Click "Customize-Interior Modeling 2.0" and select the room to enter the modeling interface. The Offset tool is to make a copy of a 2D shape at some distance in its parallel direction. Here is the instruction:
1 Select "Offset" from the top toolbar and click on the side you want to offset.
2 Then drag the mouse to determine the offset distance and click again to complete the offset (or enter a value in the "Offset-Distance" field below to determine the offset distance or hit the "Enter" key to finish offsetting).