ᐅ New construction of a single-family house with a basement and a separate apartment with its own entrance

Hello, we are currently planning a single-family house with a regional general contractor.

Here are some key details:

-156 sqm (1,679 sq ft)
-Gable roof with bay window
-KFW55 standard with central ventilation system
-Basement built as a watertight (WU) basement
-Solid wall construction with porous clay bricks, 36.5 cm (14 inches) thick (U-value 0.23 W/m²)
-Timber-framed purlin roof structure with concrete roof tiles

The site has a slight slope, and we want to convert the basement into a separate apartment to rent out.

I have made a sketch to illustrate the plot layout. According to the contractor, the terrain profile is not suitable. A separate access to the basement would not be possible. The geotechnical report requires waterproofing up to the top edge of the terrain.

Even if I cannot have a separate apartment according to the local housing regulations, I would still like to have an external separate entrance to the basement. Is that really not possible? I don’t fully understand why I can’t create a basement entrance and possibly cover it.

Here are the ground elevation levels (red numbers):
1 -4.5
2 -3.84
3 -2.68
4 -1.41
5 -0.64
6 -0.11
7 -0.83

The blue arrow shows the desired access to the basement. The main entrance is on the right side. The elevation levels are referenced from here.