Don't forget that during WW2, the availability of construction materials was heavily curtailed due to diversion to war production. A fair amount of that homebuilding would have been making up for the lack of homebuilding in WW2 and consequent housing shortages.

New construction was exempt from rent controls. So people built aggressively. But later, all of that new construction got regulated again, and construction has never picked up to that extent, due to the 'recapture'

Is there a source for this?

My first guess is period of time in the early 70's known as The Great Inflation. Primarily driven by the abolishing of the gold standard. Not sure what the gold mining to construction rate was prior to this point but thereafter they could print it faster than they could build and this type of inflation where there is little growth in actual output creates asset bubbles. Just like this latest episode of quantitative easing has led to housing price inflation around the world right now.