Most floors and walls are flat surfaces, so it’s easy to model them with straight edges and flat faces in SketchUp. In fact, chances are good that the first thing you ever model in SketchUp looks a lot like the floor and walls of a building.
Most people want to create two kinds of architectural models in SketchUp; how you approach modeling floors and walls depends entirely on the type of model you’re making:
Exterior: An exterior model of a building is basically just an empty shell; you don’t have interior walls, rooms, or furniture to worry about. This type of model is a slightly simpler proposition for folks who are new to SketchUp.
Interior: An interior model of a building is significantly more complicated than an exterior-only one; dealing with interior wall thicknesses, floor heights, ceilings, and furnishings involves a lot more modeling prowess.
Here’s the thing: Because everything in SketchUp is made of super-flat faces (they have no thickness), the only way to model a wall that’s, say, 8 inches thick is to use two faces side by side and 8 inches apart. For models in which you need to show wall thicknesses — namely, interior models — you have to use this two-face approach.
Exterior models are easier to make because you can use single faces to represent walls.
One of the biggest mistakes new SketchUp users make is attempting an inside-outside model right off the bat. Making a model that shows both the interior and the exterior of a building at the same time is, to be honest, way too hard when you’re just getting started.
Instead, build two separate models if you need both interior and exterior views. If you need a combination model later on, you can build it in a quarter of the time it took you to build either of the first two.