Wait a minute. Where do you have Wet UFH?
It does have an impact if you are overboarding the ply and need to build up the height of joining floors. For example, a Foam tile backer board would be unsuitable and instead a cement board like Hardiebacker would be OK on top of wet UFH
Ofc, its a problem for the other room. Thats not being tiled until the extension is built. Either way the ply will need another layer of something so I'll end up with a height difference. Can't lower this floor without rebuilding the sleepers.
Reply to the thread, titled "Suspended Timber Floor Tile Prep" which is posted in Canada Tile Advice on Tilers Forums.