elevated enclosed porch

I currently have a split-level house with a elevated wood deck (about 5' off ground) with a walk-out from the back. I would like to enclose a portion of the deck into a 3-season porch. I plan to attach it to the house and to the existing roof. I plan to include 3 walls of windows approximately 8'x8' for the total area. Can I just build up off of the existing deck, or should I sink new 4x4 footings? Should I rip out the existing deck for this area and treat it as a new structure? Do I have to worry about the pressure this will add to the side or roof of the house?

Any suggestions or plans would be helpful. Thanks.