I want to change the sheet flooring in a 1/2 bathroom for ceramic tiles. I know I will have to strenthen the floor with an other layer of plywood (floor creaks now) 1/2 or 5/8 ?
What goes on top of the plywood before ceramic tiles ? With the extra thickness of plywood & ceramic tiles I guess there are extenders for the toilet connection ?
Any help will be greatly appreciated.

Several folks can tell you about options for laying the tile but we need to know what you have for a sub floor. Big difference between 1/2" and 5/8" and is it tongue and groove or not, 3/4" is the norm. Explore that and come back to tell us. If your sub is not correct your tiling job will fail prematurely.

thank you for the reply. Floor is 4.5 x 3.5 ft. 1/2" plywood nailed to joists.

Clearly not enough for you to tile, is it tomgue and groove?

no. because it is so small, it is just one piece of plywood.
Thanks for answering.

My bad, of course it just one piece, what was I thinking. Since it makes noise already it may not be glued and nailed or screwed, I'd remove it and install 3/4 ply glued and screwed to the joists. You can then install Hardieborad or any similar product which is also screwed and glued according to that manufacturers instructions. You do need to take up the toilet and we can deal with that gap at the toilet drain later.

Thank you sir. That is a great start.