There are various circumstances where it would be convenient to add another light to a circuit, and connect that light directly to an existing light switch.
Examples include adding lighting underneath kitchen wall cabinets, where the existing switch will be at the exact height of the new lighting, or installing an outside light by routing the cable directly through the back of an existing switch box to the outside.
With this wiring, it is NOT possible to connect another light at the switch, because there is no neutral available.
1. Replace the existing twin and earth cable between the ceiling rose and switch with a 3 core and earth type. If the existing cable is installed in conduit or a hollow wall, it should be easy to pull a new cable in without damaging the wall.
2. Replace the one gang switch with a two gang switch.
3. Connect a new cable to the switch to provide power to the new light fitting.
The twin and earth switch cable has been replaced with a 3 core and earth cable. At the ceiling rose, Brown connects to Loop as before, Black is now the switched line, and the third grey wire is connected to Neutral. A small piece of blue sleeving indicates it is a neutral. Note that all of the other wires are left exactly as before.
At the switch, black and brown connect to the first switch to control the existing light exactly as before. A small piece of wire connects the two COM terminals together so they are both permanently live.
The grey Neutral core is connected to a separate terminal block. A piece of blue sleeving indicates is is being used as a neutral.
The cable to the new light fitting (marked NEW) contains line, neutral and earth wires. Line connects to L1 on the switch, so that the light can be switched on and off as required. Neutral connects to the separate neutral block. Earth is connected to the earth terminal.
Note that the neutral wires are NOT connected to the switch at all.