DUNBAR
You can do this, but;
If you only build the platform just for tub, you make the step out of tub very dangerous.
Also, it will definitely lurk in your mind and anyone else that sees the tub that someone tried to avoid the concrete floor.
If you tie into that drain line, you have to independently vent that shower to avoid wet-venting.
It actually is so much easier just to bust up floor and properly install shower. If this is a home-improvement project, this type of install will not add equity to your home, but take away from it.
