My neighbor had the same problem, and he decided to creat a "step-up" shower stall. I'm pretty sure the way they did this was that they positioned the drain on a metal box, and then the drain pipe so that it slanted downwards to hook into the main sewage pipe. Once in position, they poured concrete around the drain and pipe (over to the inside wall of the shower). Their only problem was the pipe that ran across the original bathroom floor towards the back, but he built a small cabinet over it for extra bathroom storage. I'm not sure if he needed a builder's permit, or an inspection, but I doubt if he bothered with them, because from what he told us, the shower works just fine.
I hope this helps, or maybe it will give you enough information to find a different way to add the shower.
Moe
I hope this helps, or maybe it will give you enough information to find a different way to add the shower.
Moe