Bring all planes in mesh to same level I have added a plane in top view. I have then extruded the plane according to a floorplan in topview.

I don't know what happened, but when I switched my view, I could see that some parts of it are for some reason high up in the air.

I would like to ask how I can bring everything back down onto the grid / floor.

And if somebody could tell me how this might have happened, I would also be glad.
Switch to object mode using the tab key, then press SZ 0 to scale the model to a uniform value on the z axis.

The result is that the selected region will be scaled to the median of all the selected vertices, effectively flattening the selection.