Ever since the patch that introduced the ability to alter the terrain was released, it is now affecting basements on houses with heightened foundations. As soon as you move the basement walls in-line with the ground floor's walls, the terrain raises around the area.
Last night I finished a house on a 20x20 lot with raised foundations and I created a few basement levels to increase the house size and I had to reduce the room sizes all because of it raising the terrain. So instead of the backyard having a nice flat surface, some of it is raised.
This is extremely annoying so please fix it. One of my other households that's on raised foundations with basement levels was lucky it doesn't have this problem because the lot was done before this terrain addition.