Legal basement apartment in Ontario: what the Building Code asks for

A basement apartment is legal when it was built under a building permit and passed its inspections. A kitchen, a lock on the stair door, and a tenant do not make it legal. The rules below are the ones Ontario building departments publish for a second unit in an existing house under the 2024 Ontario Building Code, which took effect on 1 January 2025.

We took every dimension from two municipal guides written against that code: the City of Markham's 2025 secondary suites guide for homeowners and the Township of Zorra's basement unit guide, last revised in June 2026. Where they disagree, we say so. This is planning information, not legal or engineering advice. Your municipality's building department interprets the code for your house, so ask them before anyone draws plans.

Can you add a unit at all

Since the More Homes Built Faster Act, 2022, Ontario's Planning Act stops an official plan from prohibiting up to three units on a parcel of urban residential land: up to three in a detached house, semi, or rowhouse, or two in the house and one in an ancillary building such as a garage. Urban residential land here means land in a settlement area where residential use is permitted and that is served by municipal water and sewage. Zoning still sets entrances, setbacks, and parking. Markham, for example, wants a clear 1.2 m path from the street to the suite entrance and one parking space for the suite.

Markham's guide also notes that, under the Building Code, a house with a secondary suite has two dwelling units in total. A third unit inside the same house is a different conversation with the building department, so raise it at the start rather than at the drawing stage.

The five-year line

Both guides open with the same question: how old is the house? A house less than five years old follows Part 9 of the code. A house at least five years old can use the compliance alternatives in Part 11, which are more lenient in several of the sections below. That one fact changes the fire separation, heating, and sound answers, so confirm it before you price anything.

Ceiling height

Under Division B, 9.5.3, both guides give the same two numbers for a basement secondary suite: at least 1.95 m of ceiling height, about 6 ft 5 in, and at least 1.85 m, about 6 ft 1 in, under beams and ducting. Markham notes that this includes the height over stairs.

Online checklists often quote 6 ft 11 in under ducts. That does not match what these Ontario building departments publish for a basement suite in an existing house. If a contractor tells you the ducts have to come up that high, ask which code article they are reading, because dropping or rerouting a trunk line is real money.

Getting out of a bedroom

Under Division B, 9.9.10.1, a bedroom level needs an egress window unless a door on that level opens directly to the outside, such as a walkout or an exterior walk-up stair. Both guides list the same requirements:

• It opens from the inside without tools
• The unobstructed opening is at least 0.35 m², about 3.8 sq ft, with no dimension under 380 mm, about 15 in
• It stays open on its own, without being propped
• If it opens into a window well, there is at least 550 mm, about 21.7 in, of clear space in front of it

Zorra's guide also describes an escape window, which is a different thing with larger minimums. It is used where an open exterior stair is the only way out of the suite. Ask the building department which one your layout needs before you order glass, because a window that meets one rule can fail the other.

Separating the two units

Unless the house is sprinklered, the code wants a barrier between the units. Under Division B, 9.10.9.16, the usual approach is a continuous smoke-tight barrier of at least 15.9 mm, or 5/8 in, Type X gypsum board on both sides of the walls between the units and on the underside of the floor framing above the suite. The alternative is a fire separation with a fire-resistance rating of at least 30 minutes. Zorra's guide is blunt that regular gypsum board does not count as having any rating.

Under 9.10.8.3, the beams, columns, and loadbearing walls that hold up the floor between the units need the same 15.9 mm Type X protection, or a rating at least equal to the floor they support. Doors between the units can be solid-core wood at least 45 mm thick with a self-closing device. Markham also accepts metal-clad doors.

Older houses get some relief. Markham's guide, for a house more than five years old, accepts a 30-minute separation and, for the floor, a 15-minute horizontal separation if smoke alarms are installed in every unit and common area and interconnected. That relief is in Markham's guide, not Zorra's, so confirm it with your own building department. Markham also says insulation inside fire-rated assemblies should be noncombustible mineral wool.

Alarms

Under 9.10.19, both guides put smoke alarms on every storey including the basement, in every bedroom, in the hallway between the bedrooms and the rest of the storey, and in shared areas. They have to be interconnected, hard-wired or wireless, so that one alarm sets off every alarm in the building, upstairs included.

Where the house has a fuel-burning appliance or an attached garage, both guides also want carbon monoxide alarms beside each bedroom, and inside any bedroom that shares a wall, floor, or ceiling with the furnace room or the garage. Markham allows battery or plug-in alarms in houses more than five years old.

Heating and ducts

In a house under five years old, return air from one unit cannot be recirculated into the other, which in practice means a separate heating system for the suite. In a house more than five years old, both guides allow the existing furnace to be altered to serve both units, provided smoke alarms are installed in each unit and a duct-type smoke detector shuts off the fuel supply and electrical power to the heating system when it trips.

Sound: the guides do not agree

This is the one place the two guides read Part 11 differently. For a house more than five years old, Markham lists a sound transmission class of STC 43, an apparent rating of ASTC 40, or a prescriptive Part 11 assembly. Zorra lists no sound requirement at all for that age of house, and STC 43 only for newer ones.

Markham's prescriptive assembly is the one that includes resilient channel, alongside sound-absorbing insulation in the joist and stud spaces and gypsum board on the ceiling and walls. So resilient channel is one way to meet a sound rule, not a requirement on its own. Ask which reading your building department applies before you buy it.

Plumbing and electrical

Under 9.31, each unit needs a kitchen sink, a lavatory, a bathtub or shower, a toilet, and access to laundry. Markham allows the laundry to be shared. Both guides also want a shut-off valve where the water enters each unit, so shutting off one unit does not cut off the other.

On electrical, Markham says the service to the house has to be sized for the suite and that the Ontario Electrical Safety Code adds its own requirements. We did not find an official source that says a basement suite must have its own panel or a 200-amp service, so we are not telling you it does. The Electrical Safety Authority says almost all electrical work needs a notification filed before work starts, by whoever is doing the work, and that the wiring is inspected at rough-in before insulation or drywall covers it. An ESA notification is not a building permit; you may need both.

Who draws it, and the inspections

Markham accepts drawings from a designer holding a Building Code Identification Number qualified in House or Small Buildings, a licensed architect, a professional engineer, or a homeowner who takes responsibility for the code. A building permit is required before work starts.

Markham's inspection list is a useful order of work for any Ontario city: framing, plumbing and mechanical rough-in, insulation and vapour barrier, fire separations and firestopping, and a final occupancy inspection. Some cities add a registration step afterwards; Markham requires every suite to be registered with its fire service.

Buying a house that already has one

A suite that came with the house is only as legal as its paperwork. Ontario's Fire Code has a separate retrofit section for two-unit houses, Section 9.8, which measures them against what existed on 14 July 1994. Ask the seller for the building permit and final inspection, or the fire department's retrofit file, and treat an answer of "it has always been there" as a budget line, not a reassurance. Our secondary suite cost guide covers what the work tends to cost; we have not put prices here because none of these sources publish them.