The invention relates to a computer-assisted design method for a second three-dimensional object, from a first three-dimensional object known only by its boundary surfaces (through a surface model of the B-Rep type) of the planar, cylindrical, cone, sphere, or toroid type, excluding free-forms in defining the boundary of the object, the comprising the following steps: - breaking the first object down into a set of simple volume primitives, - associating that first object with a construction graph defined from those volume primitives and a set of geometric parameters for using those volume primitives in the specific case of that first object, said construction graph enabling a compact description of all of the construction alternatives of the object through Boolean combinations of those primitives, - creating the second object as an alternative of the first object according to that construction graph, by modifying some of those parameters.