One consideration needs to be given to seepage which will take place along the joint. You may want to insert some sealing device like copper seal, rubber seal, epoxy etc to check the seepage.
I agree w/ Ishavaag. Avoid, avoid, avoid. If the expansion joint is designed to move (both structures are independent) then the cost would be enormous with all of the flexible pipe couples, etc. This does not even acount for the wear and tear of the movement on the fixtures and floor tiles. Tell the architect to move the room.
If it is possible to avoid having an expansion joint under a ceramic tile floor, then by all means do so. However, that is not always possible, particularly at doorways and passage ways between additions. It should also be noted that slab control joints pose the same problem as well as the situation where ceramic tile is laid on a topping for precast slabs. Buildings move and provisions MUST be made for that movement not only in the structure, but the finishes too!
The standard specs. put out by the ceramic tile industry call for expansion provisions in ceramic tile floors. The use of soft joints is a standard practice. A good, flexible, traffic grade sealant is more water tight than the concrete grout normally used, because it doesn't crack. Think about it, one of the most obvious ways you know that at building has moved is to see cracks in the interior finish materials.
