Needing more room to expand his warehousing operations, Captain Hatt added this two-story building in 1886 to meet growing demand for wine and spirits storage. Its brick facing came from brick obtained from the San Quentin brickyard. The first floor was built as a U.S. Government bonded warehouse for storing wine and spirits. Under Napa Milling's operation, it was used to mix feeds and bag grains milled in Hatt Hall and dropped by gravity into six bagging machines on the first floor, and the last of those bagging machines remains in place. The second floor contains Hatt Hall, which included a large meeting room, library, anteroom, and dining room where many civic and government agencies held meetings and events. Napa Milling used Hatt Hall for millstone and sifting operations. The second floor now accommodates the Hatt Suite and Hatt Hall for large meetings and conferences, while the first floor hosts specialty retail.