(n.) A building behind the main building. Specifically: A privy; a necessary.

WC archives armory arsenal attic backhouse bank base baseboard basis bathroom bay bearing wall bed bedding bedrock bin bonded warehouse bookcase bottom box bunker buttery can cargo dock cellar cellarage chassis chest closet coal bin comfort station conservatory convenience crapper crate crib cupboard cyclone cellar dado depository depot dock drawer dump earth closet exchequer floor flooring fond foot footing foundation frame fundament fundamental glory hole godown ground grounds groundwork hardpan head hold hole hutch john johnny johnny house keel latrine lavatory library locker lumber room lumberyard magasin magazine mopboard nadir necessary outhouse pavement potato cellar powder room principle privy rack radical repertory repository reservoir rest room rick riprap rock bottom rudiment seat shelf shoemold sill sole solid ground solid rock stack stack room stereobate stock room storage store storehouse storeroom storm cellar stylobate subbasement substratum substruction substructure supply base supply depot tank terra firma toe toilet toilet room treasure house treasure room treasury underbuilding undercarriage undergirding underpinning understruction understructure urinal vat vault wainscot warehouse washroom water closet wine cellar