How to Add Floors to Your SketchUp Model
Adding a second (and third, and fourth) floor to your SketchUp model isn’t as hard as it may seem. The key is to think of each level as a separate “tray” consisting of interior walls, a floor surface, and the ceiling of the level below. You model each floor as an individual group, making it easier to hide, edit, and move.
For the same reasons, you also make the exterior walls a separate group; they act kind of like a “box” into which your floor levels stack.
If the edges and faces that make up your exterior walls are not already enclosed in a group by themselves, seriously consider doing that now. If you take the time to group your exterior walls before you add floors to your building, you save hours of headache later.
Otherwise-well-meaning people who have worked with other CAD or 3D modeling programs often take this opportunity to bring layers into the discussion. Yes, SketchUp has a Layers feature. And yes, floor “trays” are a lot like layers, at least conceptually. But you should not use Layers when modeling multiple levels of the same building. Layers in SketchUp simply don’t work the way you might think they do.
Provided your exterior walls are already a group, the next step is to turn the rest of your first floor’s geometry into another group. This is how you do just that:
Select the floor and interior walls of the first level.
You can accomplish this efficiently with the Select tool: Just triple-click a face on any interior wall to select everything that’s attached to it.
Make a group by choosing Edit→Make Group from the menu bar.