Is it possible to have a window in the guest WC/guest room despite the garage?

Link the thread with your floor plan yourself, I'm too busy right now. It's easier to give advice than with just such a snippet of a question.

The floor plan will follow later. So for now, just the preliminary question. If the window hadn't worked, the whole floor plan would have been worthless.

What is the reason behind the idea of the "Hochkeller" in the first place?

Of course that's possible! Why not? There are also window slots. Almost everything exists, just not in every house exhibition.
I'm looking forward to your design: it must include every oddity ;)

But I just saw in your agenda: it has already happened.

What is the reason behind the idea of a "raised basement" anyway?

From your question, do I interpret that a "raised basement" is actually not common when building with a basement? If that is the case, I would be surprised, since a basement that is 50-80 cm above ground level can rather (in contrast to a basement below ground level) create brighter and friendlier rooms - maybe even for a granny flat - with some daylight? Put differently, what speaks against it? According to the development plan, 2 full floors with a ridge height of 7.50 m and top edge of structural elements at 11.50 m are possible. This should not contradict a "raised basement".

From your question, do I interpret that a "raised basement" is actually uncommon when building a basement?

Totally uncommon. It does help with lighting (if necessary), but it usually conflicts with the specifications in the development plan regarding building height, etc.
In addition, ground-level access to the house and garden is not possible. Who would want an elevated terrace (costs) and then have to go down several steps to get to the garden.

Who wants to have an elevated terrace (costs), only to be able to go into the garden by several steps.

But if the house was built, for example, on a slight slope, you can exit to the garden at ground level at the back. At least, that's the case with us. However, I agree with you about the house not having a ground-level access.