It doesn't have to be new, but adding more housing generally requires you to build buildings, and when a building is built where there used to be no building, i

It doesn't have to be new, but adding more housing generally requires you to build buildings, and when a building is built where there used to be no building, it is generally referred to as new construction and thus new (or newly added) housing. You could evict a bunch of people from existing houses and then re-sell those within the constraints of affordable housing parameters, but I doubt that is a feasible method of significantly scaled up expansion the pool of available affordable housing.