Boundary construction/extension obligation

Of course, we will ask the building authority again, but does anyone here have experience with this?

Shoulda woulda coulda, maybe one should have inquired about the regulations for the property before buying. Based on the photos in the other thread and the site plan, I assume that this mentioned building obligation along the street is clearly visible. For us here, it is of course not clear which buildings are new and which are existing.

Since you plan to build a two-family house...

Our new house is supposed to have a footprint of 140 m2 (supposed to be a two-family house)

That somehow seems strange to me and would be difficult for us too, since the plot is quite narrow…

... and since the plot appears very narrow to you, it is already questionable, even without the "building obligation," whether you can fit your planned footprint on the plot. Because it is like this: where there is no building obligation, setback rules apply.

Now our neighbor, to whom we have to attach, said that we are only allowed to build where the current houses "touch" and otherwise must maintain a 3 meter distance.

Be that as it may: the corridor plan reflects the building obligation well. Moreover, the neighbor seems to have dealt with it, since her house seems quite new, at least recently renovated.

I ask you whether this two-family house thing is a fact, whether you already have a plan? Because if you already know, i.e., are talking about 140 sqm footprint and not just a general space requirement, then something must already exist? But then I ask you why you buy a 15-meter wide plot if you already see problems with the building. Essentially, the building obligation is a blessing for you - so you can already forego the setback areas in some parts. 15 meters width is enough for more living space: there are nice houses that are only 6 meters wide, you would have the possibility to build 9 meters width, and there are also options in between. And as the corridor plan shows, two residential buildings are also possible.

Since we don’t know why you have the two-family house idea, for how many people and in what relationship the two units are supposed to serve (rental to others or multi-generation house), one can’t really advise anything. Also, it is probably unclear to you whether you must adapt to the roof pitch of the neighbor?

There are many possibilities, but you should first clarify what is possible and what is not and also immediately look for an architect who is somewhat familiar with the customs of the residential area.

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Since we do not know why you envision a two-family house, for how many people, and in what relation the two residential units (rental to others or multi-generational house) are supposed to serve, one cannot really give any advice.

Yes, we do know: there should be a second residential unit for the parents. And looking at the site plan, I find the solution of two houses quite attractive: where the outbuildings are, you build the parents an independent little house. Then the OP can remodel the house in front (with aerated concrete, which only the parents are afraid of stone louse infestation), and in the back the parents get their red-brick house :)

The square meter number is currently what gives us the most headache. Or rather, what it costs.

The plan is that my mother will use one living unit, but not permanently, only when she is there. And when she no longer lives, we will rent it out. Although I find that increasingly difficult. Because I don’t know if I really want someone living in my house, and the question for me is whether that is even profitable, given the current construction prices.
She still has three other apartments, one of which is her main residence. I estimate that she will be there for a maximum of half a year, not all at once, but in total.
In conversations with the general contractors it was always "No no, I’m not moving in, that is only when I’m there, then an apartment for me. Or then for old age (she is now 62... so I assume by old age she means 70-80...)"
That is basically okay, we are only annoyed that it has to be exactly 120 m2.
We have also had proposals from general contractors that the granny flat is "only" 60-70 m2. But that is too small for her.
But we can hardly deny her that since she largely financed the land.
It was agreed that we as a couple will finance the construction or what is built on the property.
As a family, we would have liked to have 140 m2, but given the current construction prices, we have to say that 260 m2 in total (so her 120 + 140 from us) is financially not feasible.
More like 200 m2.
We would like 3 rooms + a nice, large living room with kitchen (40-50 m2) and two WCs.
My mother definitely wants a very large living room (preferably 60 m2, 50 is probably also okay), a bedroom, and another room for stuff. Also a toilet separate from the bathroom.

Her house on the back part is unattractive to us because mainly we are there and presumably later will not keep living in the front and rent out the back, since it is much nicer back there. But sure, we could then move to the back.
But until then we would be looking at a basically empty house.
She also wants, if it is a single house, for it to be so big that at least 70 m2 are downstairs because she wants a huge living room and a bedroom in case she can no longer go to the second floor.

Anyway, I am personally currently burdened internally by the high demands, especially regarding the intention of not really wanting to live there.

We SHOULD have discussed some things much earlier :p

We have also already had suggestions from general contractors that the granny flat is "only" 60-70 m². But that is too small for her.
However, we can hardly deny her that since she has significantly financed the land.

That is basically okay, it just bothers us a bit that it absolutely has to be 120 m².

Anyway, I am personally currently burdened internally by the high demands, especially considering the intention to not really want to live there.

We SHOULD have discussed some things much earlier :p

You have to and should talk now. Definitely.
It’s no use. The plot is poorly shaped for her wishes, the construction project will be too expensive.
In theory, some things would work, e.g. the mom in the front house, her in the separate back house...
She is not aware that one square meter of living space roughly costs €2500.
Since she apparently only needs something near other places of residence, she could consider that “a bit smaller” would rather lead to the goal. Ask her if she is aware that you literally are stuck because of her wishes.
May I ask how expensive the plot was or how much she contributed?
If her wish costs €300,000 (120 sqm), how much has one done the other a favor here?

35k commission does not suggest a bargain. I guess about 950m2 in the Hamburg area. Still under 7 figures?

What does your budget say about the mother's plan? 280m2*3k=840k!!