Housing supply doesn’t grow by all that much though, so a “relatively small amount” is enough to tighten a market. In any case, censuses define a household as

Housing supply doesn’t grow by all that much though, so a “relatively small amount” is enough to tighten a market. In any case, censuses define a household as people living together in a unit, which would account for roommates. Portugal has gone from 3.7 in 1960 to 2.6 in 2011. Housing 10.3M people at 3.7 requires 2.8m units; 2.6 requires 4m units. And that may be just what Portuguese people have to put up with, but not their desired end state; if they were at the EU average of 2.2, then it would require 4.6m units.