What is an ideal plot size?

230 euros per sqm? Then the plot of land almost costs 200,000 euros :-O. The idea with the building body is a good one, I’ll practice sketching a bit.

It’s not expensive land on the edge of the Ruhr area or near Düsseldorf. When I think that recently a (commercial) plot of land in the middle of Düsseldorf was sold for 35,000 €/sqm

That's right, that must also be taken into account. No neighbors have built there yet, it's still pretty empty there. However, a neighboring plot has already been sold.

And how good/bad was the decision with the 830 m²? How large is your building volume?

We are more than satisfied, as it is now (relatively) spacious. We have 11.4x11.4m exterior dimensions, plus 70 sqm driveway, but no garage (didn't fit due to the building window).

You have to pay attention to the aspect ratios. If the depth is given, for example, and you can vary the width, then it doesn't make sense to say 500 sqm; with an unknown depth of 35 meters, you get almost 14 meters. You probably also have to keep 3 meters to the neighbor. If you take a house with about ± 10 meters length/width and an attached double garage, you're at about 19-22 meters. If you have to keep 5 meters to the street, and the terrace is also 5 meters deep, you are quite well off with at least 28 meters depth, with 7 meters of lawn depth and 1 meter of hedge. Anything more than that is not necessarily needed, costs more and more work, but it would also always be a bit more airy towards the neighborhood or street, so not necessarily wrong either.

In my opinion, it strongly depends on the property. We have almost 900 but! - 150 of those are a driveway and we have 6 neighbors - so I am happy for every meter of distance. If the driveway comes from the south and you absolutely want a south-facing garden, the property has to be a bit larger, because you have to drive past the house, for example, etc.

I would prefer 4000 m² in the city center with pasture for sheep and chickens and a medical center and shopping mall as neighbors.

Yes, the location of the property is important, the necessary size would result from that if it were not predetermined. For example, we only have 439m², but towards the street we have used the required 3m, the full width of 22m (including a clearance area of 3m). However, we only have neighbors on the left and right; on the side away from the street, nothing will ever be built.