It is something that needs to be built for, to my understanding -- to have no curb means that the incline from entrance to drain all needs to occur below floor level, which means that there needs to be space between the subfloor and the joists (or else you're cutting those three inches out of joists, which isn't a good idea). Had you specified this kind of shower from the outset, or just dreamed about it?

That's the million dollar question. So many details. Not sure what was spoken or just planned in my head!

Your builder is correct. What was in the contract? Add about 10K extra to do it correctly at this point. And you need a minimum of 48" to a linear drain, or else the whole room has to be done as a wet room.

elmaka - details should be in the contract, not in your head - for future reference ;)