We currently have a 3 season porch (15x8) off of the back of our house. It is supported by concrete/cement posts. The porch is about 4' off the ground. Under the porch is dirt. We want to turn this into a full, weatherized portion of the house (laundry room/bathroom). Essentially the only thing we will keep is the roof (it's new). Everything else will be replaced.
Do I need to pour a foundation? The contractors who have looked at it are saying they will use the existing posts? Is this correct? Do I need to treat the dirt underneath or pour some kind of slab/slury? Also - I'm assuming all pipes will be well insulated and covered from the elements - however, should I also seal off this area (i.e. concrete block walls) around the posts?

Help and Thank You.