I always thought it to be impossible, but maybe I am simply not aware of how to do it...

Let's say I've got a great lot in the game, but the ground is horribly uneven. I want to flatten it, yet it doesn't work, because you can only flatten the ground on which nothing is built on (with the flattening tool). Is there a way, short of destroying the whole building and re-building it anew?

First, backup in case this doesn't work. Then, type in the cheat:
boolprop constrainFloorElevation false

Now you should be able to flatten the ground underneath the building using the flatten tool.

Be sure to turn CFE back on when you're done.

I'm assuming you'd have to use the CFE cheat (boolprop constrainfloorelevation true/false) but unfortunately I'm not very familiar with how this particular build cheat works.

Thanx, I'm going to try and flatten that lot once I start up the game again. If it works, it will be a dream come true

We're assuming that you are just trying to flatten the ground under the building, and not the lot edges. You need a special program to flatten the edges.

Hello there! This might not be the right place to ask this, but I need a quick response! Thing is I just downloaded a pretty house from here and I'd like to live there, but do I have to buy that house in game in order to use it? I thought it would be mine already...It's so expensive!I'm sorry I'm new :P

Well, it's got nothing to do with flattening the ground, so no, this isn't the right place at all. And I take it you're new to the game as well as the site from the nature of the question. All the lots are "yours," in the sense of "you the player," but you've got multiple playable sims (even if you only ever choose to play one), so how could the sim live in it if he doesn't move in, which automatically entails buying it? You have a couple of options: either go into the house in build mode and take out or replace furniture, wallpaper, etc., until it's affordable, which presumably will make it less pretty, or use a cheat to give your sim enough money to buy it.

You probably need to read the gameplay guides and the brochures that came with your games before you try to play seriously; and I recommend a period of experimentation. And lurk on the boards for awhile so you can have better net manners next time.

Thanx, the CFE cheat has perfectly worked!

Another question, not related to flattening the ground, but related to the CFE cheat:

Is it possible to use it to elevate or lower the ground? Since the name suggests that there are no "elevation constraints", I thought that maybe somehow you can mold the ground fully to your liking this way... What I mean is molding it with the lot still being placed on it, of course, as it's perfectly possible to elevate or lower the ground otherwise.

For example, if I have a beach lot with the building that gets flooded because of land being a little low, what am I supposed to do after activating this cheat to elevate the building a little? If that's possible, of course, maybe I'm dreaming, and this cheat isn't so useful

Yes, that's what it can do precisely. However, if the land-raising you do under the lot is not even, it will distort the floors of the house above. Do not panic though because with the cheat turned on, you can go up a level and flatten the floors again (you'd need to do this for every level going upwards).

You need to take care with the cheat as it can cause warping problems (I'd suggest a neighbourhood back up first) and what I mean by that is don't do anything over complicated with it until you are more confident about how it works. Especially, try to avoid situations where you need to correct mistakes or use the back arrow. However, it is also the main building cheat that made possible such lots as these:

http://www.modthesims.info/download.php?t=172468
http://www.modthesims.info/download.php?t=228145
http://www.modthesims.info/download.php?t=249000

Looking at those again reminds me that Vindicare was a kick ass builder.

So I flatten the land under my house, wich works. However all my walls were a floor higer and all the objects were half in the floor, I fix all of it. But now it's full of bugs. I have the house I wanted but she can't do a lot, out of nowhere she can't do the actions, start to wave and complain she can't get there. Seems like I have indeed some warping problems, any idea how to fix it?

@ Kellay : Once when using the 'boolprop_constrainfloorelevation_false' cheat, I had some tiles from a bridge I was trying to build get stuck in mid-air.
(This was before I downloaded the 'nuke objects' tool to get rid of stuck items.)
So I exited the lot, moved my sim back into the family bin; and bulldozed the entire lot.
Then I reloaded the lot/house.

If that doesn't work; try to delete and fix one wall or area at a time, then test your sim on it to see if she can access the areas she couldn't before.
good luck !

I know this is late, but here's a solution to the floating stuff.
First ofcourse use the method above, and then when the stuff is floating, simply delete them all with that hammer tool, and then press undo. Vola!

CFE can be tricky, and while leveling a house can work it can also mess it up,especially if the builder used CFE on the house itself. For example if this house had split level stairs you could have messed them up. Sometimes you can be left with angled floating tiles and no amount of hammering with the hammer tool will budge them. If you have messed up the lot badly I would just bulldoze it and use the house how it was intended to be used.

I once used CFE to edit the Curious house in my noob days.
Not a good idea. The stairs led up to nothing since I dug so low!!

Well, there's also the GridAdjuster. You can ask to move everything up so many clicks by choosing the "Relative" option (move things relative to their current position, rather than move to a specific elevation). You'll still have to pick up and let go of all objects to convince them to go to the correct elevation.

I've tried it but it doesn't seem to work. Not in Life Stories anyway. I'll check at some point to see if it works in the Sims 2 or any of it's expansion packs.

Oh silly me. I used "boolprop constrainfloorelevation true" instead of "false". :P

I've just tried it on the Curious family household and the cheat has worked beautifully. It's nice knowing that I could adjust the terrain to street level without having to rebuild the house. Sure, I still had to remove the staircases and add them back, but that was easy, so it was all worth it. Now the Curious family can continue to live on the property that they know and love, without having to frown upon the uneven terrain surface.