Create panel > Geometry button > Extended Primitives > Object Type rollout > L-Ext button
Create menu > Extended Primitives > L-Extrustion
Use L-Ext to create an extruded L-shaped object.
Corners—Draws the object from corner to corner. You can change the center location by moving the mouse.

Side/Front Length—Specify the lengths of each "leg" of the L.
Side/Front Width—Specify the widths of each "leg" of the L.
Height—Specifies the height of the object.
Side/Front Segs—Specify the number of segments for a specific "leg" of the object.
Width/Height Segs—Specify the number of segments for the overall width and height.
Note: The object's dimensions (Back, Side, Front) are labeled as though it were created in the Top or Perspective viewports, and seen from the front in world space.
Generate Mapping Coords—Sets up the required coordinates for applying mapped materials to the object. Default=on.
Real-World Map Size—Controls the scaling method used for texture mapped materials that are applied to the object. The scaling values are controlled by the Use Real-World Scale settings found in the applied material's Coordinates rollout. Default=off.