Driveway planning for large sloped property - 25% gradient

Apart from the spelling mistakes (it must of course be 296/299 meters) I see the building layout as in @Escrodas’s proposal in post #16 and a terrace will probably be located between the house and the garage.

Just as the architect drew the first curve or as you did in your sketch, this hardly makes sense. You end up with at least a 30% incline or would have to make enormous cuttings. With a corresponding angle to the contour lines, this is alleviated.

Is the architect's implementation really that bad? In the drawing, the incline starts in the first 5 meters from 0 to 10%, then in the curve (10 meters) from 10% to 20%.
In my design, it would be the same curve, but the incline would increase only to 15% in the second step. Then it would remain at 15% for 20 meters and return to 0% within 5 meters.
That should be quite drivable and feasible, right?

Apart from the spelling mistakes (it must of course say 296/299 meters), I see the building arrangement as in @Escrodas’s suggestion in post #16 and a terrace would probably be located between the house and garage.

The design is really not bad and we will discuss it with the architect and landscape gardener! Many thanks again for that !
There are two sticking points, but maybe they can be fixed. On the one hand, the garage is still quite deep and you would have to overcome 5.5 meters with stairs to the house (if the house really remains at the height of 304.5m – this will be clear by the end of the month at the latest), and on the other hand, the road would start in the NW corner in a green strip of the city.
I know this is unfortunately not clearly visible on the plan (it is on the development plan and also not on site, since everything here is currently extremely overgrown and not demarcated from the property itself). But maybe one can talk to the city about it.

Addendum:
What might also be possible is to build a small switchback? So that you avoid the green strip and drive directly uphill in a curve from the road (just like in Escroda’s sketch)?
But then you would have to drive a quite tight curve again. It’s all not that simple.

Apart from the typos (of course it must be 296/299 meters)

Naturally. Ashes on my head ops:.

if the house really stays at the height of 304.5m

For me it is 1.5m lower and shifted to the north, so that the top edge of the finished floor of the basement corresponds with the existing terrain. The question about the pain threshold for the house height is still open, since whether

one could still look over the houses even from the garden floor

is to be understood as a wish or as a condition is not clear to me. Likewise it is open how your planner wants to solve this fact in his previous planning:

The garden floor is therefore already 2.30m above the existing terrain on the valley side

Finally, the possible terrain changes in the development plan are strongly restricted, e.g. retaining walls for wall height adjustment 0.80m, for terrain adjustment 1.30m.

If I understand it correctly and for the houses that are in our view, they should not be more than 10 meters high (4m floor + roof?).

Hard to say, since the city planners have not set building heights and wall heights depend on the terrain profile, which we do not know. But I think your estimate is correct – since the roof pitch is limited to 46°, it is probably already over at 9m.

and on the other hand the street would start in the NW corner in a green strip of the city.

It does so in the drafts from the architect and you as well. But for me it is a typical case of lacking communication between the artistically designing city planner and the technically drafting draughtsman. These "green strips" are nowhere textually mentioned in the development plan, green arrangement plan or its justification and are not separated from the traffic area by the street boundary line. This is, IMHO, a planning legal contradiction, so that I interpret the green areas from a road law perspective as street-adjacent greenery without influence on the access possibilities. If an access regulation had been intended, the city planner would have had to choose the instrument of entrance and exit restrictions according to 6.4. plan symbol regulation.

But then you would have to take a pretty sharp turn again. It's not that simple at all

A curve that is sharper in two dimensions from above can very well be the less sharp curve in three dimensions and lead to less track deviation as a result of the product of change of direction, potential energy, and tire grip. If it’s nothing more than driving slower and turning the steering wheel more strongly ...

Whereas curve & slope are quite an unfavorable combination, especially in winter.

While curve & slope are especially an unfavorable combination in winter.

That's why all the suggestions to place the garage at the base camp and not at the summit cross.