Corner plot 513m² with 130m² house and double garage

I do not see the slope in the views.

In my opinion, the floor plan is a mix of my cousin's house and that of a couple of friends

Isn’t a house design of a certain size (for 4 people, not too big and not too small) always similar to another equally sized one? You don’t reinvent the house. In this respect, a pattern always emerges that is very similar to another. Ultimately, the plot, slope, and orientation determine the pattern with which you plan a house.

So, I also see the priorities set wrongly here.

Due to the plot and building boundaries, only a narrow, elongated house is possible with a double garage.

Instead of subordinating the parking space of the cars to the house, the house is subordinated to the double garage.

South view: All windows are different and not symmetrical nor in a harmoniously appearing position.

Symmetry does not have to appear harmonious either; what matters is the amount of light, especially in winter, as well as the connection of the house with the garden.

Idea: The special window on the east side in the dining area is a window seat -> table further north/east at the window seat

Since the main garden is probably to be seen towards the east, in my opinion a terrace door is preferable instead of a window seat. I doubt whether one would want to place a dining table in front of a window seat.

Does the south side then seem as if a window is missing in the west?

As already said: amount of light, sunlight in the house. In winter it is the south; delightful evening sun in summer is reached through west-facing windows.

The plot is not large. Even if optimally planned with a southern orientation following the shape of the plot, I would also see a rotation here with an L-shape of the open space and location of the bathrooms and utility rooms in the north so that east, south, and west are used for living areas.
Upstairs also results in a nice southern orientation for the children's rooms.
Keep in mind that a roof overhang ≥ 50 cm is fully added to the built-up area.

Roughly like this:

Also, auch ich sehe hier die Prioritäten falsch gesetzt.

These do not seem to me to be priorities. Rather, a relative's house and an acquaintance's house have simply been mixed up, because if someone likes two houses well enough to imagine it similarly as their own home, then some consider the architect unnecessary. And when it then looks almost a bit like a professional building plan, the goal seems close ;-)

Bedenkt, dass ein Dachüberstand >= 50cm voll der überbauten Fläche dazugerechnet wird.

Probably not the case here, since the development plan even requires it. Just folklore. Holleradidödlduh!

With a rotated floor plan, I would rather swap cooking/living but basically rotate it like this.

Instead of a garage, I would rather plan a carport with a storage shed somewhere.