The premium a duplex commands comes from its income potential, not from square footage alone. A buyer evaluating a duplex is often thinking like an investor, checking achievable rent against the purchase price, while a buyer for a single-family home is thinking about living in it, and the Saskatchewan REALTORS Association's sale data tracks the two as distinct property types rather than interchangeable ones.
Legality changes the math substantially. The City of Saskatoon requires a permit for a secondary suite or a legal duplex conversion, and a lender will only credit the rental income from a unit that permit file supports, so an unpermitted second unit adds little to a lender's number even if it is generating rent today.
Whether a specific duplex is worth more than a comparable single-family home on your street depends on its permit status and its actual achievable rent, not a general rule, and a pre-purchase home inspection, typically $400 to $600, is worth it either way. Get your hand-reviewed valuation from Joel Dyck.