Thank you very much for the new input! Yes, we are unfortunately aware that we also lose living space because of the walls. The 5.7 m is probably not necessarily the maximum possible there; as mentioned, it was just a simple drawing to see where the house could be placed on the property and where the driveway could be.
The Tiny House is an interesting suggestion, but it doesn't seem to be very cost-effective regarding the price per square meter ;-)
From everything I have read here so far, most opinions lean towards the idea that a second house on this property doesn't make much sense. We will probably continue to look for our own property or a finished house first :)