The main difference is the size and scale. A home addition is the construction of a completely new section onto your home (either to the ground level or adding a level) to add significant square footage, usually in the form of full rooms. A bump-out is a smaller addition that extends from a single existing room to expand floor area but is not a completely new room or wing. Unlike additions that can be multi-level, bump-outs are typically done room by room on the ground level only.