Thank you Matthew,
I understand and did all this but it doesn't work. A simple rectangular hole drawn into the mesh works, but a s soon as I try to add another rectangular hole or to modify the first hole the strangest elevations occur with one or more corners extruding much higher (or lower) into space. An attempt to give these corners new numerical heights also failed and besides... that can't really be it.
Maybe it's a memory problem? I also figured I might have some mesh points within the hole that could create chaos...
The landscape was created with polylines along the height lines of the plan and then stamped points on it with the spacebar which were all given the same height. This building goes over one of these polylines, the other building I shifted between two of those lines and the problem dissappeared... Strange stuff...
I'm nostalgic about Radar, 4.5 which I "played" like a piano, all with shortcuts (partly modified with ResEdit at the time...), and stuff like this I did with roofs or library elements. Never any problem and very fast. Now it all became so extremely complex. Too many options and functions. Cheers!