Ongoing climate change is a significant threat to coastal communities. To understand potential risks during extreme storm events, detailed post-overtopping processes are investigated using DualSPHysics and SWASH with a newly developed approach. It is a calibrated-based wave generation: a target incident wave is first obtained from the validated SWASH model, and DualSPHysics creates the target incident wave by adjusting the offshore wave and bathymetry conditions. This one-way coupling process makes the DualSPHysics computation efficient enough to apply 3D simulation. With a vertical wall at the end of a room located at the end of the promenade in a mild and shallow foreshore, the present model shows a good correspondence on the wave force with the literature. After confirming the efficiency and accuracy of the present model, the 3D simulation with furniture inside the room was conducted and visualized with the state-of-the-art visualization technique. Based on the visualization, the potential risks during the extreme storm event are further discussed in this paper. The present work shows a further capability of DualSPHysics to deal with wave–object–structure interaction based on the latest developments in an efficient way. The developed model can be further used to understand the potential risks of ongoing climate change.