walk-in closet; wardrobe room; dressing room; costume room
A room used for storing clothes, changing, or keeping costumes. Can refer to a walk-in closet, a backstage costume room, or a dedicated dressing area.
The new house has a spacious walk-in closet.
I chose a costume in the theater's costume room.
クローゼット is a general closet or wardrobe, while 衣装部屋 specifically refers to a room-sized space for clothing or costumes.
更衣室 is a changing room (e.g., in a gym or pool), whereas 衣装部屋 emphasizes storage and may also be used for changing.
Compound of 衣装 (いしょう, 'clothing, costume') and 部屋 (へや, 'room'). The reading いしょうべや follows standard rendaku voicing of へや to べや in compounds.