Detached ADU 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 detached adu.

A detached unit stands clear of the main house, so it carries its own foundation, its own service laterals and its own exit - the three cost centres that separate it from an interior conversion on the same Mississauga lot. We trench services across the yard and pour to 1.2 m before the ground closes; framing, envelope and interior finishing then run through the cold months without holding up occupancy. Detached is the highest per-square-foot form of the four, and also the only one that leaves the existing house untouched - no shared walls, no disruption to the occupants during construction.