Is this financing offer good?

Yes, nobody knows that, otherwise you wouldn't have to keep working. Personally, I estimate it significantly lower. I see a similar situation to Japan, which has had low interest rates for 20 years. Therefore, I might also look at 10 more years if that's not too little for you. Meanwhile, your offer probably does not match anymore and will likely be 0.1-0.2 higher.

I see a similar situation to Japan, which has had low interest rates for 20 years.

Then I unfortunately have to tell you that you have no idea what has happened in Japan and that you cannot compare Japan and the EU, because both economies had fundamentally different starting points.

Apart from the interest rates and the government purchase program, there is nothing in common.

It is roughly like comparing a sports car with 400 horsepower to a truck with 400 horsepower. Both are driving on the same road and the power output matches. But that’s about all they have in common.

And yes, you will find a driver’s seat and a steering wheel in both, still fundamentally different. It is similar with the EU and Japan.

The problem: there is very little German-language literature on this.

Personally, I also don’t believe that the interest rate will go back to a level of 6%. The markets are interconnected worldwide and competitive pressure will take care of it. Besides, I doubt that countries like Spain will recover so rapidly that the interest rate will strongly recover within 10 years. Also, the interest rate has been falling continuously for over 20 years (except for some small spikes). I also bet on a low interest rate and took 10 years.

I have thought it over again and the building society savings option is worthwhile from an interest rate of 5.9%. Now, of course, it is not clear where the interest rates will be in 15 years, and I am no wiser than before

But that is quite an expensive option then. I just compared our building society contract setup with a 10-year one (we had also been given those conditions back then). The break-even point is at 3.7%.

The building savings contract would then be over 30 years, therefore also expensive, 15 years without repayment are just nothing

We also have a home savings contract structure over ~29 years. I compared that with a classic initial 10-year structure. And with 3.7% interest, the break-even would already be in 9.5 years.

Therefore, I was surprised about the 5.9% from which it would only start to be worthwhile.