Semi-detached house in building area 8.5m x 15m (WxD)

That would then

.. mean a house at all costs. A house that already crushes you when looking at it from your own property.

Thanks for the plan with the building plot.
I have inserted the terrain according to your specifications.
This way it can work.

We were thinking along these lines as well. However, another floor will probably be necessary, see below at

Who will get the garden plot? Is it supposed to be actually divided with a right of way?

The building planning office orally rejected an actual division. They would only approve plot divisions within the scope of development plans if both subplots reach the front building line. However, they did not want to explain the legal basis to me. They added that it is of utmost priority that the houses “all stand at the front by the street”. Interestingly, this is not described in the development plan. I have written down the textual stipulations above for you.

One must also be clear that this means garden access for larger gardening work is not free and can only take place through the garage or the house.

I think that’s okay. The inaccessible garden also has many advantages.

I know, but it’s still a good 40 cm ... and it’s not yet clear how terraces can even be distributed in such a solution.

One behind the house (towards the main garden), one in the front garden. The front garden is supposed to have appropriate privacy from the street through a tall hedge. This has also been implemented for a house nearby. However, that is a multi-family house where the ground floor apartment has its garden facing the street. That could also work with a semi-detached house, right?

The limitation of the base height primarily serves to prevent misuse in terms of building height design.

Very interesting. What kind of misuse could that be? The building height is not set in meters, but it is limited: if fully exploited, the basement floor would protrude less than 1.40 m on average, then there would be three full floors and then an attic floor with a 60 cm knee wall and 45° roof pitch. By what design misuse could this be further increased? And aside from that: we don’t even want it that tall (5 floors with living space).

Assuming 1.8 m height difference in the building window, 2.8 m floor height and 0.6 m base height/reference point, then the ground floor would protrude 3.4 m in front (0.6 m base + 2.8 m floor height) and 1.6 m in the back (2.8 m floor height + 0.6 m base but - 1.8 m terrain rise) – on average 2.5 m out of the terrain,

Here the base would be useless. At least I don’t see its purpose. It would be more reasonable to plan the ground floor at street level (without an underlying floor), whereby the rear rooms would then be largely underground and would probably only be allowed as utility space. This floor would then stand about 1.30 m above the natural terrain on average (because the slope initially rises more steeply – one might need to measure this more precisely), so it would not count as a full floor. Above that, as many floors as necessary could be built, max. 3 + attic; in our case probably rather 2 + attic.

.. would mean a house by force. A house that already oppresses you when looking at it from your own property.

What do you mean by that? The neighboring house as sketched by 11ant, only with the ground floor at street level. I have visited neighbors and did not feel oppressed at all.
I read concerns from all your posts. Probably justified as well. But I cannot recognize which direction you would tend toward?

The neighboring house as outlined by 11ant, only with the ground floor at street level.

Processing time expired. Another difference with the neighboring house: Not a semi-detached house, but a two-family house with a granny flat. In addition, the slope on the entire plot was excavated, so that you have to climb steeply into the garden behind the house, whereas the upper floor has an elevated terrace, from which a short staircase leads down about 1m into the garden. I could not find out the reason.

Very interesting. What kind of misuse could that be? Although the building height is not specified in meters, it is still limited:

Without limiting the base, there would be a great temptation to elevate the ground floor "garden-side at ground level," with the risk of resulting in a greater building height.

The building planning office verbally rejected a real division. They would only agree to property divisions within the scope of development plans if both subdivided plots reach the front building line. However, they did not want to explain the legal basis for this to me. They added that their top priority was that the houses "all stand at the front by the street." Interestingly, this is not described in the development plan. I have written down the textual stipulations for you further above.

Without making the front edge of the building window the building line, I also see no legal basis for such an obligation to build attached. The reason for rejecting the real division lies in the fact that the rear part would then legally not be "developed" and thus would have no prospect of a building permit (unless the front neighbor granted appropriate land use rights). However, I advise you not to bother with this hassle, but instead to build a classic multi-family house with apartments on different floors instead of a semi-detached house (even if the upper apartment is a maisonette).

53 m² of living area I don’t find a no-go. We also looked at terraced houses with even less. And there are also floor plans of houses with similar external dimensions. Although not as a semi-detached house, so-called "city villas" for an 8x8m building plot can be found. Of course, they have windows on all sides and can only serve very roughly as inspiration.

But there is certainly a good reason why you didn’t find anything suitable with these houses, among other things: your room program.

I have been reading your posts for years and always had the impression that you can somehow make everything fit.

Yes, but at some point you have to kickstart the thought loop whether a horrendous effort with many sacrifices is worth it to achieve something that no longer has anything to do with the original wish. I do not assume that you want to give friends a part of the property out of mercy, but it is a cost question.
How expensive was the plot? And how many euros are accepted by the building partners? One of them is certainly drawing a very short straw here.

One behind the house (towards the main garden), one in the front garden. The front garden is supposed to get appropriate privacy from the street through a tall hedge. This is also implemented in a house nearby. However, that is a multi-family house, where the ground floor apartment has its garden facing the street. That could work with a semi-detached house too, right?

Well, it is certainly a difference whether you decide on a condominium or want to build an expensive single-family house (here a "semi-detached").

Then rather plan the ground floor at street level (without an underlying floor), whereby the rear rooms would then be largely underground and probably could only serve as utility space. This floor would then stand on average about 1.30 m above natural terrain (because the slope increases more steeply at the beginning -

Here it comes: at the back, there is initially a missing floor that the front part would have.

I read concerns from all your posts. Probably also justified.

That’s not correct. You could say I tend to get into negativity. For my part, I try to see more than just stacking floors on top of each other. And with these dimensions of the floor area, I don’t see your mentioned room program. So you would necessarily have to build upwards.
But..

You could then build as many floors as needed, max. 3 + attic; in our case probably rather 2 + attic.

..You said it: in your case, namely the unit on the street side probably rather 2 + attic, but please with the ground floor extension for, e.g., technology and more.
Then you almost get your room program. But I say it clearly: no one is allowed to come with the argument that a straight staircase or a pantry or a walk-in closet has always been a dream and you want to implement at least one or two of these wishes.

So, and what about the building partners? Don’t they need their room program? They would have one floor less!
Of course, you can really put a third floor and then a creative roof on top. I believe that was freely selectable?

But I fail to see which direction you would tend to?

Many things are possible. Only I wouldn’t want to live the way you imagine it up front. I wouldn’t buy a plot for something like that.
The question that arises for me is: what did you think when you bought it? I mean, there certainly was no semi-detached house planning at that time?
I already asked about the costs. And that’s where I start: what else do I give up just to somehow save money?! I don’t think semi-detached houses or creative houses are bad at all. I also really like the topic of tiny houses and narrow houses. But that just doesn’t fit a 6-room house. And it’s about you, not me. Therefore, test your thoughts.

Moreover, the differences and negative aspects for the front residential unit are, in my opinion, very significant, which one wouldn’t necessarily want to endure for the next 20 years. The divide to the rear residential unit is very big. The little man watches how the one above drives past his house and 40 sqm of garden and up there can call his 200 sqm his own. And then there is an idealistic division where you almost have to pay for everything for the other (slope management, settling stones, etc.). No friendship will hold, I bet!

My suggestion: put up a nice two-story house there. Use the garden upstairs for yourselves. Then you also have space downstairs for a bicycle shed. Possibly you could plan a granny flat. However, you would also have to borrow a lot of money for the granny flat, which may not necessarily be worth it.

Here are the two residential units of a semi-detached house with their ground floors and garden share.

Can you please also make an elevation profile for the entire property? Not just the building envelope.