LDK stands for dining room, kitchen, living room. 3LDK means 3 rooms and 1 LDK area.
The S or WIC stands for various “storage rooms,” “service rooms,” “free rooms,” and usually refers to a small area that functions as a walk-in closet. The defined measurements for the S room are not standardized, so the storage space is large enough for a small spare bedroom. In the case of 3SLDK may actually represent 4LDK that allows S to be in another room after moving in.
Tatami(帖 or J)
The size of a room is usually measured by the number of tatami mats on the floor of the room. 帖(JOU) is referred to simply as ‘J’ and a standard sized bedroom in Japan is about 6J
The size of one tatami mat is about 180 cm x 90 cm, about 1.62 square meters.