Wait with the building application because of the child construction allowance

But that would again be subject to mandatory notification as a change - so not really a solution either



Maybe you can entice your ex with the fact that it’s about creating value for the child (topic "inheritance") and that without the Baukindergeld the financing might fail?

It is definitely a gray area. According to the KfW:

As a grant recipient, you are obliged to inform the KfW immediately in writing at the following postal address or by email at [E-Mail]424@kfw.de[/E-Mail] if you no longer use the subsidized residential property yourself, that is, if you rent out, sell, or lease your house or condominium.

You also continue to receive the child construction allowance if the children turn 18 six months after the application is submitted. To what extent it is a requirement that the children continue to live in the property is not clearly defined.

Otherwise... no idea, should I really sue him?

Why not? For you (and your child), this is about significant amounts in this case, while his only argument seems to be "lack of desire."
I would briefly explain that to him, leave the corresponding document for signature, and make it unmistakably clear that I will hand the matter over to a lawyer in X days because I have no time, desire, or nerves for games.
Apart from that, as a parent, you also make yourself vulnerable if you try to enforce such things yourself - somewhere, somehow, sometime the child finds out, and then you are the "bad one" who has written dad strange (and usually also emotionally charged) letters/emails/WhatsApp messages.

Two children, 24k€, I would like to use for special repayment (it is not included in the normal financing, but the repayment should "actually" be somewhat higher – so it would come in quite handy), and at the end of the loan term about 30k€ less remaining balance. I will ask him about it, but first the call to the KfW of course, to clarify whether there is even a problem (but I fear there is). I really hope that we can reach an amicable agreement.
And regarding lawsuits: I can hardly do that with the argument "because of the BKG," the situation has existed in this form for two years now. But I also see it critically as mentioned above, as parents you have to keep working together, and a lawsuit would be the absolute worst case for the relationship. It can only be the last resort. However, one can hint that one would go that far; maybe that alone will help. But he is actually also open to solid monetary arguments... we’ll see.
I have also wondered what happens if a child leaves for university and moves out, it is not defined on the website whether the BKG would then no longer apply. If it does, the 2x re-registration option would also be possible in my situation, I had already thought about that too.

Two children, €24k, I would like to use for special repayment (it's not factored into the regular financing, but the repayment should "actually" be a bit higher - so it would be more than welcome there), and at the end of the loan term about €30k less remaining principal. I will ask him about it, but first the call to the KfW of course, to clarify whether there is even a problem (but I fear there is). I really hope we can reach an amicable agreement. And regarding lawsuits: I can't really do that with the argument "because of the BKG," the situation has been like this for two years now. But I also see it critically, as written above, parents still have to work together, and a lawsuit would be the absolute worst case for the relationship. It can only be the last resort. But you can hint that you'd be willing to go that far, maybe that will help. He is actually also open to solid monetary arguments... we'll see.

I have also wondered what happens if a child goes to university and moves out, it's not defined on the website whether there would still be BKG then. If so, the 2x re-registration option would also apply in my situation, I've already thought about that too.

As stupid as it sounds, a lot can be solved with money. Offer him a compensation payment. Better that than no Baukindergeld.

Zaba, cool idea. But I wanted to first point out that he might very well come to reason if it costs me such huge amounts. Elsewhere he had offered to pay the amounts himself (instead of agreeing to a re-registration - but that wasn't even necessary for the tax class), but I guess that exceeds what he would be willing to pay voluntarily... He can do the math too and will quickly realize that then he would have €24k less in 10 years.