The minimum down payment rule sets the floor: 5 percent on the first $500,000 of the purchase price and 10 percent above that, and putting down less than 20 percent means carrying mortgage default insurance through CMHC, Sagen or Canada Guaranty on top of the loan itself.
Lenders also apply a stress test, qualifying you at a higher rate than the one you are actually offered, which caps the maximum mortgage regardless of how comfortable the real monthly payment would feel at the contract rate.
Getting pre-approved before house hunting turns this from an estimate into a firm number tied to your actual finances. Get your hand-reviewed valuation from Joel Dyck.