Adding floors in SketchUp is straight-forward if the model is organized correctly. We cover all the essential steps in our SketchUp training courses.
It’s best to think of each level or floor as a separate tray made up of interior walls, floor and ceiling of the level immediately below. Each floor is then modeled as a separate group, which makes it easier later to edit or hide or move. The external walls are also created as a separate group, a bit like a box in which the various floors stack as trays.
To incorporate elements into a group use the Selection tool to select the various parts. Triple-click on more complex objects to select all the associated faces and edges, then right-click, choose Edit and Make Group.
To create the next floor slab use the Line tool to trace round the inside perimeter of the walls, then Push/Pull the resultant face into a slab. Then triple click to create a Group. Then draw the interior walls and Push/Pull their height, grouping as usual. Then group the interior walls, floor and the ceiling of the level below together to form another group. Lastly extrude the external walls height upwards to match the new interior walls’ height. In this manner we create the various floors or storeys of the building.
Many useful tips and techniques can be found at the SketchUp website. And see many examples of our own clients’ work on our Facebook page.
Other related SketchUp Blog Posts:

• SketchUp background
• SketchUp introduction
• SketchUp modeling
• SketchUp contours
• SketchUp workspace