You've already learnt that you don't need to project the shadows of every element forming the building, or every plane that might catch a shadow. Before you use the tool make a new tag to ultimately be an isolated set of geometry with the rest of the model 'off' - probably making two to reflect the original and modified forms. then make a group that is assigned the appropriate tag. Edit it and add simple faced 'box' shapes that follow the main external form of the building [used as a temporary underlay]. Only add in any wall, floor or roof faces etc, that will cast useful shadows. You can probably ignore window-openings for a study this simple. Do not include any 'rear' facing elements that won't cast meaningful shadows. Do not include any wholly internal faces as they won't cast any useful shadows. It should be relatively straightforward for you to work out which parts are needed - but the algorithm will look at everything. Every superfluous bit of geometry which might not cast any meaningful shadows should be avoided - because each one will still be pointlessly considered and processed before finally being ignored - all of this only bloats the processing time exponentially and can lead to crashes as SketchUp becomes overwhelmed. Perhaps an uploaded screen shot of the form, or even a simple skp showing what you've done so far, could elicit more useful advice...