Unclear property boundary and legal consequences

Hello,

We, as a family, have bought a plot of land. Very naïvely, without a building ground investigation, but affordable, in the best location and affordable.
Current status according to the building records archive:
Originally, it was a large plot with a multi-family corner house (built in 1896, with a basement, 3 floors + attic, war damage – demolished around 1950 including the basement – unfortunately, the excavation pit was filled with construction debris/garbage up to about 3.20m). A rear building was also supposed to be built, but then there apparently was no money left. Consequently, around 1900 the plot was divided and on the neighboring plot !directly on the property boundary! a multi-family house was built (built in 1902 – basement, 3 floors + 2 attic floors – now listed as a historic monument – renovated several times since 2002).
On this house, directly adjacent on our plot, a rear house was built (built in 1903 – 2 floors without a basement – never significantly renovated). It is basically attached and does not fully have its own rear wall!

Unfortunately, from current cadastral maps and parcel maps it is not possible to see where the property boundary runs.
At first, we had considered renovating the rear house, then thought about a new building, and now we are closer again to the idea of renovation. We are unclear about what the property situation might mean for us.
We are currently trying to clarify whether there is more detailed information about the neighboring property in the building records archive. It is also possible that the plots were only divided after both houses were already standing.

About the picture:
In the background with the orange line is the aerial image with the property boundary.
The walls show the ground floor. The added walls toward the neighboring house have been installed afterwards and are not included in the building application!
In the upper floor, there are no added walls. Here, the exterior wall of the multi-family house virtually serves as an interior wall. At about 90cm height on the upper floor (under the neighbor’s 2nd floor), the wall has a step of 6cm – to save bricks – shown as a black wall. Above our roof – which goes up to the wall but is at least 6cm inside our house everywhere due to beams.

According to an excerpt from the cadastral office, there should be property boundaries marked by stones and the like. My father (a surveying technician) has not found any. A survey apparently has not been conducted at least since 1990.

Many questions arise for us here:

[*]At the neighboring house, on the gable side at the property boundary (I am quite sure after 2000 and certainly without permission), 6cm styrofoam thermal insulation and 2 windows were inserted into the fire wall. Can we demand that these windows and the insulation be removed?
[*]Do we have the right to anchor a stainless steel chimney above the height of the neighboring house on the gable side?
[*]If I hammer a nail into my interior wall, I am currently either doing it into the neighbor’s wall or into a wall that belongs to both of us. Is there anything to consider regarding renovation work? Do I, for example, have to obtain permission before plastering and replastering these walls?
[*]Can/Should I remove the added walls on the ground floor to create more living space, or is this strongly discouraged?

It has become a longer text, but unfortunately nobody has really been able to help me so far. The building permit authority only tells me that the windows and insulation are not in order but not what to do with this information, and they recommend the property be surveyed again but cannot really tell us whether it makes a difference if the wall belongs to both of us or to the neighbor.

Thank you very much for your time and help!

Jo newly measured ...

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

It has become a longer text but unfortunately no one has really been able to help me so far.

That will not change either, since your text is indeed very long but contains hardly any usable facts.

My father (surveying technician)

And then he allows you to upload such a picture? Sorry, but what information is that aerial photo supposed to convey? Regarding the problem statement, you see - nothing.

A survey has probably not taken place since at least 1990.

As a surveying technician, your father should be able to obtain and evaluate all cadastral documents. Then formulations like "probably at least" and "quite certainly" should belong to the past, because then it can be said with certainty whether and since when the boundary has been determined. If it has been determined, it can also be transferred to the location, and one can determine which building parts are on which side of the boundary.

Can we demand that these windows and the insulation be removed?

Windows yes, insulation maybe.

Do we have the right to secure a stainless steel chimney above the height of the neighbor’s house on the gable side?

Yes, §26 SächsNRG

If I hammer a nail into the wall on my inside wall, then according to current status I either do it in the neighbor's wall or the wall that belongs to both of us. Is there anything to consider in this regard during renovation work? For example, do I first have to obtain a permit or something when plastering and replastering these walls?

Seems to be a question for a lawyer. In NRW, the neighbor law is clearer. IMHO, it also depends on the result of the boundary investigation. My tip: tear down and rebuild.

Seems to be a question for a lawyer. In NRW this is more clearly regulated in neighbor law.

Does this seriously refer to hammering a nail into the wall?

The first step is to talk to each other and not to demand and sue.

The first way is to talk to each other and not to demand or sue.

Yes, that is the best.

Is this seriously referring to hammering a nail into the wall?

Yes, also, but it is about more than just a nail. In general, I wanted to say that the legal situation in Saxony does not appear so clear to me as in NRW. With us, two entire sections are dedicated to the topic of neighboring walls and boundary walls. It is clear: nail in the neighboring wall yes, nail in the boundary wall no. Are the regulations in Saxony hidden somewhere else? Does the case law there orient itself to regulations in other federal states? Who knows? A lawyer? But as I said: the factual situation is thin. When was the house purchased? When exactly were the windows installed, the insulation applied? Who was the owner at that time? Did someone perhaps agree to it? When was the property divided? Where exactly is the boundary? Are we dealing with a neighboring wall or a boundary wall? What does "rear house" mean? Are there building encumbrances? Easements? Building/renovating in existing structures including boundary construction and monument protection is highly complicated. Many architects are overwhelmed by this, so a "why is this important" should therefore seek competent advice in real life. Uncertainties in individual cases and ambiguities in dealing with concrete facts can then gladly be discussed publicly here on the marketplace.