Terrain rises when expanding basement. Help, please?

Before you expand the basement you need to lower it. Basements can be lowered independently of the rest of the house. Make sure there is enough room for longer stairs in the basement.
Once you've lowered the basement to ground level or below, you can expand it without affecting the terrain.

@Starfree , nope. Just expanded the basement. The situation was fixed according to @MDianaSims' advice above. :)

Also, one thing thats a carry-over from Sims 3 in some respects is if you have a foundation...

DO NOT EXPAND BASEMENT TO THE FOUNDATION EDGE. This also causes terrain deformation. So, make sure to leave a tile of space between basement wall and foundation edge to account for possible terrain malformation.

I realize this was stated differently earlier, but thought I would contribute by wording it differently.

@MDianaSims, thanks! Going back in to try it out.

@Wndblwz did you put a platform somewhere? I heard that can cause something like that.

"MDianaSims;c-18147419" wrote:
Before you expand the basement you need to lower it. Basements can be lowered independently of the rest of the house. Make sure there is enough room for longer stairs in the basement.
Once you've lowered the basement to ground level or below, you can expand it without affecting the terrain.

This is correct. Click inside your basement and grab the foundation lowering tool and lower it one notch, check to see if that corrects it if not do one more and keep going until your exterior terrain is level.