We have exactly this (at the moment, but will be knocking the chimney out at some point). A basic timber frame (finished with plasterboard) goes across the chimney breast, supporting a canopy cooker hood. Low profile rectangular plastic ducting runs between the kichen ceiling and the upper floorboards, all the way across the kitchen, to the outside. A fairly powerful extractor fan is necessary because of this relatively long distance (up 1m then across 3.5m).