Downdraft ducting route vent

[dannyboy123]

So, I'm 99% sure I'm going for a downdraft extractor in my new kitchen island (the joists above are going the wrong way for a ceiling extractor and there are also new beams installed). Question is when I route the duct work under the new floor I'm guessing it has to then come back up the internal wall and then through to the outside? How high off the floor does the duct need to be for building regs? Is anywhere ok along as its above the damp course?