How many KFW programs can be used?

Due to the new, low interest rates, KfW loans are all worthwhile, especially if you want to build an energy-efficient house anyway. With the KfW55, you get the last €5000 as a gift, plus the low interest rate, which, in my opinion, is a no-brainer. Unless you have to retrofit your house expensively for it, then the additional equipment (e.g., controlled residential ventilation system) eats up the good conditions again.

I wouldn't say that so generally. I agree with you on the total amount, but the lower total amount is of no use if the installments might be disrupted due to the durations and your own situation. For us, it was a significant three-digit amount per month that would have come on top via the [KfW] and thus would not have made the house building project possible.

We have the 153 and to my knowledge one could choose the duration in stages there (but I have since forgotten), ours runs very long to my knowledge and with the "only" 100,000€ that is not a significant burden per month. It is only suitable as a supplement, that is clear. The other components should then also have long durations regardless, otherwise it will be tight at the beginning, that is true.

Thank you for the numerous responses. We actually had the choice whether only program 153 (100,000€ at 0.9% interest) or a combination with 124 (50,000€ at 1.26% interest). So together 150,000€ from KFW. The average interest rate is 2.16% with a repayment rate of 3.016%. The repayment rate is fixed and therefore "our" project would have failed. Overall, we would have had to pay 200€ more per month, which we had not calculated. We have now entered into financing with a home savings contract with special repayment. We have a mixed interest rate of nominal 1.56% / effective 2.10% for the entire term. This may still change, for example if after ten years an offer from KFW is obtained or if interest rates fall further.

typo: The average interest rate should then be 1.16%, right?