Semi-detached house: Estimated costs accurate and feasible, experiences?

I even know the neighbor. He will not change anything structurally, neither will we. The cellar was wet on one side before the heavy rain,

You only know the current neighbor, but not his heirs or other subsequent owners. I meant the worst case, that your houses share a single party wall and have a low or partial cellar, and only one of the co-owners is not satisfied with the space offered and wants to extend, add a floor, or demolish. If both of you now only have the expectation that essentially newer cables would suffice, that is not guaranteed permanently and is rather atypical nowadays. Almost every buyer or heir of an older u100sqm semi-detached house has "higher" ambitions. At the latest after realizing "wow, even so little already costs so much money," most want more or want to demolish immediately.

You only know the current neighbor, but not his heirs or other subsequent owners. I meant the worst case, that your houses share a single party wall and have a low or partial basement, and only one of the half-owners is not satisfied with the space and wants to extend, add a storey, or demolish. If you both only have the expectation that essentially newer cables would suffice, that is not guaranteed permanently and is rather atypical nowadays. Almost every buyer or heir of an older u100sqm semi-detached house has "higher" goals. At the latest after realizing "wow, even so little already costs so much money," most want more or to demolish immediately.

The residential area is designated as a commercial/industrial area with residential restrictions. The residential buildings approved by the building authority enjoy protection of existing structures. Take a look at the picture regarding the wall. The neighbor is a family member.

Look at the picture regarding the wall.

That looks as expected like a one-piece communal wall.