We built an L shaped pool - didn't have the amount of space you have (couldn't have fit a 20x40) - our 'L' is the sunshelf and is the same width as the actual pool area. Since we had such a small area to work with, we kept the depth of the pool shallow enough that average adults can stand in the deep end... so, sunshelf is 6" deep at the water line (same depth as top step), pool starts at 3'6" and goes to 5'6".
Note: At 5'6" deep I do not even recommend cannon balls...but I can put the basketball hoop out and actually shoot from most of the pool (I'm 5'9" tall) - in the deepest part of the pool I have to jump a little.
The pool planning started out as a 12x25 rectangle and then we added a 12x14 'L' for a sunshelf (shelf is 12x9). The pool actually goes around the shed in our yard. See pictures here:
Pool build - Southwest FL - Tampa - Page 10
..and here:
Trouble Free Pool
We thought about having the sunshelf a different color than the rest of the pool, but, because the first step is at the same level as and connected to the sunshelf, couldn't find a good way to transition from one color to the other. (we would have used PebbleSheen Desert Gold on the shelf and Blue Granite for the pool - this would have given a beach color to the shelf and nice blue for the rest of the pool. Instead we chose White Diamonds with cobalt blue glass bead for the entire pool - because it gave the blue color to the water while keeping light enough color for the shelf.
The 6" depth of the shelf is perfect for us, as the adirondack chairs we planned are right at 6" seat height.
If I had a chance to go back and ask for anything different, I think I would have asked for an additional return on the shelf. The return is only on one side and I always have leaves/bugs stuck in the corner opposite the return. The skimmer is directly across the pool from the shelf, so the additional return would have increased flow to the skimmer as well. This is a minor inconvenience and it only takes a minute to sweep this area with the pool brush. (I push the debris off the shelf where my robot can get it - 6" depth is too shallow for the robot to run.)
I am really happy that I asked for the light added to the shelf - the effects of the color LED lights can set the entire 'mood' of the back yard and the shelf would have been dark or barely lit without the light)
I wish you the best in making this pool perfect for you!