Wow. What's your budget? I'm hoping for something touching 6 figures. That's getting into some SERIOUS engineering. The "going under" the existing floor is what is going to kill this financially and structurally. The floor structure you would have to be rebuilt to handle the weight of tile AND BE thin enough to handle a void (space where the teak floor has to go when bath is in use) is going to take some EXTREME effort. I mean high end structural engineering tricks to get this to work.

I'm thinking "metal deck" for the tiles with metal trusses/rails all sitting ABOVE the floor level you have right now. You would have to lift UP the entire bathroom floor by a few inches (4-6 inches???), or you would have to DIG DOWN a few inches (into the lower level ceiling????). Somehow you would then get the ceramic tile to sit on that metal deck (not easy...most tile setters would leave before you finished your sentence). The metal trusses/rails would be strong enough to hold the metal deck+tiles "up" so that you have a void where the teak floor would sit. Whew! How much do you have to spend? How much of a 'step up' into the bathroom do you want? A step up like that is usually very dangerous for the elderly.

Serious investment. Serious. It would be cheaper to completely reconfigure a new bathroom - including new plumbing runs, etc. A newly created bath would probably be cheaper - and safer.

Thank you for thinking that through. We are adding sq. ft to accommodate the below floor space needed for a shallow soaking tub....level entry shower. I have seen this done and it is like a pocket door on its side. Will post again if we actually get it to work....again thanks for your wisdom.

Did you ever get this to work economically? It is a great idea! I would love to try it in a new build if workable within a budget.