Secondary Suites in Mississauga - Design, Permits, Construction

Mississauga's ARU by-law caps accessory structures at 10% rear-yard coverage, which on a typical Erin Mills lot works out to roughly 645 sq ft - enough for a two-bedroom garden suite but not for the three-bedroom units clients often ask about first. We size the concept to that percentage before drawing anything. Mississauga permits additional residential units under its 2022 ARU by-law - up to two units in the main dwelling plus one in an accessory structure, subject to a maximum 10% rear-yard coverage.

Large 1970s-90s subdivision lots with attached garages; garage conversions and rear coach houses are both common outcomes. Lake-moderated winters in Mississauga - 112 cm of snow, -3.4°C average - give one of the longest build seasons in the region (April-November), which shortens the permit-to-occupancy timeline on secondary suites.

"Secondary suite" is the Ontario planning term for a second self-contained dwelling inside the existing house, so the review in Mississauga turns on the interior - separate kitchen and bath, an exit that does not pass through the primary unit, and rated separation between the two. Because the shell already exists, the schedule is driven by inspection sequencing rather than by weather: rough-in, framing and insulation inspections gate one another regardless of the April-November season. Bill 23 makes up to three units as-of-right on most serviced Mississauga lots, so a secondary suite generally proceeds by building permit alone without a rezoning application.