So when were they built do you ask?
Well, the largest growth period took place in the 1950's when nearly 1 in 5 units were built.
Every decade before and since has seen housing construction in a tight range of 11-14% of the total supply - until the 2000's.
Interestingly, only 13% of all housing units were built earlier than 1940.
Home construction is again picking up - which is good news.
And now you know.