You can extend the exterior face of a wall down by editing the wall’s properties. You may, for example, want to extend the veneer down on a composite wall to cover the exposed floor structure below.
To extend the exterior face of a wall:
Select the wall whose properties you want to change. You can select multiple walls using Shift+click if you want.
Right-click and select Properties, or select Edit > Modify Elements > Properties.
In the Walls dialog, select the Basic tab.
Edit the value in the Drop Exterior Face edit box. The value you enter is the distance that the exterior wall face will be extended down below floor level (or the terrain if the wall is a retaining wall).
Click OK.