Building a single-family house with/without a basement on a small plot

If the building line is stated in the legally binding development plan, it also applies. To deviate from it, you would need at least written, i.e. formally approved permission, which means it would void the exemption normally possible in BY. However, I would not want to retreat from it either. It seems to me that there is an urban development concept behind it, so in my mind’s eye the recessed upper floor on the street side is not set back. "With no basement" I see a utility room, a roof terrace, and in between a "studio"-called storage room as a passage room on the upper floor, that's it. I see no room for romance in the plan, you can forget about the nine times nine, and you will easily fill the nine times ten and a half. Does your wife smoke? (then I would set up the study in the studio).

Whether garage north or south ... unfortunately, the site plan cannot be seen clearly. How much space is still left on the north side of your building plot? And maybe the property can be measured ... so total width/length. Maybe you could also take a screenshot of the relevant section from your PDF, enlarging the PDF view significantly and then taking the screenshot there.

2.5 floors should just be enough for 4 people in my opinion. But if 2 children come instead of 1, it gets tight. Then I think the basement should definitely be included. (You have to be able to afford 3 children. Then the whole space and children discussion kicks in again.)

Basement yes or no also sounds so casually loose. How about the finances? If that's not a problem anyway, I would always build it.

Whether the garage is north or south ... unfortunately, the site plan cannot be seen clearly. How much space is there still north of your building plot? And maybe the property can be measured ... so total width/length.

I strongly estimate about 15m x 15m = 225m².

The plot does not look square, only the building envelope does.

Whether garage north or south ...

It seems that building rights for "ancillary facilities: garage, carport, parking space" are planned both to the north and to the south. To the north, I estimate 3m, to the south (dimensioned) six halves, so also 3m, thus a total width of 15m, with a building plot 9m wide and 10.5m deep.