The Smart City is a possible evolution of each City. With the rapid development of information technologies applied everywhere in city life, cities are becoming smarter and smarter. However, this context has multiple orientations, supported by an architecture of the assistive system and the associated implementation process. In this paper, we propose a design methodology with different approaches and illustrate them by a case study that attempts to transform a coastal city into a Smart Coastal City. All the design methodologies and technologies can be referred in other smart city applications.