Not hard at all - but it requires some interesting plumbing that means your slab will be cut a bit. If the new island is in the opposite direction of where the plumbing drains now, it is just and extension and a rejiggering of the slope. There is a loop vent instead of a vent through the roof that needs to be accomodated in the island. If the drain for the sink now heads across (to drain) where the new island will be, it could be even simpler. A pipe guy with the right equipment will come out separately and mark out all your subterranean lines. The plumber will know how to read this code and then can make a definite estimate (barring finding something unusual underground). The water service to your sink can be piped over from underground while open. Your slab will be cut open, plumbing reworked, sand fill and then concrete repoured .. . do new flooring at the same time of course, and you will need to pull out the sink cabinet in full to get to the old plumbing behind.