The first Custom House, where documents are brought to custom officials by shipping agents, was built by the side of the Thames in the 14th century. After the Great Fire of London, Sir Christopher Wren, built a new Custom House. Finished in 1671, the main office ran for nearly the entire length of the building. It was called the Long Room and it eventually became the tradition to call all clerical offices in custom houses throughout Britain by this name. In 1715 Wren's Custom House was destroyed by fire. The replacement Custom House was built to a similar design to Wren's building.
The office is built with brick and stone, and is calculated to stand for ages. The Custom House is 189 feet in length, the centre is 27 feet in depth, and the wings considerably more. The Custom House consists of two floors, in the upper-most of which is a magnificent room, 15 feet high, that runs almost the whole length of the building: this is called the Long Room and here sit the officers of the customs and their numerous clerks.