If there are 5 houses in an area, populated by 1 family each for a total of 5 families, and those families have 1 child each (50% below population replacement rate), then with no new houses, there is no way for any of those children to have a house of their own until their parents die. But when their parents die, they get to have a home of their own.

At 2 children, the population replacement rate, only one child can have a house of their own after their parents die, if no new houses are constructed.