A duplex is usually the simplest version, since each unit is already a separate legal dwelling under the zoning. A secondary suite added inside a single-detached home needs its own permit for things like a separate entrance and fire separation before a lender will count the rent at all.
CMHC's lending guidelines allow qualifying with documented rental income once a suite or unit is legal, and the minimum down payment on a qualifying purchase is 5 percent up to $500,000, then 10 percent on the portion above that.
Joel Dyck can walk you through which of these structures actually fits a specific property before you make an offer on it. Get your hand-reviewed valuation from Joel Dyck.