OAKVILLE, Ontario — Janet Barber doesn’t dwell too much on the view from her driveway.

Many of her longtime neighbors up and down the block have cashed in on the area’s soaring house prices and moved away. Where their modest wood homes once stood, much larger architect-designed houses of stone, steel and glass fill the lots. Across the street, a big sign plastered with “Sold” stickers means that one is probably next to go.

But no matter how much Ms. Barber and her husband, Michael, might now be able to get for their own three-bedroom bungalow, they are not about to join the rush.

Why? Though they are old enough to be empty-nesters, their nest isn’t empty. Their 29-year-old daughter, Sarah, has been living with them since she finished a graduate degree in 2013, because she can not yet afford a place of her own. Her older sister, Jennifer, did the same for six years.

The more house prices rise, the longer it will take Sarah to save up enough to move out. But the longer she and thousands like her stay with their parents, the fewer houses are put up for sale — and that scarcity is a big reason prices are soaring.