The freestanding residential building in the former Rheingasse now houses the museum library with the collections of the Historical Society and the museum depot. Built in the second half of the 18th century as part of a larger farmstead, it came into the possession of the Baltic Baron Gustav Johann von Mengden in 1836, who also acquired properties within the Imperial Palace. In 1855, he had to sell the entire area due to financial difficulties. The buyers were the Dutch consul general John David von Harder and his wife Natalie, née von Stieglitz from St. Petersburg. Natalie von Harder is considered the founder of the Ludwigsstift due to her financial and organizational support, which served as a home for the elderly, the needy, and children until 1909.
After changing owners, Karl Mett bought the estate and had it remodeled in 1919. During this process, the entrance was moved from the middle of the street wall to the southern narrow side. The house is an example of the regionally typical late Baroque architectural style.
