If you are drawing an extension plan you might find it useful to have the existing building shown but not estimated.
The easiest way to do this is to draw the existing structure first and select all of the existing structure by clicking and dragging your mouse around the entire existing structure.
With the existing structure selected go to the Modify & Selection tab and then click Set Selection as Non-Estimated.
You will see a prompt asking "Are you sure you want to set the current selection of items to be non-estimated?"
Your existing structure is now set as non estimated. You can draw the extension on the house and it will be the only construction estimated when you save and import the plan into ex.