The shipbuilding industry in recent times has witnessed a decline in the demand for commercial ships and a sharp increase in the demand for offshore plants. In response to this change in the industry, shipbuilding companies have sought an optimal product mix for producing commercial ships and offshore plants with minimal additional investment in facilities. This study was aimed at optimizing the pre-erection (P. E.) area partition for ship and offshore blocks based on the forecasted product mix of the planning horizon. The offshore P. E. area requires additional investment in facilities because the blocks are usually much heavier than those of ships. For given production schedules, and taking the flexibility of the forecasted change into consideration, we determined the optimal P. E. area partition ratio using the dynamic layout problem methodology. In this study, material handling cost, operating cost, rearrangement cost, and additional P. E. area construction cost were used to determine the optimal division. The optimal division of the P. E. area for offshore plants was then determined for a 10-year planning horizon. The results were applied to a shipyard to evaluate their business feasibility.