Decks, Patios, Porches and Docks - Workspace under existing deck

I have a 20 x 20 deck on a house I've bought. I'd like to enclose the underneath as four-season so that I can use it as a workshop. I talked to a deck guy and he is not sure it can be done effectively - attractive walls that I can insulate and won't offend the neighbors, door, window, and the real challenge is how do I deal with rainwater on the deck top that goes through that I need to wick away so it doesn't get into a structure? Essentially I'd like to build a separate space of sorts under an existing deck. Thanks.
Loren

The big box stores have an "under deck" roofing system designed to attach to the bottom side of the deck to dela with the water issue.