First, about the development plan: it looks to me like it's from the 80s, and the plot probably fits into a building gap within it. The essential organizing intention was probably that the houses have a clear axis and are gable-end oriented to the street, as well as that the garages do not stand around lost. I suspect the specification about the house axis is misquoted; the aspect ratio of 1.2 : 1 (other – mostly Bavarian – development plans more often speak of 5:4) is supposed to be "not significantly undercut."
Yes, valid question. I know that the upper floor is more important,
"More important" is a wrong word, the upper floor is rather "more delicate" (and thus more demanding of planning skills).
but decisive is whether the wing has to go on the upper floor or if it fits on the ground floor. And to answer the question, I first tried on the ground floor.
You should have developed the upper floor first. (Not only) amateur planners only drive themselves into despair when they start with the simpler ground floor. I still haven’t understood why: does the popularity of this mistake come from the fact that one usually enters the house on the ground floor on reasonably level terrain, or because the "good parlor" traditionally lies there, or because the masons start downstairs? Or is it actually blindness, or an addiction to frustrate oneself? – in any case, such amateur planners who are not satisfied even after seven designs regularly start the eighth obviously incorrigibly again with the ground floor. I have to watch whether it is a male thing, this uncomfortable way of trying to break through the wall with your head, even though the back door is not locked.
In your place, I would first try two ground floors, one with and one without the wing. From both you will then derive almost by itself developing ground floors, only that with the one without the wing it won’t fit down there again. So do the upper floor with the wing first. You really only have to pull the nipple through the loop, then it cranks much easier.
