Method for manufacturing two types of central structural-body understructure parts of two respective types of motor vehicle, said central parts being intended to delimit the lower part of the passenger compartment of each vehicle, each central understructure part comprising a longitudinal structure tunnel, characterized in that it comprises the mutual assembly: - of a floor of the central understructure part, - of a one-piece central profiled section which is attached to the floor, is oriented in a longitudinal direction of the vehicle and has a length dependent on the length of the central understructure part in the longitudinal direction, the central profiled section of the vehicle of the first type having a length that has a first value such that the central understructure part has a length that allows it to receive, on the passenger-compartment side, two seating positions that are offset from one another in a lateral direction of the vehicle, the central understructure part provided with the central profiled section of the vehicle of the first type that has the length corresponding to said first value constituting the first type of central understructure part, the central profiled section of the vehicle of the second type having a length that has a second value such that the central understructure part has a length that allows it to receive, on the passenger-compartment side, four seating positions that are distributed in two rows that are offset from one another in the longitudinal direction and each include two seating positions that are offset from one another in the lateral direction, the central understructure part provided with the central profiled section of the vehicle of the second type that has the length corresponding to said second value constituting the second type of central understructure part, - and at least one end part (14, 15) that is arranged in line with the central profiled section and is configured so as to have an interface (141, 151) for connecting to and providing a function of reacting and transmitting loads with a structural part (100, 200) of the body that is separate from the central understructure part and/or to/with said floor, the end part (14, 15) used in the central understructure part of the first type being identical to the end part (14, 15) used in the central understructure part of the second type, the method comprising, through the use of the identical end part, a step of adapting the length of the central profiled section depending on the length of the central understructure part in the longitudinal direction of the vehicle. - of a floor of the central understructure part, - of a one-piece central profiled section which is attached to the floor, is oriented in a longitudinal direction of the vehicle and has a length dependent on the length of the central understructure part in the longitudinal direction, the central profiled section of the vehicle of the first type having a length that has a first value such that the central understructure part has a length that allows it to receive, on the passenger-compartment side, two seating positions that are offset from one another in a lateral direction of the vehicle, the central understructure part provided with the central profiled section of the vehicle of the first type that has the length corresponding to said first value constituting the first type of central understructure part, the central profiled section of the vehicle of the second type having a length that has a second value such that the central understructure part has a length that allows it to receive, on the passenger-compartment side, four seating positions that are distributed in two rows that are offset from one another in the longitudinal direction and each include two seating positions that are offset from one another in the lateral direction, the central understructure part provided with the central profiled section of the vehicle of the second type that has the length corresponding to said second value constituting the second type of central understructure part, - and at least one end part (14, 15) that is arranged in line with the central profiled section and is configured so as to have an interface (141, 151) for connecting to and providing a function of reacting and transmitting loads with a structural part (100, 200) of the body that is separate from the central understructure part and/or to/with said floor, the end part (14, 15) used in the central understructure part of the first type being identical to the end part (14, 15) used in the central understructure part of the second type, the method comprising, through the use of the identical end part, a step of adapting the length of the central profiled section depending on the length of the central understructure part in the longitudinal direction of the vehicle.