We often have split level projects and we've had a lot of trouble isolating the stem walls and footings from the rest of the model for use in a foundation plan... that is until Revit 9.0 came along. You can now use Revit Filters to isolate your foundation from the rest of your project.
1. Duplicate your Basic Wall: Foundation - 12" Concrete for each stem wall and footing size you need. You could use a Continuous Footing for your footings but we avoid Continuous Footings because their intersections don't clean up as nicely as Basic Walls.
2. Add the Type Comment "Foundations" for each of your new walls. Go to the wall's properties... Edit/New... Type Comments.
5. In Visibility/Graphics go to Model Categories and uncheck everything except Detail Items and Walls.
You can also use phases or worksets to isolate foundations, but we prefer the filter method.