How do I verify that a basement apartment or duplex unit is legally registered before I buy?

Start by requesting the property's building permit and zoning history from the municipality, which can typically show whether the second unit was ever permitted, inspected, and approved as a legal secondary suite or duplex configuration under the applicable zoning and the Ontario Building Code, rather than simply added without approval at some point in the property's history. A municipal zoning compliance letter or similar confirmation, where available, adds another layer of verification beyond what the listing or seller tells you.

Because municipalities and their record-keeping practices vary, and older additions sometimes predate current permit systems, this is not always a simple yes-or-no answer from a single document, so it is worth having your lawyer help interpret what the records actually show for your specific property and municipality. A qualified inspector can also flag physical signs, such as fire separation and egress issues, that suggest a unit was never brought up to the standard required for legal status. Confirm this during your due diligence period, before waiving conditions, since resolving uncertainty afterward is far harder.