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

I can probably manage that.
X 3470577.8589 Y 5753309.5880
X 3470585.3739 Y 5753308.7858
X 3470593.6803 Y 5753301.5770
X 3470570.4185 Y 5753278.9416
X 3470558.0615 Y 5753285.4520

I hope this fits.

Another difficulty with you is that the building boundaries in the development plan are neither dimensionally nor geometrically defined, especially at the turning area expansion. I have transferred them graphically. IMHO, a 9m*12m floor area is not opposed, regardless of whether it is parallel to the northwest or southeast boundary.

The architect, however, said that a maximum of 8.5x10.5 would fit

Have you already looked for a new one?

Well, according to BPL it is not really clear, since it still concerns the original property, which was divided into two properties.

Well, according to BPL it is not really clear

What do you mean by that?
Do you have any more questions?

No, I don't think so. You already mentioned that the edges of the turning circle at the BPL are vaguely marked without dimensions. And in my opinion, the size of 9x12m should still be sufficient. Thank you very much to you first!! Regarding architects, we are still looking.

I've now tried 4 different CAD freeware programs

That such programs as freeware don't quite reach the moon seems quite reasonable to me, insofar as freeware users generally have processor performance typical of private computers. The typical shareware user doesn't have a BladeCenter in the cooling chamber.