Ask for the permit number and pull the file yourself rather than taking the seller's word for it. A finished, good-looking second unit with no permit on record still fails a lender's test for legal secondary income, which changes what you can actually qualify to borrow.
If the suite is legal, CMHC and other insurers apply specific rules for how much of the rental income can count toward your mortgage qualification, and those rules differ from a standard single-family purchase. Confirm with your lender early, before you are committed to a purchase agreement.
A home inspection, typically $400 to $600, should still cover both units before your conditions come off. Get your hand-reviewed valuation from Joel Dyck.