Height of the fence to the neighbor and windows in the boundary construction

Hm... I think I'll ask our architect (he simply built unauthorized windows into the boundary development next to his neighbor) and, if necessary, consult a lawyer knowledgeable in neighborhood and building law to find out what is approved. That's one thing.

The other was about the technical execution. Like how to properly build such a wall there. There must be some kind of sealing to the existing structure, right?

Less effort might be a tall wooden wall?

First check if you are even allowed to build something there: keyword building boundary from the development plan, if it exists.

Then look into the state building code to see what you are allowed to build within the setback area without a permit: keyword privileged buildings. Here in Hesse, for example, trash sheds, garages, systems for heat and energy supply, dog houses, or wood stacks. No walls!

Walls are often described separately; here in Hesse they are permit-free up to 1.5m height.

Finally, check the development plan again regarding enclosures. Here: enclosures only up to 1.5m height and camouflaged on both sides with hedges.

So I wouldn’t be allowed to brick up the windows here. The most you could do would be a free-standing firewood stack. But its dimensions are also limited...

As I said: like a garage wall, just without a garage.

is probably right there. Walls are not buildings. And privacy fences, in turn, must be installed with the appropriate distance (height/distance). How big is the window? I would plant a tree in front of it.

How big is the window? I would plant a tree in front of it.

Yes. About a fucktus: *duckandcover*

There has to be some kind of sealing to the old structure, right?

Yes. Bavaria is very sparing with its neighbor law. With us, it is quite clear in the legal text (NachbGNRW):
§ 22 Erection of a second boundary wall
(1) If there is a building directly on the boundary on a property and later a building is erected on the neighboring property at this boundary but attached without structural connection, the builder thereof is obliged to close the resulting gap at their own expense in a suitable manner so that damage in the area of the gap, especially due to building movements and weather influences, to the first erected building is avoided. The necessary connections must adapt to the materials used in the existing building.

No walls!

HBO §6 paragraph 10 (privileges)
6. Enclosures, privacy fences, and terrace partition walls in commercial and industrial areas, outside these construction zones with a height of up to 2 m above ground level, ...

Here in Hesse no permit required up to 1.5 m height

Appendix to §63
Building permit-free projects according to § 63
...
7.1 Enclosures, terrace partition walls, and privacy fences up to 2 m height ...
For Bavaria, I already mentioned the regulations in #2.
If the window is illegal, nothing stands in the way of a wall.