Financing land and house construction

I find it positive that the interest rates have come down quite a bit and that the installment for your net income is basically okay. However, I also see the problem that you barely pay off any principal. Saving money and investing it elsewhere is a nice intention. But: in the first few years, various “little things” will come up here and there and eat up the money. It is very difficult to consistently refrain from spending and save the money. In 90% of cases, that doesn’t happen, and in the end the mountain of debt grows large. I would strongly recommend putting a little more money each month into the Commerzbank loan. A 3.31% interest saving, tax-free and guaranteed, is not bad, and the forced saving means you won’t be tempted to fritter the money away elsewhere. What absolutely makes no sense is to currently save funds at 3.5-4% in the hope of being able to repay the KfW loan. The general consensus right now is that the peak of interest rates has already been reached. That might not be the case, but if it is, that is definitely the worse idea.

whether you choose 5 years interest-only to consume or to save the money and invest it profitably (our case). At 0.53% interest, I will do the devil's work to repay more than the absolute minimum

and put aside everything we don’t currently need for the installment

Discipline

Increasing the repayment rate on the Commerzbank loan would indeed be desirable, but unfortunately, it is difficult for other reasons. The Hamburg development bank would like to see that the loan installment plus housing costs remain under 40% of net income. So, the only options are generous special repayments and/or changing the repayment rate. You then have to see how expensive you make that in terms of interest. I still maintain, better to set up a standing order for an extra savings rate into a daily/term deposit/government bonds or similar, and then access the money after 10 years.

Then ask. I am sure that an increase in repayment right at the beginning won't cost you much. The thing with the savings rate and discipline is wishful thinking for most people, and in the end the money doesn't go into special repayments, but into a new car or ski vacation. Unfortunately, people are mostly undisciplined and prefer to reward themselves concretely and immediately rather than abstractly sometime in the future.

People are unfortunately mostly undisciplined