Narrow plot, max possibilities, single-family house with low knee wall

Well, maximum Hornbach. It wouldn't be enough for OBI anymore
Didn’t know how else to explain those things. By [Gaube] I would rather understand a small little thing where just a window fits.

Under dormer, I would rather understand a small little thing where only a window fits.

Ok, so this here:

We would like to build a nice little "bourgeois" single-family house on the, among other things, property.

... actually doesn't exist at all

Didn't know how else to explain those things. Under dormer, I would rather understand such a small little thing,

In your case, they are also cross-gabled dormers, but they have as much to do with Bauhaus style in the sense of the art school as a red Golf has with a Ferrari.

Quick question to the group, I plan to inquire which surveying office was responsible in order to possibly obtain exact details of the precise building envelope. Does a surveying office do other more precise measurements? Because on the upper preliminary survey plan it says "Spannmaße sind aus Koordinaten ermittelt und örtlich zu prüfen!" What would such a thing cost and how does an architect proceed to know exactly how wide one is allowed to build?

Does a surveying office do other more precise measurements?

Yes.

How much does such a thing cost?

Which thing exactly do you mean? If you can do something with coordinates, they should be provided to you free of charge. Calculating four right-angled measurements should also be a service, with the expectation that you commission the site plan and/or the building survey. A penny-pincher might charge about €100.

How does an architect proceed in this case?

An architect obtains the coordinates—in the ideal case, asks about their accuracy or is informed by the surveyor—and imports them into his CAD software. This way, he obtains any desired measurements at any point within the accuracy limits of the measurements.
The note

"Span dimensions are determined from coordinates and must be verified on site!"

indicates that the planned parceling has so far only taken place on the computer and slight changes may still occur through on-site surveying. These usually happen below 5 cm. If you want to be absolutely sure, you have to wait until the cadastral office has recorded the new boundaries in the land registry.

Thank you, I have contacted the surveying office. Let's see what comes of it.