Building a new single-family house with KfW - costs?

This is roughly the interest rate development of the last 10 years. In 1982, it was even at 8-9%. Even 10 years ago, people argued that interest rates had to rise because they couldn’t possibly fall any further. But they could continue to stagnate. I currently see no development that would cause a trend reversal in interest rate development.

I can understand that you want to fix the interest rate for a longer period. I have also financed a part with a 30-year fixed interest rate. If I possibly continue with some special repayments, I will be finished paying off by the latest at 60. If things go well, even earlier. If after 10 years the interest rates are still that low, you can also terminate and take out a new loan. However, sometimes I also wonder whether a 20-year fixed interest rate would have been enough. Since your financing amount is relatively low, I would finance with KFW in your place and then finance the rest with a 10 or 15-year fixed interest rate. Of course, it also depends on how high the monthly installment should be and how much outstanding debt is left at the end of the interest rate fixation. But with that amount, the risk should be manageable.

This is roughly the interest rate development of the last 10 years. In 1982, it was even at 8-9%. Even 10 years ago, people already argued that interest rates had to rise because they couldn't possibly fall any further. However, they could continue to stagnate. I currently see no development that would cause a trend reversal in interest rate development.

The trend reversal has long since occurred! Bond yields have risen well, and consequently mortgage rates have increased. If inflation takes hold sustainably, this trend will continue. In a worst-case scenario, extremely high mortgage rates can occur if hyperinflation sets in and the currency crashes. It is certain that this will happen; the question, as always, is when. For this reason, one should finance sensibly because otherwise, one will certainly get into big trouble. The high Bitcoin price is an indicator that trust and thus the currency system is on the verge of collapse.

The turning point has long since arrived! Bond yields have risen significantly, and with them, construction loan interest rates. If inflation takes hold sustainably, this trend will continue. In a worst-case scenario, mortgage rates could become extremely high if hyperinflation sets in and the currency collapses. It is certain that this will happen; the question, as always, is when it will happen.

For this reason, one should finance sensibly, because otherwise, one will certainly get into big trouble.

The high Bitcoin price is an indicator that trust and thus the monetary system is on the verge of collapse.

hm I also think it’s going in this direction. What do you estimate by “on the verge of collapse”? Short-term this year? Or rather within the next 5 years?

I'm going to leave out the whole apocalypse theory with "currency system collapse" for now; you probably can't plan sensibly for that case anyway. For that, I tied a Nokia 3310 with duct tape to a stick.

For the more realistic scenarios, I initially also considered a longer fixed interest period. I then calculated the total costs in an Excel spreadsheet. If you choose the longer fixed period, you already pay higher costs in the first 10 years. With the lower interest rate, you can repay significantly more with the same installment – so I calculated in Excel what each of the two loans would cost me overall. Once with the full fixed interest period (I think we were offered 30 years), and once the short one.
I then continued calculating with hypothetical follow-up interest rates after 15 years (various scenarios). And came to the conclusion that only from a follow-up interest rate of about 3.8% onwards would the 30-year offer be cheaper in total costs.
That would have looked something like this in the graphic, and personally, I considered the interest rate development unlikely. Not impossible, of course, but unlikely.

hm I also think it’s going in that direction. What do you estimate by "almost at the end"? Short-term this year? Or rather in the next 5 years?

Nobody can predict that reliably. I just read about it again yesterday, many currently expect a crash in 2002 or 2023. Others see one more longer final boom cycle before it crashes. I am preparing broadly for it, such a gigantic crash will offer unique opportunities. You can see it with the pandemic and cryptos. Getting rich has never been as easy as it is now. And the big thing is yet to come. But you also have to be liquid.