Building with a knee wall of 50 cm?

I am a bit resistant to the idea of a "residential basement." I am also afraid that we actually cannot afford a basement at all, since our entire equity will go towards the land. I also have not really figured out whether you "have to" build with a basement at 10%, or if you can level the land (or only the part where the house will stand).

whether you "have to" build a basement at 10%

You can’t say that in general, because a slope usually isn’t linear. And it doesn’t only depend on the building envelope, but also on the rest of the development plan, the surroundings, orientation, etc. "You don’t really" take a piece of house and put it on a plot – you have the plot and try to plan the house on it in harmony with taste, wish, and budget.
The basement thing can be influenced, whether it becomes a full story or not. As long as less than 1.40 meters of the exterior walls protrude, it is not considered one.

I’m a bit resistant to the idea of a "residential basement."

Why?

If you provide more information about the property, it is easier to help. has already pointed out what matters.

How expensive is the building gap?

In my opinion, a 50 cm knee wall is not a deal breaker. Other conditions in the development plan can influence the decision. Unfortunately, we do not know the development plan.

The surroundings have managed it with a 50 cm knee wall. There must be another reason why the property is still undeveloped.

The following applies in BW: "Storeys are above-ground storeys if their ceiling upper edges protrude on average more than 1.4 m above the ground surface; otherwise, they are basement storeys."

With a 10% slope, the basement should become a residential basement anyway. Already because of the costs.

Does the development plan specify reference heights?

This is what a residential basement can look like.:

In BW the following applies: "Floors are above-ground floors if their ceiling top edges protrude on average more than 1.4 m above the ground surface; otherwise they are basement floors."

With a 10% slope, the basement should be a residential basement anyway. Already because of the costs.

Does the development plan specify reference heights?

This is how a residential basement can look:

I would really like it if the lower side faces south/west. The storage room can also serve as a utility room.
If not, I would put the kids' rooms and bedrooms downstairs.

I have sketched it once with a 10% slope of the original terrain (blue). Terrain modeling is then green.