@ Dogger: I do not want to participate in the discussion about which installation method and direction and which visual effect are considered positive or negative. But regarding your comment "Problem: straight expansion joint across the third-bond pattern of the tiles." The drawing you inserted elsewhere is quite expressive! I assume that the blue line represents a fixed wall, while the red lines represent the joints considered problematic. Regarding this: If the blue line is indeed an intermediate wall, then (because it is a heated screed) a movement and field delimitation joint (in the ceramic covering) must be inserted in the extension. Very likely there will also be a movement joint in the screed, which is functional and must be preserved. You should quickly abandon any thoughts about working with silicone and other things, as this is neither proper nor professional. All other joints, shown with red lines, are very likely "fear joints" only in the ceramic top layer. These will also not be needed in the future! That means: only one single movement joint is necessary, which must be taken over in the screed and at the same place in the top layer and must under no circumstances be filled or closed!! Good luck: KlaRa