Build a 150-160 sqm single-family house in the parents' garden - overwhelmed!

Then make sure that you only ask the questions for which no authorized draftsperson is required.

Would you like to explain that to me in more detail? I would have used my site plan now, placed the house to scale on the desired spot, including roof orientation and boundary distances. And a short description of what the house approximately looks like. Is something missing? Or is that already too much?

Is something missing?

Yes. The building application form with the check marks in the correct places and the question(s) to be clarified.

Or is that already too much?

Maybe. If you do not ask any concrete questions at all or if you ask questions about the extent of the structural use, i.e., for example, how large or how high the building may be, or questions otherwise requiring answers beyond the three aspects without authorization for submission mentioned in the state building code (type of structural use, construction method, buildable plot area), the authority will probably reject your application due to formal errors.

... that an architect is definitely required for the preliminary inquiry.

This statement is neither false nor true but requires further explanation.

for example 8 m x 10 m

As soon as the dimensions appear without comment, this can be interpreted as a question about the size of the building volume and thus require building submission authorization. If, on the other hand, you only ask about the permissibility of the location of the sketched building volume, it concerns the buildable area, which does not require a building submission authorized person.

As you can see, it is not quite simple, and the authorities therefore advise early involvement of professionals rather than struggling for weeks with deficient construction submissions and incomprehensible builders. With the statement

that it concerns development according to Paragraph 34.

and the images shown here, I consider the plot to be buildable with 99.9% probability and therefore the investment in someone who knows about this worthwhile.

What do you want to achieve with such a preliminary building request? It is meaningful only to the extent that your sketch is accurate. Go to the architect and have the thing planned; everything else is a waste of time. a) unplanned inner area = building law exists b) building density of the neighborhood suggests that building is possible, the property feels like a gap c) property size also allows compliance with distances and setback areas d) distance to the street results from an imaginary line between the neighbors