&gt;  Consider a couple in a 4 bedroom house who&#x27;s kids have moved out, but don&#x27;t move into a smaller place (say 2 bedroom condo)<p>This would theoretically deal with the problem of having young couples with many children who can only afford to live in a 2-bedroom condo, because all the bigger housing is occupied by old people with empty nests and the only new housing is 2-bedroom condos or smaller. But, if you&#x27;re not interested in forcing the old people to <i>leave</i>, it doesn&#x27;t solve the problem of housing supply -- you&#x27;ll still need some housing for the old couple and some more, separate, housing for the new large family, and if you don&#x27;t build additional housing to accommodate the incoming people you&#x27;ll end up with a housing shortage, regardless of whether old people shift from their existing homes into smaller ones or not. As long as you&#x27;re building housing for the people who move in, you might as well build it in the sizes they demand.