A motor vehicle chassis including a rear central cross member, a hollow beam positioned over the rear central cross member along this rear central cross member, and a floor seat-cushion support part including rear seat fittings, supported at the rear by the hollow beam positioned under the seat-cushion support part. The beam includes a rear raiser including a horizontal plate fixed to the rear central cross member and further includes a blanking sheet fixed to the horizontal plate of the rear raiser and including at least one anchoring reinforcement assembled on the inside of the hollow beam. The reinforcement, which forms a hollow box section, connects the rear raiser and the blanking sheet, an upper face of the anchoring reinforcement supports a connecting piece, the connecting piece also being assembled, on the outside of the hollow beam, with the rear raiser.