Key facts
| Requirement | Number |
|---|---|
| Units allowed as of right | Up to 3 per lot: 3 in the primary building, or 2 in the primary building plus 1 in an ancillary building |
| Rezoning | Not required in settlement areas on full municipal water and sewage |
| Parking | A municipality cannot require more than one parking space in connection with these units |
| Development charges | Cannot be imposed on a second or third unit in the primary building, or a second unit in an ancillary structure where the primary building has no more than two units |
| Basement ceiling height | 1.95 m (6 ft 4¾ in) |
| Attic ceiling height | 2.03 m (6 ft 8 in) over at least 50% of the required floor area |
| Fire separation | 30 minutes between units, and between units and common areas |
| Smoke alarms | Every level of the house, plus every bedroom in the second unit |
| Permit decision | Within 10 business days of a complete application |
Do I need rezoning to add a second unit?
No, not in most cases. Ontario made up to three units permitted as of right on parcels where residential uses are allowed, inside settlement areas with full municipal water and sewage. No rezoning application, no committee meeting, no neighbour notification.
The exceptions matter, though: lots on hazard lands, houses that are already a legal non-conforming use, and properties outside serviced settlement areas do not get this. Conservation authority rules still apply near ravines, creeks and shorelines.
Will the city charge me development charges?
Not on these units. Municipalities cannot impose development charges — regardless of unit size — or require parkland dedication or cash in lieu, for a second or third unit inside the primary building, or a second unit in an ancillary structure where the main house has no more than two units.
That is real money. Development charges on a new dwelling in the GTA run into five figures, and their absence is most of the reason a basement suite pencils out at all.
What ceiling height does a basement apartment need in Ontario?
1.95 m, which is 6 feet 4¾ inches. Measure from the finished floor to the lowest thing overhead in the living space — and the lowest thing is usually a duct, a beam or a run of pipe, not the joists.
This is where most basement apartment plans die. A 1970s bungalow with 6 ft 2 in under the ducts either needs the mechanicals rerouted, or it needs the floor lowered by underpinning or bench footings — structural work that starts around $30,000 and needs its own drawings and permit.
Converting an attic instead? Then 2.03 m (6 ft 8 in) over at least half of the required floor area.
What does the Building Code require inside the unit?
- A 30-minute fire separation between the two units, and between each unit and any shared space such as a common entry or laundry room.
- Exits. Separate exits for each unit are preferable, but a shared exit is allowed if the exit area itself has a 30-minute fire separation.
- Emergency escape windows in bedrooms, sized to the Building Code minimum. In a basement this usually means cutting the foundation and building a window well — budget $3,000 to $6,000 per window.
- Smoke alarms on every level of the house and in each bedroom of the second unit.
- Ventilation and heating that serve the unit properly, which often means the existing furnace return has to be reworked.
A designer who does second units regularly will spot the expensive items in one walkthrough. That hour is worth paying for before you sign anything with a builder.
Do I have to register the unit with my city?
Often yes, and this is a separate step from the building permit. It also varies by municipality, which is why two neighbours in different cities get different answers.
Mississauga requires every second unit to be registered under its Second Units Registration By-law. Registration itself is free — you email proof of ownership, a signed-off building permit and a completed registration form, and the unit goes on the city’s public Second Units Registry. Brampton runs registration of additional residential units under by-law 157-2022 and has its own two-unit registration process.
Check your own city before you start. An unregistered unit can be a problem when you refinance, sell, or make an insurance claim — and tenant insurance and fire insurance both care whether the unit is legal.
One honest caveat about the rules
Ontario’s own homeowner guide to second units carries a note that parts of it may not reflect the land use planning changes brought in by the More Homes Built Faster Act, 2022. The province is still catching up with itself in places. Treat the numbers here as the starting point for a conversation with your municipality’s building department, not as a final answer for your specific lot.
Red flags when you hire for a second unit
| What you see | Why it matters | What to do |
|---|---|---|
| “We do legal basements, no permit needed” | A second unit always needs a building permit. This sentence means the work will not be inspected | Walk away. An uninspected unit is not legal, whatever it looks like when finished |
| Nobody measured the ceiling on the first visit | Height is the one number that decides feasibility and the biggest cost | Ask for the measurement in writing, taken under the ducts, before you pay a deposit |
| Fire separation not itemised in the quote | Drywall, sealing and rated doors are real cost and they will be added later | Ask for fire separation, egress and alarms as separate lines |
| No mention of registration | The unit can pass inspection and still not be registered with the city | Ask who handles registration, and confirm the requirement with your city yourself |
| Drawings by “a guy we use” | Second unit drawings need someone qualified under the Building Code | Ask for the designer’s BCIN or the architect’s or engineer’s stamp |
| Timeline with no permit stage | A permit decision takes up to 10 business days after a complete application, and applications are rarely complete first time | Ask for a schedule that names the permit and inspection dates |
Frequently asked questions
Insurers ask whether a property has a rental unit and whether it is legal. An unpermitted or unregistered unit can affect coverage and claims, so tell your insurer and confirm in writing what your policy covers.
Your assessment can change when you add living space or a rental unit, which may affect property tax. MPAC handles assessment in Ontario, so ask them rather than your contractor.
Published cost guides put a legal secondary suite well above a standard basement finish, commonly starting around $90,000 and rising with underpinning, egress windows and a new bathroom. Get local quotes — the spread is wide and depends on your ceiling.
A separate exit for each unit is preferable, but the Building Code allows a shared exit when the exit area itself has a 30-minute fire separation. Your designer will confirm which arrangement works in your house.
1.95 m, or 6 feet 4¾ inches, measured to the lowest projection in the living space. Ducts and beams count. If the house falls short, the options are rerouting mechanicals or lowering the floor through underpinning or bench footings.
On most residential lots in serviced settlement areas, yes — three units in the main building, or two in the main building plus one in a laneway or garden suite. Hazard lands, unserviced areas and legal non-conforming properties are excluded, so confirm with your city.
Sources
- Government of Ontario, Add a second unit to your house — ontario.ca, checked 18 September 2026
- Environmental Registry of Ontario, notice 019-6197 on permitting up to three units as of right — ero.ontario.ca
- City of Mississauga, Second units and Second Units Registration By-law — mississauga.ca, checked 18 September 2026
- City of Brampton, Additional Residential Units Registration By-law 157-2022 and two-unit registration process — brampton.ca
This guide summarises published Ontario and municipal rules. It is not legal or design advice. Confirm the requirements for your own lot with your municipality’s building department before you commit money.
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