Knowing the credit limit one year in advance – is that possible?

a) My possibly incorrect assumption that financing our demand 100% with a fixed-term contract + probation period could lead to unfavorable loan conditions. b) My possibly incorrect assumption that my parents might have non-rational problems with "rent" in the formerly owned house. If both a) AND b) are wrong, objectively nothing.

Then just clarify that with the bank. The first conversation is non-binding anyway, nothing more than "now is rather unfavorable" will happen. There won't be a Schufa record or anything similar. And with the parents, you can also agree on the "rent" so that it is deducted from the purchase price. If it's only about the payments

If the delta of 40k is too much for your parents, I would advise having the property officially and factually appraised ([Sach/Verkehrswert] AND [Marktwert]). In my experience, banks often only assess the [Sach-/Verkehrswert] based on very rough parameters. That won't help you then.

For me, the logical order would therefore be: commission an appraiser/expert with the valuation, then check with the bank whether that would be within your range.

if you only save the 10000.-- ancillary costs (3% notary and land registry) after 1 year at the salary, I would not buy. Or did I misunderstand something?

A very approximate one. The free(!) super-fast analysis that can be obtained online from various portals resulted in a range from 340k to 380k in September 2021.

So with these explanations, the whole thing becomes somewhat more plausible. My advice: Contact one of the major online real estate agents and tell them that you (or your parents) might possibly want to sell. They will then carry out a somewhat more detailed valuation than purely online and send you the corresponding report. You will then have a probably quite rough price range, but you of course do not have to hire the agent. If you do this with two or three providers, a good picture should emerge; at least that was the case for us at the time. Then there is the bank. It will probably not finance the targeted selling price, but I think if you deduct 20 percent from that, it could already be suitable. If necessary, you can also give the valuation to the bank to get more precise figures.

if you only save the 10,000.-- ancillary costs (3% notary and land registry) after 1 year with the salary, I wouldn't buy. Or did I misunderstand something?

? It is a purchase within the family, so the 6.5% real estate transfer tax on the purchase price does not apply. Notary and land registry still have to be paid, there is no broker. In the end, the ancillary purchase costs are reduced by purchase price*0.065, which is certainly >20k.

I have now scheduled initial consultation appointments with Interhyp and the local Sparkasse. Let's see how much clearer I will see afterwards.

I find the following tip

Contact one of the large online brokers and tell them that you (or your parents) might want to sell. Then they will carry out a somewhat more detailed valuation than just online and send you the corresponding report. Then you will probably have a rough price range, but of course you don't have to hire the broker

logical, but having someone do preparatory work when I already know for sure beforehand that I will not take their product is not quite my style, which I would feel comfortable with.