4) smaller households. You can add 50% more housing, but if more people are spending more of their years single, and desiring to live in their own abode, then increase in demand could easily outpace increase in supply.
The number of people per dwelling went up though, so that hasn't happened, at least not on average.
But I think the average could be misleading if the household size had gone up significantly in council homes, hiding a slightly falling household size in the private sector. But that's pure speculation.
