Let's see an example about the constrainFloorElevation cheat. We are going to build a green-house.

First, let's build a rectangular room (again, I'll use the glass walls, to let you see the ground).

Let's add a tile roof (here I'm using, for testing purpose, a transparent floor tile by RGiles; it can be found at ModTheSims2: http://www.modthesims2.com/showthread.php?t=34012)

Let's add some flowerbed fences...

I've used a regular pattern, but the layout of the fences is irrelevant: it's just for aesthetic purpose.

Now open the cheat box by pressing SHIFT+CTRL+C and enter the cheat:
boolprop constrainFloorElevation false

Then, select the raise terrain tool and start raising the terrain inside the room. In this example, I'm raising the ground by four "clicks". You will notice that the roof raises, according to the ground.

Let's level the central ground tiles, to raise them all to the same height of the first one.

Done!

Let's raise some more the very central tile: 5 more clicks (or any other height you may like...)

Let's level the top tile. Again, notice how the roof has the very same shape of the ground underneath.

Now, we have to deactivate the cheat, by typing in the cheat box:
boolprop constrainFloorElevation true

Then, we can level down the terrain.

As you can see, when the cheat is deactivated, modifying the ground won't modify the roof!

Finished! Add some floor tiles, and a door, and the green-house is usable.