Legality is the gate. The City of Saskatoon requires a building permit for a secondary suite, and a lender will not count income from an unpermitted unit the way it counts income from a permitted one, regardless of how much rent it actually generates.
Down payment and debt ratios also shift once rental income enters the picture. CMHC still applies its gross debt service and total debt service guidelines, roughly 39 percent and 44 percent, but the rental income offsets part of the housing cost side of that math rather than being ignored.
A two-family purchase is financeable, but only when the paperwork backs up the income a buyer is counting on. Get your hand-reviewed valuation from Joel Dyck.