n.a building that is subordinate to and separate from a main building
英汉双解大词典
outbuilding/ˈaʊtˌbɪldɪŋ/(outbuildings)
1.
N-COUNT（可数名词） Outbuildings are small buildings for keeping things in or working in which are near to a house or main building but separate from it. (储存物品或作为工作场所的)外屋 [usu pl]