What house construction options are available under these conditions?

Stupid question in between.

A loan within the family (via parents / aunt / uncle etc.) Is this actually possible without any further ado? Do common interest rates have to be observed here or is this individually negotiable as long as there is a small contract?

Of course, this is possible, as is a land charge on the parents' property as substitute security.

Where appropriate and possible, it should always be considered, as long as the family circumstances allow it or it is even desired by the parents.

Assuming a person from the family is "very liquid". An amount of 500k can be provided without any problems. Interest of 0.5 p. a. is agreed. Could there be any problems of any kind with this constellation? Sorry for the interim questions here.

Then it counts for the bank as equity.

They officially declare it as a loan, which is not necessarily required, simply externally financed equity.

Sorry Noelmaxim, it was not meant to be offensive.

:
In principle, a private loan works just as simply or complicated as a bank loan.

BUT, what happens if the private loan can no longer be serviced? Is there a land charge? If yes, will the family member initiate the forced sale? What will the relationship within the family be like after a forced sale, or even worse, if the lender does not get everything back after the forced sale?

For a bank, every loan default is statistically factored in and they can deal with it. Usually, the family cannot.

Oh yes: stocks or anything else yield more than 0.5% p.a. For the lender, this is economically not attractive at all.

Everything is okay, I partly understand how you think since I am a financing broker, but you do not know me, that always annoys me, and especially my way of helping consumers.