, laying the drain pipes in the gravel bed below, and patching the concrete. This is skilled work that typically costs $3,000–$7,000 for a complete 3-piece bathroom rough-in and is the single largest cost component of a basement bathroom addition.

Each fixture must have a trap (the U-shaped pipe section that holds water to block sewer gas). Trap sizes must match fixture drain sizes, and the trap must be located within the maximum distance to a vent as specified by the OBC — typically 1.5 metres for a 1.5-inch pipe and up to 3 metres for a 3-inch pipe.

Backwater Valve Requirement

This is critically important for GTA homeowners. The City of Toronto requires a backwater valve on the building's main sewer line to prevent sewage from backing up into your home during heavy rainfall or municipal sewer surcharges — events that Toronto experiences regularly during summer storms. If your home doesn't already have a backwater valve, adding a basement bathroom is the right time to install one. Cost is typically $1,500–$3,000 installed, and Toronto offers a subsidy of up to $3,400 through the Basement Flooding Protection Subsidy Program that can cover the full cost.

For basement bathrooms specifically, a sewage ejector pump may be required if the bathroom fixtures are below the elevation of the main building drain. The ejector pump collects waste from the toilet, shower, and sink in a sealed basin and pumps it up to the main drain line. An ejector system typically adds $2,000–$4,000 to the project cost.

Venting Requirements

Every basement bathroom fixture needs proper venting to function correctly. The vent system provides air pressure equalization that allows drains to flow freely and prevents trap siphoning (which would allow sewer gas into your home). Basement bathroom venting typically ties into the main vent stack through vent piping routed up through interior walls. Your plumber may use wet venting — where an oversized drain pipe serves as both a drain and vent — to simplify the installation, which is permitted under the OBC.

Fixture Clearances

The OBC specifies minimum clearances that affect your basement bathroom layout: 15 inches minimum from the toilet centreline to any side wall or obstruction, 21 inches of clear space in front of the toilet (24 inches recommended), and 21 inches in front of the vanity. These clearances are code minimums and will be verified during inspection.

Permits and Inspection

A new basement bathroom always requires a plumbing permit from the City of Toronto Building Division. There is no exception — any new drain connection, new water supply connection, or new fixture installation requires a permit. An electrical permit is also required for bathroom lighting, the exhaust fan, GFCI outlets, and any heated floor circuits. Expect permit fees of $300–$600 total for plumbing and electrical.

The plumbing inspector will visit to examine the rough-in work before the concrete is patched and walls are closed up. Never allow your contractor to cover up plumbing work before it has been inspected — if it fails inspection, you'll be tearing everything open again at significant additional cost.

Get matched with a bathroom contractor for a free estimate on your basement bathroom project through Toronto Bath Remodeling.