Out of the box REVIT just doesn't make it easy to create stepped footings.
We need to:
1. Create some Reference Planes
2. Model the stepped footing using an In-Place family
3. Split the walls
4. Apply the Concrete Material and then Join Geometry
Below are 5 CADclips embedded in a single viewer on how to create the stepped footing you see above. This is good for both for REVIT Architecture or Structure.