Is house construction financing possible at all?

Seriously? I consider forecasts regarding pensions in 30 years to be a look into a crystal ball... nothing more.

Yes, seriously. The pension information is needed by the bank anyway to check whether you can still afford the loan in retirement. You can keep it ready.

It states how high the pension will be with constant contributions, each at 0%, 1%, and 2% annual adjustment. You can already draw conclusions from that. In the last 10 years, adjustments were more than 2%, this year more than 5%.

Do you really still believe that we will get 48% at retirement in 2050? I would already be happy if it becomes 25%.

The 48% is a very statistical figure (or pub number) that says nothing about the personal pension.

I do agree that the pension will not be great, but only because it increases less than inflation.
But that doesn't matter, the debt is fixed and does not increase. Therefore, it will already be possible to say with the notification whether the pension will be sufficient to service the loan in 30 years.

It’s basically like paying rent, only then to the bank.

Sort of. You’re still responsible for all maintenance, which is something you should keep in mind with such arrangements.

Sure, that's the crux. Without reserves, a new heating system will require new loans. And the new heating system will definitely need to be replaced at some point. It's like a car; you don't drive one for 30 years or more.