Garden suite permits in Ontario rest on one provincial rule and a different set of local rules in every city. The province requires municipalities to allow a third home on most serviced residential lots, and that third home can sit in a detached building in the backyard. How big it can be, how tall, how far from the house and what you file before the building permit are all decided locally, which is why a design that works in Mississauga may be too large for Brampton. That local check is the first step in any garden suite permit, before a drawing is started. The table compares the current garden suite rules in four GTA cities, taken from each city’s own zoning documents.
| City | What the city calls it | Maximum size | Maximum height | A rule that often decides the design |
|---|---|---|---|---|
| Mississauga | Detached additional residential unit | 55 m² on lots up to 550 m²; 10% of lot area up to 100 m² on larger lots | 4.6 m one storey, 6.8 m two storeys (sloped roof) | Rear yard only, 1.8 m from the rear lot line |
| Brampton | Garden suite | 35 m² in most residential zones | 4.5 m, or the house height if that is lower | Planning review before the building permit, then registration |
| Vaughan | Additional residential unit in an accessory building | No larger than the main dwelling unit | Set by the accessory building rules | 4.0 m from the main building, one parking space |
| Markham | Garden home (coach house if on a lane) | 15 m² on lots up to 500 m²; 60 m² on larger lots | 4.0 m on most lots | 4.0 m from the main building, one per property |
Ontario garden suite rules under the Planning Act
The provincial baseline came with Bill 23, the More Homes Built Faster Act, passed in November 2022. On a lot with a detached house, semi or townhouse in a settlement area with full municipal water and sewers, the Planning Act now lets owners have up to three units: either three inside the main building, or two inside and one in an ancillary building such as a garden suite. A city can add more permission, but it cannot take this away.
The province also limited what cities can demand. A municipality cannot require more than one parking space per additional unit, and it cannot set a minimum unit size. Second and third units of this kind are exempt from development charges, parkland dedication and cash-in-lieu, a cost that a new house on the same lot would carry. What the Planning Act does not do is set the dimensions. Height, floor area, setbacks and yard location stay in each city’s zoning by-law, and the building permit is still reviewed against the Ontario Building Code like any other dwelling.
Garden suite rules in Mississauga and Brampton
Mississauga’s zoning allows one detached additional residential unit on lots in its main low-rise residential zones, as part of the same three-unit limit. The suite goes in the rear yard, or on a corner lot in the rear and exterior side yard. Floor area scales with the lot: 55 m² on a lot of 550 m² or less, and 10% of the lot area on bigger lots, capped at 100 m². Setbacks change with height. A one-storey suite needs 0.6 m from the interior side lot line and a two-storey suite needs 1.8 m, while the rear setback is 1.8 m for both. A flat roof is held lower than a sloped one, at 3.0 m for one storey and 5.8 m for two.
Brampton is tighter. Outside its hamlet, estate and agricultural zones, a garden suite is limited to 35 m² and 4.5 m in height, and it cannot be taller than the house. It has to sit in the rear or interior side yard, 2.5 m from the rear lot line, 1.8 m from the side lot line and 3.0 m from the house. Brampton also adds a step before the building permit: a Custom Home – Garden Suites Review application to the Planning Division. Once the suite is built, it must be registered with the City, as every additional residential unit in Brampton is. A lot can have the house, one attached second unit and one garden suite, and no extra parking space is required when the garden suite is the only additional unit.
Garden suite rules in Vaughan and Markham
Vaughan’s current rules came into force on March 25, 2025, through By-law 082-2025. One additional residential unit can be in an accessory building, as long as there is also a unit in the main house, and only on a lot with municipal water and sanitary services. The building has to be at least 4.0 m from the main building, its floor area cannot exceed that of the main dwelling unit, and one parking space is required for the unit unless the lot is in a protected major transit station area.
Markham’s April 2026 building standards separate two types. A coach house needs access from a lane, while a garden home does not, and a property cannot have both. A garden home is limited to 15 m² on lots of 500 m² or less and 60 m² on larger lots, and to 4.0 m in height unless the lot is over 4,000 m², where 5.5 m is allowed. Both types sit at least 4.0 m from the main building and 1.2 m from the side and rear lot lines. Markham also spells out a servicing detail many owners miss: each unit needs its own water shut-off and a self-contained ventilation system.
What every garden suite permit in Ontario still needs
Whatever the city, a garden suite is a separate dwelling, and it needs a building permit. The application shows the suite on a site plan with the setbacks the local by-law requires, plus floor plans, elevations, sections and the construction details for a house. Water, sewer and electrical service usually run from the main house, so the route of those lines and any trees in the way are part of the design rather than an afterthought. Fire department access to a backyard building is another common question, and the answer depends on the lot and on the municipality’s own requirements.
Toronto deserves a note of its own because it has a more detailed system, with separate rules for garden suites and laneway suites and a set of pre-approved plans. A Toronto lot is checked against those rules first, the same way a Brampton lot goes through the Planning Division review first.
For any garden suite permit in Ontario, the practical order is the same: confirm which by-law applies to the lot, measure the yard against the local size, height and setback limits, and only then draw the suite. The province guarantees the right to a third unit on most serviced lots, but the city’s numbers decide whether a 35 m² studio or a 100 m² two-storey home fits behind the house.