Security deposit according to §650 of the Building Code

We did it there ourselves as well. Our deal with the construction company was that this is the only security, meaning no other assignment, and we receive a completion guarantee of the same amount. This makes the payment of the installments much more convenient, and I receive the money directly from the bank.

We are in a similar situation. Our contract states:
"To secure the final payment, an irrevocable financing confirmation (bank guarantee) for 10% of the total amount must be submitted. Alternatively, a so-called and-account can be set up for this amount at the house bank of XXX (general contractor)."

For this guarantee, we would now have to pay about €200: It does not seem fair to us to have to bear costs caused by our general contractor (we could just waive a guarantee).
Is a general contractor allowed to demand that the client pays the guarantee fees themselves?

, in your case, are the guarantee fees covered by the general contractor?

May a general contractor demand from the client that he covers the guarantee fees himself?

The fee is not charged by the general contractor, but by your bank. They could also offer it for free. Conversely: Would you pay for a warranty bond of the general contractor? That would be just as unfortunate as your request.

I may not have made it clear: we ourselves do not need the guarantee, we could easily do without it. That is why I find it inappropriate that the building owners have to pay for it. I would be interested to know: Do all general contractors proceed this way, or are there alternatives to offer them financial security? (preferably without a bank: We are not taking out a loan)

Without credit, the answer is usually: advance payment as security.

In our long search for a construction loan, we have not found anyone who builds without security. Most want an assignment of the loan (which is usually formulated very vaguely, Town & Country even sells this as extra security for the builder...). Our construction loan only provides coverage of 10%, and others require assignment plus bank guarantee. In addition, with general contractors there is usually a high overpayment in the first installments, which ultimately is nothing other than additional security.

We have categorically excluded an assignment, which even led to the termination of an already signed construction contract. Nobody allowed any leeway, no concessions.

we ourselves do not need the guarantee

That's true, it is stated in the contract, but you can also deposit the money. Then you don't have to spend money on the guarantee.
You have to accept one hardship – otherwise, you wouldn't have been allowed to sign the contract.

P.S. When you buy a car, you also have to pay for everything you order and sign for. It's no different here. A contract is a contract. And yes, this is quite common.