Buy and renovate a semi-detached house from 1959? Suggestions/tips welcome

You should definitely get something in writing from the ground lease grantor regarding the planned increase in the interest rate.
A current market interest rate would be ground value * 4% / per annum, so a slim 19k.

Aside from that, it will be difficult to find a bank that finances a property with a ground lease contract with such a short term. This might even be a disqualifying factor.

The only certain, feasible way:
The orphanage now concludes a new ground lease contract with you for 99 years.
This way you have planning security (the interest rate should then only be adjusted within the scope of general inflation / cost of living increases) and you will also find a financing partner.

The house itself is not a bad basis, the floor plan is just typical for the year of construction and not very suitable for a family.
If the statics allow it, your renovation plans would be financially feasible, I would say, and the order of the changes as intended also seems reasonable for now.

: the condition of the house hardly plays a role in the location. Calling it a wreck is nonsense anyway; the house is a solid basis for a major renovation, some things can also be done gradually.

... probably negligible compared to the leasehold issue, but at least keep it in mind: These grey PVC tiles with the red tiles in between (in the kitchen) look very much like Floor-Flex panels (we had exactly this pattern in the house), i.e. asbestos-containing. It might be sensible to have an (hazardous materials) expert inspect such a house at least once.

: the condition of the house hardly matters in relation to the location. Calling it a wreck is nonsense anyway, the house is a solid base for a complete renovation, some things can also be done gradually.

Well. Yes. For me, it does. At some point, you have to draw a line. And not agree to every price. 610,000 € for foundations on a rented piece of land. Plus whatever is added on top for the renovation. I would never participate in this madness. Ok, I couldn’t anyway.

Difficult matter. The house itself makes a relatively good impression in the pictures and the location looks pretty idyllic for Frankfurt conditions (it is probably in FFM-Eckenheim). However, I would seriously consider and especially calculate the ground lease history. The interest rate increases at regular intervals and will certainly continue to do so in Frankfurt. Conclusion: If I absolutely want to be in this location and time is pressing, then I would probably go for it instead of letting another 5 years of rent payments and further price increases on the market pass by. If I am flexible regarding the location, then I would rather keep looking. With your budget, not everything but quite a lot is possible in the Frankfurt area.

I recommend, for the purpose of decision-making, to mentally rise from the chair of the homeownership romantic for a moment and sit on the chair of the investor.

I would find the price reasonable for this year of construction with a purchased plot, but not with a leased plot. And measured by today’s architectural taste, I honestly see - admittedly very well-maintained - junk.

I would first like to thank everyone for the help and your opinions. My husband and I have now read all the comments several times and discussed them.

We really like our current apartment, even though we are renting and transfer €2000 monthly. Therefore, the pressure to buy a home is not so huge. We have enough space even if two children were to come immediately. We definitely want to own a home, but we are not under any time pressure right now. Of course, we know that prices can continue to rise.

We have decided not to buy the house. The seller already wanted to go to the notary this Friday, even though our bank has not yet 100% confirmed the financing. We also found this strange. All the wording regarding the [Erbbaurechts], which you all mentioned, is causing us great concern. Both of us also had a strange feeling about the construction. Therefore, we made the decision not to buy the house. Rather a smaller plot, but entirely ours and no worries about what happens with the [Erbzins].
Over the weekend there is a bidding auction for a small end-of-terrace house, exactly 3 minutes away from this house. It is not a leasehold and the same size, of course the plot is much smaller. Maybe we have to get used to the fact that we cannot buy 600 sqm within our budget. Especially not in this location or maybe consider further out.

I thank you very much for your help and all the notes about [Erbpacht], asbestos, and the chimney. This has been very helpful and we have looked at the house much more critically. Thanks again. I will gladly write as soon as we have viewed the house over the weekend. This house search is really a nerve-wracking matter.