A custom house or customs house was a building housing the offices for the government officials who processed the paperwork for the import and export of goods into and out of a country. Customs officials also collected customs duty on imported goods.
The custom house was typically located in a seaport or in a city on a major river with access to the ocean. These cities acted as a port of entry into a country. The government had officials at such locations to collect taxes and regulate commerce.
Due to advances in electronic information systems, the increased volume of international trade and the introduction of air travel, the custom house is now often a historical anachronism. There are many examples of buildings around the world whose former use was as a custom house but that have since been converted for other use, such as museums or civic buildings.
In the United Kingdom since 1386, the phrase custom house has been in use over the term customs house. This was after a "Custom House" was erected at Wool Wharf in Tower Ward, to contain just the officials of the Great Custom on Wool and Woolfells. The singular form was used even though in later years the Custom House was the location of other Customs officials as well.(Wikipedia)
"U.S. Custom House"
"US Custom House"