Need your assessment for the renovation of a rental property

Repayment as quickly as possible since the depreciation is spread over 5 years

I deliberately refrained from nitpicking because I believe it makes no difference in this case.
If you break the whole story down to the essentials, you have €350,000 that you want/can invest and are currently considering whether an additional €200,000 as a (loan-financed) leverage would make the whole thing even more profitable.

My point is that a house or the renovation of a house in your case (relatively short-term horizon, high need for security) is no more a "sensible" investment object than putting the money in a current account. If at all, then without "leverage," i.e., without (major) renovation; that is unnecessary risk.

The chimney is not in use; there is already one for the heating system, therefore no weak spot on the roof and no elaborate cladding if it is removed down to the floor of the attic

I can’t follow you. There is a chimney that is currently not in use and is to be removed. Which chimney are the existing systems connected to? Or are there two chimneys, one of which is redundant and is to be dismantled?

With 115 efficiency house with 2 residential units, a 15% grant means for me 2 x 15 K = 30 K.

My mistake, I had overlooked the second residential unit. But this does not change anything regarding the individual measures and would at most double the repayment grant to €25,000 in the case of the loan variant (151). However, things like dormers, bathrooms, or flooring still do not qualify.

It will become important in the future to have an efficiency house, because no one wants to live without insulation anymore.

I don’t see it that way. Of course, everyone would like to live in an energetically renovated building simply because the ancillary costs are much lower there. However, if the landlord pockets the saved ancillary costs in the form of a higher cold rent, as a tenant I have gained nothing. Therefore, I couldn’t care less whether the place has KFW standard or not.
That would only be an argument for buyers, and one would have to ask the same question there: Does the initial investment actually yield a profit in the end?

I would evaluate the whole thing differently if you planned to live in the building yourself and with a time horizon of 20 years or more. At your age, however, I wouldn’t tie myself to that anymore. Especially given the fact that a loan would still have to be taken out for it. There are worthwhile investment opportunities with comparable risks without having to put all your eggs in one basket.

The measures can very quickly exceed the €200,000. Have you consulted a planner or architect and an energy consultant? I also see this very critically. Far too much investment for a rental. Is it really such a desirable location?

The location is very good, a quiet residential street and very central in a pleasant way. There are no new buildings, but almost all houses in the street and surrounding area have been renovated. The house is free. It makes no sense to leave the house unoccupied, but there is also no sensible investment for the sale proceeds. One could practically pay it off in 5-6 years until retirement, tax-free, with additional depreciation, which would now provide a nice tax advantage. So let's say an advantage with the KFW BAFA subsidies of 35-40 K. So the substance is okay, in 5 years the house could continue to be operated like this or possibly be sold. It is clear that a direct sale would probably not realize 100 K. of value increase in the sale proceeds, that only works in the long term. But I could live with that.

It is clear that without an energy consultant and more precise figures, I will go in circles to decide whether to do it or not. In conclusion, thank you to you, you see it as critically as I do - but of course, such topics are always difficult to handle on the internet. I might have expected more inquiries about individual points of the building fabric, but it is always a coincidence who is currently reading along. I will certainly ask many more questions and then finally report which decision I have come to...

The house is available. It makes no sense to leave the house unoccupied, but there is also no meaningful investment for the sale proceeds. You could practically pay it off in 5-6 years until retirement, tax-free, with additional depreciation you would now have a nice tax advantage. So let's say an advantage with the KFW BAFA subsidies of 35-40 K. So the substance is okay, in 5 years the house could continue to be operated like this or possibly sold. Clearly, with a direct sale, about 100 K would probably not realize any increase in value in the sale proceeds; that only works in the long term. But I could live with that.

That reads difficult, I can't understand it. So I'm asking directly: The house is worth €100,000 today, right?

€200,000 planned renovation

Okay, but that’s like with the new price of a car, driven one kilometer (occupied by tenants for one month) and it’s only worth half as much.

Sale of the house brings approx. 350-380 K

100k value + 200k renovation is not equal to 350k.

Where is my thinking error?

The value of the house was estimated by the real estate department at Bank 350 - 380 K. The plot of land is already worth 200 K according to the standard land value. This certainly does not correspond to the current sales value, which will be somewhat higher.
The forest adjacent to the garden is the small, tranquil local recreation area Nachtigallntal. You can reach the highway, the shopping center Essen, or the Reinruhrzentrum in 5 minutes. It doesn't get any better...
The upper apartment has already been renovated. Therefore, I was surprised that the contribution from "Galdreth" with his 120 m², purchase price, smaller plot, not located in a big city, and yet elaborate renovations was rated much more positively by almost everyone than my inquiry. This is about an investment of private money 170-180 sum + funding -25-35, which brings the value significantly above 500 K...

Unfortunately, the staircase to the attic is not wide enough at 75 cm to become a DIN living area. However, the renovation costs are already such that the rent + apportionment costs of 8% cannot be charged to a single tenant - then the rent would be too high. Nevertheless, the room is quite nice with direct access from the apartment via this staircase. Not planning it right away now would be silly. The attic has screed and 10 cm of Styrofoam and then wood; laminate or carpet could be laid directly on top of that. 5 cm of Styrofoam would probably have sufficed, but someone hadn't thought that through properly. The emergency exit to the roof / dormers is then possible, as an inspection has already shown. Whether laminate or carpet, that is then up to the tenant...