The ex-Warenhuis, built in 1919, is one of the remained heritage buildings in the city center of Medan, North Sumatra, Indonesia. It has been abandoned for years and experiencing deterioration; therefore, Medan Municipality Government is conducting an investigation and planning to adapt and reuse the building. The study aims to find opportunities and new functions which able to enhance the economic value of the building by carrying out a field survey to collect the data. Noteworthy that it considers creating new functions while advancing social development and environmental sustainability. In addition, the adaptation of this building continues to prioritize the aspects of authenticity and the principle of minimal intervention. This study performs the highest and best use assessment which consists of five stages, namely 1) Location Analysis, 2) Market Analysis, 3) Combination of stages 1 and 2, 4) Financial Eligibility, and 5) Best Alternative Recommendation to obtain new alternative functions that are following market trends and locations. The result of the highest and best uses analysis highlights the highest and best use as an adaptation of the ex-Warenhuis function as a commercial building consisting of multi purposes such as hall, retail, working space, restaurant, café, gallery.