There are two ways to build a bump-out addition. The difference shapes the project. Cost, timeline, permitting, and how invasive it is.
A cantilevered bump-out extends the room outward without a new foundation. The new floor is supported by extending and reinforcing the existing joists. The new space hangs off the side of the house. This works for projections up to about three or four feet. No excavation. No new footings. The rest of the house stays untouched.
A foundation-supported bump-out is what you need when the projection is larger than the joists can carry. It requires footings, a foundation wall, and a new floor system. It's a bigger project. Closer in scope to a small addition. Still meaningfully less invasive than expanding the full house.
We walk through this decision early. Most kitchen and bath bump-outs are cantilevered. Most family room and bedroom extensions need a foundation. The right answer depends on the room, the depth you need, and what's underneath.