I would like a long-term fixed interest rate, and unfortunately that doesn't work with KfW loans. I'm a bit anxious about that.
The repayment calculator shows that with 2 residential units after 10 years of repayments of €789/month, you would still owe €107,000 which you would then have to refinance. I don’t really expect KfW to demand exorbitant interest rates after the end. But I can't say that from experience. On top of that, there would be a loan of around €120,000 from a regular bank which you could, for example, repay at 1% (280€/month). Overall, this would result in the same monthly rate, but you could sign the contract immediately instead of wasting half a year now and thereby creating a waiting time of one year. Also, you could soon finalize a financing now. Your whole construct is based on the fact that you can finance now and apply for the subsidy in July. As I understand it, however, the subsidy is also applied for by the bank when applying for the loan. This means that before July, you can neither apply for a construction contract nor a loan. Honestly, the risk that your interest rate in July could be 2.5-3% would be too high for me. That will also cost you much more over 30 years than if the interest rate for the follow-up financing at KfW then is 2% instead of the previous 0.95%. Or that the general contractor in July no longer feels bound by his offer from February because the window maker and electrician raise their prices. Then you’ve wasted another €5,000. Caution is nice and good but risk aversion can also lead you to take a risk that is less obvious but in truth much more dangerous.
Why actually 75k?
BEG would be €37,500 per residential unit at KfW40+