@Reitanna
If you have a basement, then go to build mode and go down to the basement level. Click on the floor and you should see a double arrow (up/down) somewhere around the middle of the floor along with the arrows to move walls.. Grab that arrow and slowly drag down on it to lower the basement. That should fix your problem. You can flip between the ground level and basement level as you do it so you don't lower the basement any more than you need to.
Given the way it looks in your pic, your basement is probably one tile wider than the ground floor.
Hope this helps.