10 years ago I put 2 decks on my house and did exactly what you want to do. At that time I bought the software "Punch Ultimate Deck & Landscape" from Punch software. I see there is newer versions still made. Don't think its a download but price seems reasonable. The version I used was fairly simple and was very helpfull in coming up with the best design. And you can use the walk through mode to virtually walk on your new deck. Was impressed how the program veiw looked very much like the deck did when finished. 
There are probably others but this one did what I asked of it.
Good luck.