Once the site plan is complete with camera views for elevations and perspective scenes, its ready to start modeling the building.
Working with your client you can setup the floor plan, doing this either on paper or in a blank SketchUp file. This is to get the general layout of the building and location of each room. Then refine it on your own and clean it up.
Next go into your site plan and create a large plane (enough to fit all the setback lines within) and start sectioning the rooms off of that with a single line to divide them and adding no detail to it.
Make sure the building is within the setback and property lines and make the thickness of the floor. To set the thickness of the floor you want to find the complete width of the supports of the floor not including the finish, such as wood or tiling.
If the terrain is a lot higher in one area than another for a room you can select that separately and lift or lower it accordingly.
Once you have your outline of the building you want to extract that and put it into another file to work within. If not the layers could get mixed and messed up. Then bring the building model back into the terrain file as a component and just update it when needed.
When copying it out you must copy the property line with it and copy from the origin and paste to the origin in the new file.