Floor plans of a single-family house on 640m² with low eaves height

Such a forum is also there for learning.

you want to calculate your knee wall? You can't do that exactly. pi times thumb a few centimeters if you want to build with UG, EG, DG.

Plan your lower floor as living space. That works. You don't need a basement as a heating, laundry, and storage room, which you then partially dig into the ground so that there is a knee wall above in the DG.

Especially with a slope, it doesn't count to be bigger than it has to be. Every excavator bucket costs

In your case, the upper floor must not be a full storey, meaning: it may only have 2/3 of the area of the full storey below it (here the ground floor), although this definition varies from state to state.

I'll inform myself about this for NRW:

Building Code NRW 2018

"Building Code 2018 §2(6) Full storeys are above-ground storeys that have a clear height of at least 2.30 m. A storey is only a full storey if it has the height mentioned in sentence 1 over more than three quarters of the floor area of the storey below it."

3/4 already sounds better than 2/3.

you want to calculate your knee wall? You can't do that exactly. Roughly a few centimeters if you want to build with basement, ground floor, and attic.

Plan your lower storey as living space. That works. You don't need a cellar as a heating, laundry, and storage place, which you then partly dig into the ground so that a knee wall results in the attic above.

Especially on a slope, don't count bigger than it has to be. Every excavator bucket costs

I then also had to learn that "painfully". Not being able to calculate things scratches my ego. But there are simply too many factors that I cannot assess. It didn't fail because of the pure arithmetic.

We will decide on an architect in 1-2 weeks and he will show me options. In the worst case, there will be several designs.

Look around Pinterest, here in the forum, at various house building companies, etc.

Stay open.
Every excavator bucket costs, every sqm costs

The house itself gets 2 VG. Urban villa appearance, development plan observed.

Development plan stipulates 1 full storey.

It would be practical to know the textual stipulations of the development plan

There are none.

You have a slope to the east.

No, hill to the east.

From the street I see a single-storey building with a gable roof, then to the back it goes into the basement

Exactly the opposite. The basement would then be free towards NW and SW. NW is bad for living rooms and SW is bad because of the street.

3/4 already sounds better than 2/3

Sorry to disappoint you. The development plan is from 1979, meaning that the definition of the state building code valid at that time must be applied and there it was 2/3, moreover referring to its own floor area, not that of the floor below.

But there are simply too many factors

Here not so many at all, since the reference points, even though not described quite clearly in the development plan, are obviously not related to the site. IMHO knee wall = 0.

Thank you for this sobering clarification.