Hexagonal plot, place a single-family house with carport/garage

Explain 1+!

That is probably another way of writing 1+D.

So it will probably be a maximum of 1.5 stories with a 1m knee wall,

You read that correctly.

German is not my native language,

Oh dear, I wouldn’t want to have to read a development plan in a foreign language. I don’t see any chances to circumvent this here – reasonably, even height specifications per property have been made, which do not create an advantage on the valley side. By the way, slope is more of an accomplice than an enemy if you want to build an interesting house. I wouldn’t think of leveling, and barely of terracing.

I see no indication of goodwill in the neighborhood "Am Stollen" to allow typically two-story "city villas":

According to the OP's wish for a two-story building – yes, but unfortunately the development plan doesn't allow it: it states 1+.

You should read that before making a reservation.

A two-meter slope across the width of the plot is also no joke...

I overlooked that :/

I see no indication of goodwill in the neighborhood "Am Stollen" to allow typical two-story "city villas":

No, I don’t see anything either. Just lots of dormers and third gables.

I see no indication of goodwill in the neighborhood "Am Stollen" to allow typical two-story "city villas":

Well, in the new development area to the left of your picture section, which belongs to the development plan posted by the thread creator, there are a few staggered floors.

Two meters of slope across the width of the property is nothing to sneeze at either ...

Yes, it's about 6%. You definitely have to think about it a bit and plan for additional costs. With a single garage, 9m house width, and 4.5m conservatory, it would be a 16.5m distance from the access road in the east and thus 99cm. Not really dramatic.

I don't see any way around it here

The OP already said so. Ground floor 4.5m larger than upper floor, results in 41m² roof terrace and thus not a full story. Or no roof terrace but rather a conservatory of corresponding size (the OP hasn't said anything about the budget yet). Maybe someone can explain to me the reference under C 4 to §2 para. 4 of the Lower Saxony Building Code. (4) Residential buildings are buildings that contain only apartments or rooms serving their ancillary purposes, such as garages. I suspect an error in the textual stipulations that could at least invalidate this stipulation.