windowless storage room; non-habitable room; non-conforming bedroom
In Japanese real estate, a room that legally cannot be called a bedroom because it lacks a window or other building-code requirements. Often used as a storage room, walk-in closet, or small study. The term is wasei-eigo; English 'service room' does not carry this meaning.
wasei-eigo (Japanese-coined English), from service room
The floor plan of this apartment includes a windowless service room.
Because a service room is not recognized as a bedroom, the rent can be cheaper.