Unclear property boundary and legal consequences

What does your future neighbor say about your plans and why do you want to start with the dismantling of the insulation and windows?

He hasn’t said anything yet. It is a rental building with many different tenants who are managed by an administration, and I have just recently identified the owner (a company from West Germany). I just wanted to first check the legal situation before I politely ask what they think and say. I assume that in this setup, not all matters can be settled amicably. So I consider having a certain negotiating position to be advisable.

Yeah, new surveying ...

That is currently my plan: to ask the owner (he owns both adjacent plots) if he has more precise information about the boundary line and if not, whether we can share the cost of an official land survey?! Or is he obligated to do so anyway?

That will not change either, as your text is indeed very long but contains hardly any usable facts.
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And then he allows you to upload such a picture? Sorry, but what information is that aerial photo supposed to convey? You can’t recognize anything related to the problem.
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As a surveying technician, your father should be able to obtain and analyze all cadastral documents. Then phrases like "probably at least" and "quite certain" should be a thing of the past, because then it can be stated with certainty whether and since when the boundary has been established. If it has been established, it can also be transferred to the actual site and it can be determined which building parts are on which side of the boundary.

The picture is the official aerial photo with cadastral boundaries from the city. It serves as a tool for me. The only more precise source seems to be the excerpt from the land register. 1:500 is the most accurate scale available there. I have now added that. But even there you don’t really see more about the property boundary. The building application is still interesting. The line shown there (which might represent the property boundary) runs midway through the "double" and along the outside of our wall right through the property. The alleged boundary marks are definitely not (easily) to be found. But I will inquire at the surveying office.

To me, it also looks like you won't be able to avoid a new survey. Basically, you only know whether and what problems you have once you know the exact boundary line. An owner from elsewhere will probably have no interest in this, and certainly not in sharing the costs; but I don't know to what extent you can obligate the neighbors. I think I would start with a survey and build on that. Everything else seems like crystal ball gazing to me.

I added that once more.

Yes, that’s good. You can recognize much better what it’s about, as the essential things, buildings and boundaries, are much clearer to see.

But you really can’t see more about the property boundary there.

No, you can only determine the exact boundary line based on the cadastral documents (survey sketches), but your father should be able to help you with that, unless he hasn’t dealt with cadastral surveying for 40 years.

Or is he obliged to do that anyway?

Yep, §919 Building Code. Whether the two boundary points will actually help you, I don’t know. After all, you have to determine inside the house where you currently stand in relation to the boundary. Solvable, but not easy.

The building application is still interesting. The line there (which possibly represents the property boundary) runs centrally in the "double" and just outside our wall straight through the property.

Yes, that could be an indication of a party wall. Do the dimensions correspond with the current building? Nevertheless, the cadastral extract strengthens my opinion: Cut down two trees and build new! Freestanding. The property is big enough, you don’t even have to demolish.

To me, it also looks like you won’t be able to avoid a new survey.

My father (no longer in service himself) is currently getting me an offer. So it will very likely come down to a survey/official boundary determination.

I don’t know if those two boundary points will actually help you. After all, you have to figure out inside the house where you currently are in relation to the boundary. Solvable, but not easy.

One boundary point is about 3m in front of the wall. It should be possible to tell very precisely whether it runs through the middle of the wall or through our planned living room.

Yes, that could be an indication of a party wall. Do the dimensions match the current building stock?

Yes, the dimensions fit very precisely. If the building application from 1903 said "property boundary" at this line, we would actually have certainty. Then we would only have to clarify whether the boundary was ever moved. But there is nothing written on the line and it can only be suspected that this is the property boundary.
It definitely seems as if the neighboring house was actually planned and built as a detached house and the plan to build our house next to it was only created afterwards. Reason: the gable side actually had 2 windows on each floor in the gable wall in 1902. While stripping the plaster from the wall, I found an old poorly bricked-up window area. Poorly means that a brick came loose and behind it I found a free space of about 80*160cm. About 10cm deep with some insulation material inside. Then a drywall. So I have to assume that the neighbor’s apartment is already behind it. I don’t think it functions as a firewall.

Nevertheless, the cadastral extract reinforces my opinion: Cut down two trees and build new! Detached. The plot is big enough, you don’t even have to demolish.

We had actually considered that in the meantime. But I opened a second thread. Topic there: result of the soil survey. Conclusion: Sulfate Z2 and solid lead >Z2 in fills up to about 2.75m. So it would practically be planned with a basement (but groundwater level after a very dry year 2018 at 3.20m without a test for concrete aggressiveness) and the excavated soil would have to be taken to the landfill. That probably breaks our financing plan. And basically, we had always planned to renovate an old building. The 3-5 months of searching for houses were not exactly pleasant.

By the way, thank you for the extremely helpful answers!

It should be possible to tell quite precisely whether it runs centrally in the wall or through our planned living room.

It depends on who you have tell you that, and what that person has done to arrive at their statement. Tell the surveyor on site what it's about for you. In any case, just the two boundary points will hardly help you.

If the building application from 1903 said "property boundary" on this line, we would actually have certainty.

No, because even then you still wouldn’t know if it was actually built that way and whether the architect back then even cared about the correct boundary line.

The gable side actually had 2 windows on each floor in the gable wall in 1902.

So it would indeed be possible that the existing windows are legal. IMHO, however, the neighbor bears the burden of proof.

After that, a drywall.

Tell the surveyor to include this point in the measurement. The hole is a great opportunity to determine the real wall thicknesses inside the building and their position relative to the boundary.

That probably blows our financing plan.

The unclear legal situation regarding the window openings in the boundary wall might convince the neighbor to buy you the strip of land with your old building and then demolish it. You’d have to be able to negotiate well ...

For the building application, you need an official survey anyway, so you can just commission it right away, there’s no need to guess so much.