Thank you very much for the feedback, I will try to sort out and comment on some things.
First, a note on my own behalf, there was actually supposed to be a reference to the development plans here:
The plot is not within the scope of the provisions of the 2nd amendment.
Attached are the development plans.
Development plan from 14.12.1967
Development plan 2nd amendment from 13.04.1987
Dear Katja,
thank you very much for your contribution. I want to try to answer your questions.
As a first planning aid, the current setup in the living room:

We still have two bird cages and the four feathered creatures love to look into the garden, but at the same time they need a certain amount of visual protection. As flight animals, they would otherwise go crazy with too much activity outside the window.
Is that actually a seat window down to the left front in the living room?
One could sit on the windowsill if there were no flowers there by now. In addition, the radiator under the windowsill had to be removed for the time being, so it is quite cold from the stone bench:
what do you want to spend?
I cannot say that yet, only that it will be a project in 5..10 years. Regardless, we could currently manage over €60k with equity.
This is mainly about the living room extension if I read that correctly. Your family planning is unfortunately a bit confusing. That’s why the need in the upper floor is unclear.
Normally, the upper floor could stay as it is and you enlarge the balcony into a roof terrace - but you don’t want that, right? Why now? That would save some costs.
I will try to sort it out:
[*]Family planning: No. 1 is on the way and will move in January, No. 2 is conceivable and not excluded, then a room would be missing
[*]The small room (former shower) would suffice for the home office, it doesn’t have to be a big office (it’s not tax-relevant anyway)
[*]Guests are occasionally present but would always be satisfied with the living room, an additional guest shower is not necessary
[*]In summary, this means: a further room should be created on the upper floor that is not a walk-through room. It should function as a children’s room; if not needed for that purpose, it would be a guest and work room. Neither the current balcony nor a roof terrace is needed there.
[*]Living room extension: The current state is not optimal (partly also due to existing furniture). The conservatory is poorly insulated and ready for demolition. The question here is of the optimal extension, it doesn’t have to be at the place I have in mind. Possibly it can be better extended in other directions. The extension will depend on the upper floor expansion.
[*]General:
[LIST]
[*]I am not a fan of flat roofs
[*]We do not want an open kitchen; the current location is fine (laying water and electricity to the living room would have been a minor problem, I just recently did that for the kitchen)
[*]It would be nice to preserve the parquet flooring in the rooms
[*]The drainage is due, it also runs under the current conservatory, which makes replacement impossible before demolition. Hence also the desire for a basement under the extension.
[*]Windows and furniture orientation should not primarily face to the right on the plan. The neighboring house is not very attractive so far and there is also a rather tall "hedge" in that direction.
Next, I will try to get the 3D drawing with the slopes/heights.