You can toggle whether walls are measured from the outer or interior edge in the Dimensions dialog found under the Design tab.
Select a room or house structure and click on Design -> Dimensions to bring up the Dimensions dialog.
The Measure Wall section in the Dimensions dialog controls whether SmartDraw displays the dimensions of a room based on the outer edges or the inner edges of the walls. By default, this is set to From Interior.
If your selection isn't a closed shape or wasn't created with walls, this setting will appear grayed out.
Walls have a thickness. By default, interior wall are set to be 4" thick and exterior walls are set to 6". This a setting you can adjust under Set Wall Thickness in the Adjust Wall area of the SmartPanel (the panel left of your work area).
When you change how to measure walls:
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From Exterior: The dimensions displayed reflect the distance from outer edge to outer edge of the walls. This measures the entire footprint of a room or structure.
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From Interior: The dimensions reflect the distance from inner edge to inner edge. This measures the usable interior space within the walls.
For example, assume your walls are 4" thick and you set Measure Wall to From Exterior, SmartDraw will show a room as 20 feet wide. However, toggling to measure From Interior, you'll see 4" inches subtracted from each side (8" total) to account for the wall thickness on both sides of the room and the same room will show as 19 feet 4 inches wide.
This setting is useful for ensuring your floor plan displays the right kind of measurements depending on whether you're planning a construction (exterior) remodel or layout/interior design (interior).