Traditionally the dressing room in stately homes and grand house was attached to the master bedroom and for the man of the house. Women had their own rooms, boudoirs.
A dressing room typically contained storage for clothes, both hanging and folded, shoes, and a single bed, a day bed for occasional use and afternoon naps.
A dressing room in estate agent parlance may describe rather fancifully a walk in wardrobe or a large cupboard, or a small room dedicated to clothes storage, or more realistically a dedicated room, part of a bedroom suite, or complex of rooms that include gym bathrooms bedrooms etc.