This chapter outlines some of the fundamental stages of the design process specific to Modular Floating Structures (MFS), which encompass sustainable methodology and spatial optimization in open water conditions. MFS are multibody configurations consisting of several floating modules designed to provide a sustainable urban development space in the adjacent marine environment to coastal cities, which suffer from land scarcity. All modules considered herein are assumed to be rigid elements that comply with the acceptable box dimensions in the shipbuilding industry. They are arranged in proximity, moored on the seabed, and may or may not be interconnected. The specific configuration of each module depends largely on its urban function, which determines both the shape and geometry of the superstructure and the main particulars of the substructure, including the global hydrodynamic characteristics that influence human comfort. The design process of a MFS starts on a module level and continues to a multi-body arrangement in accordance with zoning, metocean conditions, and other physical and spatial considerations.