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

That is not "to your disadvantage."

Hm, I don't quite understand. If I were to base the height profile on the TIM, then by adhering to the highest FOK in the living area <7m (i.e., to avoid building class 4), three floors above garden level would be possible. However, with the height profile as it apparently appears, only two floors. That’s what I meant by "disadvantage." I also wonder, on this occasion, why a height profile with 10 cm accuracy is published in the TIM if it is irrelevant in every way.

And there too lie rather opportunities to use uncertainties in favor of the applicant.

That is true. Nevertheless: on the points where there is room for interpretation, the architect tends more towards the negative.
How is it actually if you then submit a building application and it is rejected because of some minor detail? Do you then have to pay, make corrections, and wait another 12 months?

One more argument for my proposal of the classic multi-family two-family house.

That is correct, and this option is still not excluded.

If I were to base the elevation profile on the TIM, with compliance to the highest FOK in the living space <7m (i.e. to avoid building class 4), three floors above garden level would be possible. With the elevation profile as it apparently appears, however, only two floors. That’s what I meant by "to your disadvantage".

So the terrain has meanwhile been changed to your disadvantage? - normally, reality is more favorable than theory.

I wonder on this occasion why an elevation profile with 10 cm accuracy is published in the TIM if it is without any relevance.

Here one could argue from a legal expert’s perspective that a document from a state office publisher should be regarded as an official testimony. Within certain limits, I could also imagine a "classic multi-level two-family house", where the rear rooms are, for example, three steps higher. And/or you adjust the reality before the surveyor honors the actual terrain as the standard.

So the terrain was changed in the meantime to your disadvantage? - normally reality is more favorable than theory.

Yes, as described above. In the TIM the terrain is documented as higher than it actually is, because by the latest in the 1950s it was excavated forward and in the construction area down to street level.

To some extent I could also imagine a "classic split-level two-family house," where the rear rooms, for example, are three steps higher.

That supposedly doesn’t help because with split level the highest occurring FOK counts, but the reference height level to determine the building class is uniform for the entire building. And with that, the garden-side split-level part of the 2nd floor would again be over 7 m above the reference height and thus the whole building GK4.

And/or you adjust reality before the surveyor elevates the as-built terrain to the standard.

Throwing 2 m of topsoil from the garden over the front yard and garage driveways? :-D Haven’t thought of that yet. Sounds good. That would also eliminate the slope in the garden right away.

That supposedly doesn't help because with the split level, the highest occurring FOK counts, but the reference height level for determining the building class is uniform for the entire building. And thus, the garden-side split level part of the 2nd upper floor would again be over 7m above the reference height and therefore the entire building would be GK4.

Take a look at the neighbor: if the split level is not feasible (although I don’t necessarily see it that way, since architects have learned exactly THIS, to plan a house legally according to the relevant regulations, which a layperson cannot do due to lack of knowledge) ... it continues further down.
Basically, I feel like your duplex idea is getting in your own way.

However, I advise you not to bother with the struggle, but instead to build a classic multi-story two-family house rather than a duplex (even if the upper apartment is a maisonette).

This was advised to you several times. Perhaps that advice indeed included the knowledge about the escape route, which, however, was not communicated yet because the time was not right.

They show that even very extensive internet research and years of reading house building forums have their limits.

Yes, we mentioned a lot. Unfortunately, one does not always arrive at the right result through deductions and assumptions.

However, you were also asked to show the plot. That is unknown to us.

Maybe the forum knowledge would have already taken the edge off here and there if more knowledge and actual conditions had been shared.

The rather steep incline from the street given in TIM cannot be found on the plot, since this plot, like those of both neighbors, has been excavated down to street level over about 20 meters depth and across the entire width.

How is it now?

The building planning office verbally rejected a real division. They would only approve plot divisions within the scope of zoning plans if both subdivided plots reach the front building line.

There is already quite a lot behind that.

If a natural terrain is no longer recognizable,

How is it now?

To some extent, I could also imagine a "classic multi-story two-family house" split level, where the rear rooms, for example, are three steps higher.

I have also thought about that several times.

By the way, the neighbor has dug away the slope all around the house. I do not quite understand the motivation yet.

Whether the neighbor has a split level or not:
The motivation will be that he can set windows in a lower rear floor at the slope.

Also, the slope was excavated over the entire plot area so that one climbs steeply into the garden behind the house, whereas the floor above has a raised terrace from which a short staircase about 1m leads down into the garden. I couldn’t find out the reason.

... and then the floor above opens onto the garden.

How is it actually if you then submit a building application and it is rejected because of some minor detail? Do you then have to pay, make corrections, and wait another 12 months?

This can be prevented with a preliminary building inquiry.

If you or your planner are responsible for the rejection, then it can indeed start all over again. What does the city's fee schedule say about this? The architect should also know that.

Look at the neighbor: if the split level is not feasible (although I don’t necessarily see it that way, since architects have precisely learned to legally plan a house according to the relevant regulations, which a layperson can’t due to lack of knowledge) .. it continues further below.

Well, one neighbor certainly has the FOK of his attic more than 7m above the street and also above the average terrain height. But I don’t know what kind of rooms he has up there. It is a single-family house with a granny flat, but already several decades old; maybe back then before building class 4 you didn’t have to worry. He doesn’t have a split level.

Basically, with your semi-detached house idea you are standing in your own way, that’s the feeling I get.

The semi-detached house is not a must, but the idea also has many advantages (especially the independent usability and the non-overlookable garden), so I don’t want to dismiss it immediately. If it turns out to be not (sensibly) feasible, then it’s off the table. It is currently losing points but not dead yet.

How is it now?

If you take the driveway and garage floor as a basis, then as shown in orange in the attachment. What terrain height would a surveyor use where the entire width of the property is built on or sealed?

The motivation will be to be able to put windows in the one lower floor at the back on the slope.

He has those there, correct. It is just pitch dark because above that is the elevated terrace of the garden floor and even below the terrace the terrain already rises towards the garden.