A vehicle body lower part structure includes: a floor panel; and a vehicle body frame member including a frame body, an inner side wall, a joint flange projecting inward in a vehicle width direction from an inner surface of the inner side wall in the vehicle width direction, and having a joining surface on which an end of the floor panel in the vehicle width direction is stacked and to which the end of the floor panel in the vehicle width direction is joined, and a recess provided to the frame body, recessed outward in the vehicle width direction from the inner surface of the inner side wall in the vehicle width direction, and having a surface that is continuous and flush with the joining surface of the joint flange.