New funding for family home ownership

I was told by that this is not so attractive.. the banks want some kind of security in this regard and you would have to invest something for it? So, you can’t just say "by then I inherit the money or find a treasure".

Correct. There must always be a good reason for a bullet loan.
This can be, for example, a due life insurance policy or something similar. Alternatively, it also works if you bring enough equity at the beginning of the financing. The bank doesn't care whether you start with an 80% loan-to-value ratio that falls to 60% after 10 years of repayment, or whether you start directly with 60% and end after 10 years of interest-only with 60%.

By the way, this is an absolutely common approach with institutional investors such as funds, insurance companies, pension funds. They all bring 45-60% equity to the purchase price of a property and then do not repay for it.

My financier would at least calculate the option with the bullet KfW loan in combination with a home savings contract. Actually, I was always rather skeptical because of the high interest rates. But with a KfW loan, it might not be so uninteresting after all.

I would also be interested in details.

We are building a 180 m² city villa, without a basement, without attic conversion. The exterior walls are made of 17.5 cm RDK 2.0 KS-Quadro bricks. RDK 2.0 is somewhat exaggerated, but our shell builder was able to obtain the Quadros quite inexpensively and the structural engineer had no problems with it. The insulation of the exterior walls will be done with 22 cm 032 EPS. For the appropriate CO2 balance, a photovoltaic system is planned. According to the certificate, it must be at least 4.4 kWp. When we planned the house with Poroton bricks, the value was 3.3 kWp.

4.4 kWp

That's not really much. That means if I were to install significantly more photovoltaics (around 18 kWp at 30° southwest on our side), I could, for example, reduce the wall thickness or insulation?

could I, for example, reduce the wall thickness or insulation?

No. You need the photovoltaic system to meet the requirements for the ecological balance. The KfW40 insulation requirements must still be met. With KS stone, you simply have to insulate better because it is terrible in this regard. It is the energy consultant's task to provide the appropriate insulation.