Construction financing - mortgage instead of equity?

Hello

We (two civil servants, not married) want to finance our own home.
It is supposed to cost 430,000 euros, plus the obligatory additional purchase costs of approximately 22,000 euros. There is no broker fee in this case.
We have already been to Interhyp and felt quite well looked after there, but I would still like to know what you basically think of the following plan:

We have equity of about 25,000 euros but of course would not be unhappy if we could keep a certain buffer.
The nice gentleman from Interhyp said that there is also the possibility of registering a land charge on our parents' house and basically splitting the loan - that is, once 100,000 from the KFW (it will be a KFW 55 house), then 50,000 with the land charge and then almost 300,000 through another loan.
The interest rates would thereby change significantly, quite independently of the equity provided (at least according to his calculations). In fact, my father has now offered me to even use both already registered land charges on my parents' house, which are 50,000 and 69,000 euros, of course free of encumbrances.

Would this approach be "normal" or would the banks throw their hands up in horror?

Sorry but that is not a good idea! If something happens to you, your parents' house could also be lost under certain circumstances.

First, it would need to be clarified whether the land charges are in first rank. Then your parents should be aware that they are guaranteeing a loan with their house. As has already written, the idea is not optimal

Sorry, but that is not a good idea! If something happens to you, your parents' house could also be lost under certain circumstances.

As far as I understand, my parents' house is only at risk if the credit with the associated land charge cannot be covered by the auction proceeds for our house - which I consider almost impossible with 50,000, right?

First, it would have to be clarified whether the land charges are in first rank. Then your parents should be aware that they are acting as guarantors for a loan with their house. As has already written, the idea is not optimal

I actually don't know in which rank the land charge is; I need to ask again about that. My parents are fully aware of this and would like to do it. Our situation is a bit special: they have their house and a pretty large plot on which I could have built, or alternatively, I could have expanded in their house. But as life goes, I first fell in love with my boyfriend and then also with the area he comes from. Our house would therefore be built about 250 km away from my parents' house. However, my parents are more than happy to support us, and with two civil servants, the default risk is quite manageable.

If your parents act as guarantors, you should also contractually regulate the internal relationship with your friend before you start the adventure of building a house.

Parents are only at risk if the auction proceeds for our house could not cover the loan with the associated land charge – which I consider almost impossible with 50,000, right?

Oh, you think so

Just think, purely hypothetically, everything goes to ruin in the 4th year. You probably haven’t paid off 50k€, but the prepayment penalty for the bank on your over 100% financing is still added.
The auction then yields 80% and your parents lose their house. Quite simple.