In recent years, the number of exhibits and collections in museums that were built in the past are on the increase. As a consequence museum-related facilities and services expand to meet the demand. In addition, the museum has become a complex cultural space by adding the social communication element to its existing functions of preservation and exhibition due to the change with the times. As one option to solve this issue, the demand for museum extension continues to increase. However, despite the rapid increase in demand, the number of research related to museum expansion is relatively less. Therefore, this study aims to build basic design data for future museum extensions by analyzing the spatial hierarchy, mainly focusing on the type of extension and the space used by visitors for the museums belonging to the National Museum of Korea. For the objective, this study selected the museums for which an actual extension was completed as an analysis group among the museums belonging to the National Museum of Korea. In particular, the scope of analysis was narrowed down to the museums that have an independent extension which is the most typical method of museum extension. This study then performed a quantitative analysis on the accessibility, recognition, and utilization of the space used by visitors before and after the extension using spatial syntax. As a result, the museums belonging to the National Museum of Korea tend to have relatively more public or other spaces for visitors than the exhibition space after independent expansion. This means that the functional change of a museum leads visitors’ movement to public and other spaces after space expansion. Such a change demonstrates that more diverse options can be provided for visitors.