This paper presents a new method for synthesizing structural assemblies directly from the design specifications, without going through the two-step process. Given an extended design domain with boundary and loading conditions, the method simultaneously optimizes the topology and geometry of an entire structure and the location and configuration of joints, considering structural performance, manufacturability, and assembleability. As a relaxation of our previous work utilizing a beam-based ground structure, this paper presents a new formulation in a continuum design domain, which enhances the ability to represent complex structural geometry observed in real-world products. A multiobjective genetic algorithm is used to obtain Pareto optimal solutions that exhibit trade-offs among stiffness, weight, manufacturability, and assembleability. Case studies with a cantilever and a simplified automotive floor frame under multiple loadings are examined to show the effectiveness of the proposed method. Representative designs are selected from the Pareto front and trade-offs among the multiple criteria are discussed.