We closed in a concrete porch. It was a concrete pad that had a footer and block foundation. The concrete pad was in contact with the ground. The walls and new roof were put up, and electrical outlets were installed. The windows were put in before the outlets. Then insulation was put in and drywall hung. The interior flooring and trim work was last. We needed a vapor barrier down on the concrete and a foam underlayment on top of that. The floor had to be leveled with floor leveler in a couple of spots. We started with an even and dry surface. The vapor barrier and foam were needed to protect the floor from condensation moisture that could be caused by the concrete pad and to eliminate any noise of the flooring.
It turned out really nice. The foam underlayment also acts as a nice insulator. Concrete can feel really cold. I was concerned about it feeling really cold in the winter even on the hardwood floating floor above the concrete. It actually feels nice. We had a large patio out back that we took some of the space of and made a bathroom. A window was framed out to door size and a large bathroom was made that took up half the patio. For that renovation we installed heated flooring over concrete and below ceramic tile. It is really toasty in the winter.
There are so many cool things you can do with flooring nowadays. Did you know that they even make non-organic flooring that can be used in basements and other areas where moisture could be a problem? In fact, they have entire wall, floor and ceiling systems to be able to finish basements. The technology even controls water from formerly wet basements. It is amazing how far construction materials have come since I was a kid.