To create a basement or below grade walls in drawing.tool follow these steps:
Navigate to settings panel and add a floor
Type in a negative value for the elevation
Draw the geometry on the 2D mode.
Watch this video that walks you through the steps:
Important Notes :
You cannot change Floor 1 to a negative value.
You can select all geometry and copy/paste to a new level by using the copy/paste icons in the settings tab under the floor section.