TLDR: A legal second unit comes down to things you cannot see in listing photos: a fire-rated separation between the units (30 minutes for a secondary suite in a house, 45 for a full duplex), code-sized exits and egress windows, alarms in the right places, and permits on record with your municipality. The rules now come from the 2024 Ontario Building Code, in force since January 1, 2025. In Toronto, a complete permit application for a house with up to two units is reviewed within 10 business days, so the legal route is faster than most people assume.
Stand in a finished basement apartment and try to tell whether it is legal. Nice flooring, a tidy kitchenette, a separate entrance. You cannot tell, and neither can anyone else, because almost everything that makes a second unit legal is buried inside the walls and ceilings or sitting in a permit file at the City.
That gap is where Toronto homeowners get hurt. Buyers pay for "income suites" that turn out to be unpermitted apartments. Owners collect conversion quotes that price the kitchen and skip the fire separation. This guide covers what the code actually requires, what changed in 2025, and the order of operations that keeps a conversion on budget.
What Most Homeowners Get Wrong
Three misconceptions do most of the damage.
"Duplex" and "secondary suite" are not the same thing, and the difference changes your requirements. A secondary suite is a subordinate unit inside a house, like a basement apartment, and the code permits a 30-minute fire separation between it and the main unit. A full duplex is two principal dwelling units, and the standard separation between dwelling units is a 45-minute fire-resistance rating. Homeowners use the words interchangeably. The Building Code does not, and neither does your plans examiner.
"It's been rented for years, so it must be legal." We hear a version of this constantly, and years of rent cheques prove nothing. Legality is specific: the rated separation, proper exits, alarms in the required locations, and permits on record. On one of our projects we caught an unpermitted basement apartment through a City records search before our client had spent a dollar on design. The records, not the rental history, told the truth.
"The rules are whatever my contractor says they are." The rules are published. Since January 1, 2025 second units fall under the 2024 Ontario Building Code, which brought Ontario in line with the national code and its secondary suite provisions. A contractor quoting you from memory of the old rules is quoting you the wrong rules.
The Requirements That Actually Make a Unit Legal
Fire separation: the part photos never show
The wall and ceiling assemblies between the units carry a fire-resistance rating: a minimum of 30 minutes for a secondary suite in a house, 45 minutes between dwelling units in a full duplex. In practice that usually means Type X drywall and mineral wool insulation built as a continuous barrier, with rated closures where the units connect. Sprinklering the building can change what is required, which is a conversation for your designer, not a reason to skip the assembly.
This is the heart of a legal conversion, and it is invisible in a finished space. It is also the first thing to look for in a quote. The red flag in a conversion quote is usually not what is in there but what is missing, and a quote with no fire separation line is telling you how the project will go.
Exits and egress windows
Every unit needs its own way out. That can be direct access to the outside, a shared exit with proper fire separation and interconnected alarms, or bedroom egress windows with at least 0.35 square metres of clear opening and no dimension under 380 mm, openable from inside without tools. Basement conversions often need new, larger windows to get there, and window wells need enough clearance to actually climb out of. Budget for the concrete cutting early; it is one of the more common surprise costs.
Smoke and carbon monoxide alarms
Under the current code, smoke alarms go on every storey and in every bedroom, interconnected so an alarm in one unit is heard in the other. New work is hardwired; sealed 10-year battery alarms are permitted in the existing parts of a house when a suite is added. Carbon monoxide alarms go near sleeping areas and wherever fuel-burning appliances live. If both units share ducted heating, the system needs a duct smoke detector that shuts the air handler down, or the units need separate systems.
The small stuff that fails inspections
Self-closing devices on doors between the units and into shared spaces. An Electrical Safety Authority permit and inspection for every electrical change, and many conversions trigger a panel upgrade the same way adding a bathroom does. A backwater valve where a basement unit's plumbing sits below the sewer line. Ceiling height, which the current code relaxes to roughly 1.95 metres for suites in existing houses. None of these are expensive by themselves. All of them are expensive to discover after drywall.
Permits: faster than their reputation
A second unit needs a building permit, with drawings prepared by a qualified BCIN designer. In Toronto, a house with up to two dwelling units sits in the City's House Stream, where complete applications are reviewed within 10 business days. Read that again, because it undercuts the most common excuse for skipping permits. The review is not the slow part. Assembling a complete, code-compliant application is where the time goes, which is exactly why the designer matters.
The BVM Approach
We build second units permit-first, and we get involved before the drawings exist. Our advice to every homeowner is the same advice we give on additions: do not get baited into the design process until you have a real gauge of construction costs and of what is possible on your property from a zoning perspective. For a conversion that means confirming the zoning allows the unit, walking the house for the expensive realities (egress, ceiling height, panel capacity, plumbing depth), and putting real numbers against them before a designer is engaged.
Then the sequence is boring on purpose: BCIN drawings, permit, construction, inspections, records. Boring is what you want from a project whose entire value depends on being provable on paper. A unit we build is one you can hand a buyer's lawyer without flinching.
We do this work across Toronto, and the bungalow streets of East York and Scarborough are where we see the most conversion potential sitting unused: deep lots, solid post-war structure, and basements with workable ceiling heights.
Key Takeaways
A secondary suite in a house needs a 30-minute fire separation; a full duplex needs 45. The split is set by the Ontario Building Code, not by your municipality's mood.
The 2024 Ontario Building Code has governed second units since January 1, 2025. Quotes and articles written against the old code are stale.
Bedroom egress windows need at least 0.35 square metres of clear opening with no dimension under 380 mm.
Smoke alarms go on every storey and in every bedroom, interconnected between units; shared ductwork needs a duct smoke detector that shuts the system down.
Toronto reviews complete permit applications for houses with up to two units within 10 business days. The paperwork is the slow part, not the City.
Rental history proves nothing. Permit records do.
Frequently Asked Questions
How do I find out if an existing basement apartment is legal?
Ask the City, not the seller. Toronto's building records search shows every permit on file for the property, and we use it before clients commit money to anything. If the unit does not appear in the records, treat it as unpermitted and price the work to legalize it into your offer.
How long does a second unit permit take in Toronto?
The City reviews complete House Stream applications within 10 business days. The real timeline lives in everything before that: zoning confirmation, the site visit, and BCIN drawings typically take a few months done properly. Plan the project around the drawings, not the review.
Can we live upstairs while the basement is converted?
Usually yes, and it is one reason conversions are gentler than additions. The disruptive stretches are concrete cutting for windows, plumbing rough-in, and any panel work. We sequence those tightly and tell you the noisy weeks in advance.
Thinking About a Second Unit?
Before you spend anything on design, find out what your property allows and what the conversion really costs. We review the zoning, walk the house, and put honest numbers against the invisible work first, so the design money you spend is spent on a project that can actually be approved. Book a call through bvmhomes.com and we will tell you, plainly, whether your basement is a good candidate.