If you've come to SketchUp without any prior knowledge of 3D CAD, one of the features that will absolutely amaze you is components. When you create a component, you can copy it as many times as you like. These copies are linked to the original component and are called instances. If you change something in the original component, all the instances will also change! Likewise, if you change something in any of the instances, all the others will change too.
Imagine how much difference this single capability will make when modeling your building: