Is this no longer possible? I used to have a stone oven with a wall in front of it that disappeared so it looked as if the stone oven was in the wall. Looked very nice, but after I customized something in my house it disappeared and I can't get it the way I had it before. Also noticed that I was no longer able to place a ladder in a spot that was fine before.
Haven't played for a a year or so...
If you mean this:
It may be still possible. This is a 2 tile add on.
Remove the walls around and above the oven.
Place a couple gozas where you want the oven to place and raise them at least 2 tiles.
Place the oven on the gozas and lower one of the gozas until the oven "breaks" (You will see what I mean when it happens.)
Lock down
both of the oven pieces. You will get a locked down message when you do this.
Go back into custom mode and replace the walls you removed. While in custom mode, the oven should still be visible as you placed it.
Commit your house design and head back to the oven with your deco tool.
Now you should be able to lower the oven section back down to line up with the other.
Once the pieces are lined up and the oven becomes whole again, you can not raise or lower each piece separately. So be sure you have one piece of the oven lowered or raised where you like. Then adjust the height of the 2nd piece to align.
It has been years since I built this home. So if you try this, let me know if it still works. Good luck!