How to raise the ground / lower an object so that it does not hover?

when using the rake use the right mouse button to raise the terrain level.

Quote from WickedSchnitzel

I know that. But it never raises it so that it touches the object, the terrain always stays lower so that there's a gap and the object is floating.

It's floating in your screenshot because the terrain under the back half of the smelter isn't level

It's not as simple as raise or lower the ground.

You need to start from the higher point with right mouse button towards the lower ground. That will raise it.

Same if you want to lower the ground, start from the lower one with left mouse button towards the higher

I would simply pick up the furnace and reset it (long press F and select pick up)

or just make a large block ( your choice of texture) and just slid it in underneath also