A note on engraving walls between rooms: it only increases the value on the side of the wall that the engraver has engraved, and he will not engrave both sides of a wall. Watch where your engraver goes. Whatever side of the wall he goes to in response to the engrave order is the side that is engraved.
This is incorrect, and hints at a failure in understanding the underlying mechanics of the game. Tiles do not have 'sides'. The entire tile is the wall and the entire tile is engraved.
Any tile or building on a tile, such as installed furniture, contributes a portion of its value to all the rooms that occupy that tile. The value contributed is the total value of the tile and it's building divided by the total number of rooms that occupy that tile.
So, depending on orientation and the disposition of surrounding rooms, an engraved bedroom will typically contribute somewhere between a quarter and a half of its value to any one bedroom.
This I have tested and confirmed. If you disagree, you are free to outdo me by posting some screenshots, doing some math, and showing your work.
What I'm wondering, though, is if statues prevent the value of the engraved floor tile on which they sit from being added to the room value. They count as walls for the sake of room-making, so they
can prevent the value of engraved wall tiles from being added to the room, depending on placement and room design.