Plot with existing old building, new construction not possible

Then it will just be a conversion where each wall is rebuilt individually.

That's how it is: conversions, renovations, and modernizations are allowed. Even if in the end there will be a splendid new house standing :)

Conversion, renovation, and modernizations are allowed.

Caution! For a major conversion and especially for a (core) renovation, you need a building permit just like for a new construction. And not everything is permitted. The boundaries are not obvious to a layperson; good advice is important.

Caution! For a major renovation and especially for a (core) refurbishment, you need a building application just like for a new construction. And not everything is allowed in this case. For a layperson, the limits are not obvious; good advice is important.

Nevertheless, not prohibited. Existing structures, as here, may be rebuilt or renovated, etc.

there is also a dilapidated house from 1950 on it

Tell me more about the house. What do you mean by "dilapidated"? Would a renovation be possible?

oh, sorry that I am only replying now, I had only received the first response here via email. So basically I am serious about it, but I have only been dealing with this specific issue for a few days. Before that, I rather assumed I would find a plot of land with a somewhat clear development plan (I’m an optimist).

The house has been unoccupied for years, has only 90 sqm of living space, an oil heating system, which wouldn’t be usable anymore. But the plot is really beautiful and has its own cadastral number.

Be that as it may, today I received an email from urban development that included, among other things: "The possibility of exceeding the building boundary would best be assessed based on a sketch or a plan, we cannot generally provide any dimensions here." And a few more notes on maximum wall heights. That’s why I am now looking for an architect, tomorrow morning there is a preliminary phone call with one who was recommended to me. Whether it will then come down to a "construction project; application for a preliminary decision" or initially just a sketch, he should be able to tell me. I am very curious to see how it will develop!

The house has been uninhabited for years, has only 90 sqm of living space, an oil heating system, nothing could be done with that anymore.

Despite the problematic year of construction, I primarily see the many years of unoccupancy as critical here, and otherwise consider your pessimism unfounded.

Be that as it may, today I received an email from urban development that among other things contained: "The possibility of exceeding the building boundary would best be assessed by means of a sketch or a plan, we cannot give any dimensions in general." And a few more comments on maximum wall heights. That is why I am now looking for an architect, tomorrow morning there is a preliminary phone call with one who was recommended to me. Whether it then comes down to a "building project; application for a preliminary decision" or initially just a sketch, he should be able to tell me. I am very curious to see how it develops!

We are also curious, and you are left in suspense, do you find that fair? You can now complete all your homework here after all!