A room for changing clothes and preparing, often in a theater, gym, or traditional setting.
The actors changed into their costumes in the dressing room.
楽屋 is a dressing room specifically for performers in a theater or backstage area, while 支度部屋 is a more general term for a room used for preparation and changing.
更衣室 is a general changing room, often in public facilities like pools or gyms, whereas 支度部屋 implies a room for preparation beyond just changing, such as putting on makeup or arranging equipment.
Compound of 支度 (したく, 'preparation') and 部屋 (へや, 'room').