Capital for new construction available - but

The KFW40+ houses cost correspondingly more, and indeed so much more that it would not be profitable without the subsidies... therefore, the business case is questionable.

Absolutely legitimate approach! The subsidies are for everyone, it doesn't matter at all how much capital one has.

Anyone who builds a KFW40+ house just spends a lot of money that will never pay off. Even the additional €12,000 will not compensate for the loss due to the higher price of KFW40+. It doesn't matter whether you intended to build KFW40+ anyway or are doing it because of the subsidy...

I wouldn't call it a loophole... you can also take out the €100k loan and repay it as quickly as possible through special repayments, then you also save part of the 0.75% interest. Also legitimate!

Thank you for all the contributions above. I am building a low-energy house because I expect energy prices to rise continuously in the medium term and in old age – when income is likely to be smaller than today – I want to spend as little as possible on it. The money in the bank doesn’t bring any returns anyway; you might as well put it into a (today perhaps still financially unprofitable) house. If the government gives a small "compensation" as well, you might as well take it; that’s my opinion. If in 20 years the kWh from the electricity supplier costs 40 cents or more, it may have been worth it in retrospect. In 1990, for example, a liter of Super cost about 1.- DM, if I remember correctly...

Well, whether you have €60 or €50 energy costs... The topic has been discussed up and down by now. What you save on energy, you pay again on electricity. Anyway!

Can the KFW loan be repaid without limit?

Can the KFW loan be repaid without limit?

KFW 153:

Special repayments
During the first fixed interest period, you can repay the loan amount in full or in partial amounts from 1,000 euros without additional costs.

I am currently on Tapatalk, but if I remember correctly from your profile... you are no longer in your twenties. So any extra effort for some KfW probably won’t even pay off for you. And the passive house is something for ideologues with way too much money.

Honestly, if you have that much money to "give away," you will surely find one or another good cause ;-)