The Old Custom House is the oldest surviving government building in California, built in 1827 by Mexican authorities in Monterey, then the capital of Alta California. The former custom house is the first designated California Historical Landmark, marking the site where U.S. Commodore John Drake Sloat raised the American flag and declared California pa…The Old Custom House is the oldest surviving government building in California, built in 1827 by Mexican authorities in Monterey, then the capital of Alta California. The former custom house is the first designated California Historical Landmark, marking the site where U.S. Commodore John Drake Sloat raised the American flag and declared California part of the United States in 1846 during the American Conquest of California.