This kind of foundation can be an alternative solution for direct foundations.It solves the problem where oversized ordinary raft foundations are needed. In fact, the box foundation gathers the gravity loads resting on the peripheral walls, so that the global soil reaction reduces its eccentricity. In most cases the elements constituting the box structure,such as peripheral walls, raft, and ground floors, are already part of any actual building; they only need a proper seismic design, which is herein proposed. A simplified theoretical approach is presented to allow a correct proportioning and design of each structural element, such as reinforcing ribs and panels constituting the box.