Adding Drain for a Wet Room

I'm looking at my downstairs bathroom on my plans. We decided to put in an extra shower. Its now coming back to haunt me. The room doesn't look big enough for a shower tray, even size 700 or 800, and a sink. I'm wondering though how hard would it be to add a drain for a wet room? The finished floor and underfloor heating have both been put down so is there a risk of damaging them?

I'm now at the stage where I'm at second fix electricity. The shower trays and that will need to go in for tiling soon.

Would it be that much work to add a drain?