Excellent question, what happened is there are less people in a household than there used to be and households consume more m2 per household member. In reply to your second point, percentagewise there has been less building, in absolute terms there is more because of population growth since the 60s. This translates to ‘seeing more building projects’.

https://www.umweltbundesamt.de/daten/private-haushalte-konsu...