Hi, I have read many of the replies and think many of the so-called electricians are useless tradesmen. The cable should go down the cavity wall of the extension. This is the easiest and best means of getting the cable down to the outlet switch. If you need to, you padsaw a hole in the plasterboard above the switch and or in the ceiling, keeping the cutouts to place back in position after fishing the cable down. A couple of battens fixed back in the holes allows you to screw the cutout boards back in and a little filling plaster finishes it off.