Basement walls pop out of the ground.
That's not a bug, and it's not related to any specific lot. (Play, this probably isn't your bug which seems like a bug. I could add a basement to the original pre-built house no problem.) It's because of the new terrain tools. I wanted to add a basement to my Vacuous Green house -- because money vault -- and where the basement extended beyond the walls of the floor above, it was flattening the terrain to that height.
Before the terrain editing update, basements would always, and forever, sit beneath the foundation. And the stairs would awkwardly slice through the foundation. Now, since the update, if you start drawing your basement while clicking on the ground floor -- the top of your foundation -- the ceiling of the basement will be set at that height. It won't sit beneath the foundation, the basement will be in the foundation.
And, where the basement level extends beyond the house, it has to flatten the terrain to that height. It has no choice. It
must do that.
You can decide if some more terrain editing to hide the boxy shape is worth it for the big basement, or, you can still draw the basement on the level of the flat lot. And stairs placed from the basement to a room above the foundation will awkwardly slice through the foundation as before, but without disfiguring the flat terrain above.
