This room needs to be the shape and size of the desired roof so you may need to combine several empty rooms together.
Sims 4 add another floor without removing roof.
Once there is a room on the upper level you can add floor covering and a wall trim to the outer edge.
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.
Here are a few tips to help you graduate beyond just making a box with windows.
Hey in this sims 4 video you see how to delete walls floor doors and how you can remove the ground.
I hope this short tutorial can help you if yes you ca.
Zoom out the camera select the removing tool hammer from the upper panel and click on the roof.
To add a flat roof to a building you will need to add an empty room on the upper level.
Hit page up on your keyboard to move up to the second floor view then build the walls floors etc from there.
Although the sims 4 has made building houses better than ever it s still a bit tricky to make your dream home.