I looked at handicap accessible showers when I first began building the house because I knew my wife's arthritis was going to get worse as time went by. I wanted to be able to roll a wheel chair into it without getting bumped by a lip or edge and shower curtains have always just bugged me so I thought and thought and decided I could build one specifically to my tastes from tile for the same or less as a prefab one would be. I asked a few people and was told the way they used to do it was with a shower pan, which is basically a thick sheet of rubber like a pool liner that you use to seal the floor and then cement over and finish with tiles. They make a special drain that secures around the hole you need to cut in the rubber water tight and then a second drain screws into it to allow for about an inch or two of cement and tile putting the finished drain flush with the surface.
The house took me seven years to build so by the time I was ready to actually tile the room they had come out with some heated floor systems for warming tile floors and I splurged and went with a single ribbon system instead of the mat system since my floor was oddly shaped.
I got it on EBay and can't remember what I paid but I think it was around $600 for the entire kit including a thermostat that keeps the floor just a few degrees above the room air temp. You can get bigger systems to actually heat the floor but that seemed to expensive and unnecessary in Florida.