Street about 50cm above the property - backfill or basement

I have attached a section of the property (including required and planned contour lines).

Unfortunately, property boundaries and measurements are missing – better tell me the name of the development plan so I can read the original myself, this is useless like this.

(The only thing was that my wife did cartwheels when she heard about 1.5 meters knee wall)

Pretty naïve. One and a half meters knee wall means she will hardly be able to "look over the steering wheel," i.e. will only see the bottom edges of the window frames. You only wish for that if you can’t count to Pi.
In the plan section I see a dotted line. Does it mean the eaves height on the valley side or the mountain side?

Thanks already for the abundant input
: Rappenberg 4, Gemeinde Kirchberg Murr
Attention: in the BB the division of the plots is not yet final. There is also the construction division document available as a pdf in parallel.
The height of the eaves is determined by the given maximum floor height (304 for us)
: 2 full floors are specified. How this should technically work with the eaves is beyond me

We will have the soil survey done as soon as possible.

Had/have a similar situation, although only 30cm on our 840m2 plot.. We will roughly do it like this: "80cm compacted gravel and a floating slab on top" unfortunately can't explain it more precisely, but with this we are exactly 10cm above street level. The basement was also too expensive for us at about 60k €. We're getting a 12x6m wide/deep driveway + excavation (driveway/house/rainwater cistern 9000l) and have to have it hauled away because with that we can compensate more than the 30cm.

The planned road is at Visierbruch at 303.49, i.e. at least 50cm above ground level.

But that is also the highest point. Afterwards, the road descends at 1.99%. Since the reference point is to be understood as the maximum height, I fully agree with the architect's statement.

How this is supposed to work technically with the eaves is beyond me

With dormers and/or cross gables.

What did you recognize the 1.99% from now?