The rule changes by municipality
Start with the city where the property is located.
The City’s ARU program treats legality as a combined permit, inspection and registration workflow. A standard basement-finish permit is not the same thing.
The City publishes a step-by-step ARU process with a two-business-day screening target and a 10-business-day building-permit review once the application is complete.
An ADU is a separate self-contained residential unit that can be within, added to, or detached from the principal dwelling. Internal ADU work is explicitly included in Hamilton’s permit-required list.
This is a separate project route from ordinary basement finishing because the work creates another dwelling unit.
London treats ARUs as a dedicated permit path with zoning review, site planning and construction drawings.
A secondary suite can be in the basement or another part of the house. It is a distinct permit and registration workflow, not simply a finished-basement project.
Mississauga treats an additional residential unit as more than a basement finish. Conversion drawings, applicable-law information and registration are part of the route.
A basement-to-accessory-dwelling conversion is treated as a change that needs zoning approval; the zoning certificate is not itself permission to construct.
The guide is for basement secondary suites in existing buildings and distinguishes those projects from detached accessory-building units or third units.
Ottawa’s published guidance is deliberately scope-based. The permit route changes with the work, so define the proposed construction before assuming every secondary unit uses the same application.
A basement apartment is a common example. This differs from a finishes-only basement project. The City’s guide covers the drawings and related mechanical or plumbing submissions.
Vaughan’s process begins with property eligibility and zoning. Newer units use the Building Code permit route, while older existing units can involve Fire Code review.
For basement or main-floor ARUs, the City also directs applicants to confirm zoning, parking and driveway requirements before filing.
Whitby accepts principal-residence and detached ADU permit applications and requires registration with Enforcement Services. A temporary incentive program is available for qualifying permits under 2026 deadlines.
Ontario sets the Building Code. Municipal rules, zoning, processes and project guidance still differ.
We publish a city only after attaching municipal sources to the project answer. Missing coverage stays missing instead of borrowing another city’s rule.