A raft is as the name suggests, designed to float on the ground. It's not just about case of settling, but regular bobbing up and down over the seasons.As a lay man, my question is why isn't it best practice to match the foundation types... So in this case, build the extension on a raft so that they all float together. Seems like a recipe for trouble doing anything else?
Two rafts, especially of different sizes, loads and locations, will move differently not as one. So you're no better off matching the foundation type.