Headquarters House / Flagstaff House [1846- ]
Flagstaff House in Hong Kong Park Central is the oldest existing Western building in Hong Kong. Built in 1846 for Major General George Charles D'Aguilar, it was formerly known as Headquarters House. The building was renamed Flagstaff House around 1932 and remained the office and residence of the Commander of British Forces until 1978. It now houses the Museum of Tea Ware displaying Chinese teaware and related material of different periods. (from AMO website)
There is some debate over whether it's the oldest. The chapel in the colonial graveyard may be older