The low birthrate is primarily driven by college-educated women having few or no children (over 30% of all German women who were born between 1964 and 1968 and had a college degree did not have any children). It has historically been difficult to be a working mother in Germany; it was both more challenging (school being out at lunchtime) and society frowned upon you being a working mother.
This has slowly been changing; reunification happened and East German women weren't having any of this nonsense; politicians realized that the net effect was that the fertility rate dropped; and the abortion debate and a subsequent judgement by the Constitutional Court forced legislation that made having children easier. As a result, it is now a whole lot easier to be a parent (single or married, male or female) than it used to be.
This did at least stabilize the fertility rate, which had been dropping before. While nominally, Germany's fertility rate is 1.4, this does not account for the fact that the great majority of German children are being born after their mothers reach the age of thirty; current estimates set the actual fertility rate -- i.e. the average number of children a woman has over her lifetime -- at 1.6. More importantly, this number did stabilize in the aughts and has actually been increasing since then for college-educated women.
It is normal here (for my parents/grandparents/friends) to live in a rented flat and perhaps move sometime on to a own house/apartment.
I've heard that getting into the house ownership market in Germany is extremely hard, with high requirements set on your own savings to be granted a loan, meaning most people don't until they are well into their 30s and maybe even their 40s.
To me that certainly also sound like a contributing factor. Any comments or perspective on that?