Here are step-by-step instructions that replicate what's driving me nuts.
1) In a NEW project work space, select the WALL tool (any one will do) and draw a rectangle in roughly the ratio of the width twice the height.
2) Using the SELECT tool, select the top wall to have the outer and inner vertical dimensions displayed.
3) Click on the outer vertical dimension on the left side of the drawing and enter " 10' " in the Edit Dimension box and either press the Enter key or click the OK button.
4) Select the vertical wall on the left side of the drawing to reveal the horizontal dimensions and click the bottom outer dimension then enter " 20' " in the Edit Dimension box and press Enter.
AT THIS POINT YOU SHOULD HAVE A STRUCTURE THAT SHOWS OUTER DIMENSIONS OF 10' (vertical) BY 20' (horizontal).
5) Select the WALL tool and draw a vertical wall near the lower left corner of the structure that protrudes from the lower, long horizontal wall about 1/3 the length of the 10' wall. Click and continue drawing a section of wall toward the left edge of the screen making it turn 90 degrees and stop making its length end approximately even with the 10' side of the structure. (You should have something that looks similar to Example A.)
6) Select the vertical wall segment below the 20' horizontal wall to reveal the dimensions and click on the dimension for the distance between the end of the rectangular structure and the edge of the vertical wall segment and enter " 3' " in the Edit Dimensions box and click OK.
7) Select the bottom horizontal wall segment to reveal the dimensions and click on the dimension for the distance between the bottom of the rectangular structure and the bottom horizontal wall segment and enter " 3' " in the Edit Dimensions box and click OK.

Select the 3' vertical wall segment to reveal the horizontal dimensions and click on the dimension showing the distance between the end of the bottom horizontal wall and the left edge of the vertical wall segment and enter
" 3' " in the Edit Dimensions box and click OK.
OBSERVE THAT THE 3' VERTICAL WALL SEGMENT MOVES AND THE LEFT END OF THE HORIZONTAL 3' WALL SEGMENT STAYS IN PLACE.