On your lot use either the basement room tool to create a basement or navigate to the basement level with the page down button and use the standard.
Sims 4 add another floor without removing roof.
How to remove basement ceilings in sims 4 1.
Building another floor if that is not enough for you or you just want to build your house upwards instead of sideways you can build another and then yet another floor.
Hit page up on your keyboard to move up to the second floor view then build the walls floors etc from there.
Zoom out the camera select the removing tool hammer from the upper panel and click on the roof.
Here are a few tips to help you graduate beyond just making a box with windows.
To add a flat roof to a building you will need to add an empty room on the upper level.
This room needs to be the shape and size of the desired roof so you may need to combine several empty rooms together.
To that end you should first remove the roof.
Hey in this sims 4 video you see how to delete walls floor doors and how you can remove the ground.