What deck? What home? Pictures may help you get advice.

I interviewed three contractors. Our deck was very old and needed to be replaced. The existing supports would not have passed code inspections for a new deck, let alone a screened porch. Chances are you need a permit, so probably not a DIY project. ETA. Ours was a raised deck/now porch because house is basically a 1960 ranch built on a slope with a walk out basement. The porch is about 10 feet above grade on one side and lessens to about two feet at the top of the slope.