A method for the modular production of motor-vehicle structures for two different segments or classes of motor vehicles, for obtaining structures of different dimensions in the transversal direction, wherein for each motor-vehicle structure an underbody structure is provided including a floor structure and two side members, which are welded to two longitudinal lateral edges of the floor structure, said method being characterized in that: - said floor structure is in common and suitable for producing both motor-vehicle structures belonging to a first lower segment of motor-vehicles, and motor-vehicle structures belonging to a second upper segment of motor-vehicles, wherein said lower segment is characterized by a smaller dimension of the motor-vehicle at least in a transversal direction, - in order to form motor-vehicle structures of the first segment and of the second segment, side members are used which are different one from each other always in combination with said common floor structure, - in such a way that the space defined between the side members above said floor structure is considerably increased in the case of said motor-vehicle structure of the upper segment compared to the motor-vehicle structure of the lower segment, despite the use of a common floor structure, - each side member comprises an inner element (19, 20) of side member, welded to a respective longitudinal lateral edge of the floor structure, and an outer element (23, 24) of side member, welded to the inner element (19, 20) of side member, - said elements (19, 20, 23, 24) of side member have cross-sections each defining a vertically oriented omega shape, with cavities facing each other and upper flanges (F1, G1) and lower flanges (F2, G2) welded together, - the two inner elements of side member used for forming motor-vehicle structures belonging to the lower segment each have a substantially vertical intermediate wall and an upper wall and a lower wall which are slightly inclined with respect to a horizontal direction, one upwardly and the other downwardly, in the direction of the upper vertical flange and of the lower vertical flange; - the two inner elements of side member used for forming motor-vehicle structures belonging to the upper segment also each have an intermediate wall and an upper wall and a lower wall which are slightly inclined with respect to a horizontal direction, one upwards and the other downwards, in the direction of the upper vertical flange and of the bottom vertical flange, said intermediate wall having a lower portion which is substantially vertical and an upper portion which at least for a part is markedly inclined outwardly in the direction of said upper wall, said upper portion of the intermediate wall occupying the greater part of the vertical extension of said intermediate wall, - each lateral edge of the floor structure has for a prevalent portion of its length a flange vertically bent upwardly, welded to the intermediate wall of the respective inner element (19, 20) of side member. - said floor structure is in common and suitable for producing both motor-vehicle structures belonging to a first lower segment of motor-vehicles, and motor-vehicle structures belonging to a second upper segment of motor-vehicles, wherein said lower segment is characterized by a smaller dimension of the motor-vehicle at least in a transversal direction, - in order to form motor-vehicle structures of the first segment and of the second segment, side members are used which are different one from each other always in combination with said common floor structure, - in such a way that the space defined between the side members above said floor structure is considerably increased in the case of said motor-vehicle structure of the upper segment compared to the motor-vehicle structure of the lower segment, despite the use of a common floor structure, - each side member comprises an inner element (19, 20) of side member, welded to a respective longitudinal lateral edge of the floor structure, and an outer element (23, 24) of side member, welded to the inner element (19, 20) of side member, - said elements (19, 20, 23, 24) of side member have cross-sections each defining a vertically oriented omega shape, with cavities facing each other and upper flanges (F1, G1) and lower flanges (F2, G2) welded together, - the two inner elements of side member used for forming motor-vehicle structures belonging to the lower segment each have a substantially vertical intermediate wall and an upper wall and a lower wall which are slightly inclined with respect to a horizontal direction, one upwardly and the other downwardly, in the direction of the upper vertical flange and of the lower vertical flange; - the two inner elements of side member used for forming motor-vehicle structures belonging to the upper segment also each have an intermediate wall and an upper wall and a lower wall which are slightly inclined with respect to a horizontal direction, one upwards and the other downwards, in the direction of the upper vertical flange and of the bottom vertical flange, said intermediate wall having a lower portion which is substantially vertical and an upper portion which at least for a part is markedly inclined outwardly in the direction of said upper wall, said upper portion of the intermediate wall occupying the greater part of the vertical extension of said intermediate wall, - each lateral edge of the floor structure has for a prevalent portion of its length a flange vertically bent upwardly, welded to the intermediate wall of the respective inner element (19, 20) of side member.