You have an existing single (1-way) light switch and you want to add a second switch position somewhere else to control the same light. Common situations: adding a switch at the top of stairs so you can turn the landing light on and off from both top and bottom; adding a second switch at the opposite end of a long room or open-plan space; or putting a bedside switch in addition to the one by the bedroom door.

This is a retrofit. You keep the existing switch position, replace the 1-way switch with a 2-way switch, and run a new cable between it and a second 2-way switch in the new position. This guide covers the standard UK retrofit method using 3-core and earth cable between the switches, in accordance with BS 7671:2018 (the IET Wiring Regulations, 18th Edition). If you are wiring a 2-way circuit completely from scratch (new install rather than retrofit), see our How to wire a 2-way light switch (UK) guide instead.

The existing 1-way switch must be replaced. A 1-way switch has only two terminals (typically marked COM and L1, or just “1” and “2”). To support 2-way switching, both positions need a switch with three terminals: COM, L1, L2.

Notifiable work: adding a switch position by extending an existing lighting circuit is generally not notifiable under Part P of the UK Building Regulations, provided you are not modifying the consumer unit and the work is not in a special location such as a bathroom or kitchen. If in doubt, use a Part P registered electrician.

Before you start: safety

1. Isolate the lighting circuit at the consumer unit and verify dead at the existing switch using a GS38-compliant two-pole voltage tester. Prove-test-prove on a known live source.
2. Lock or label the consumer unit so the circuit cannot be re-energised while you are working.
3. Non-brown conductors used as strappers, switched live, or permanent live must be identified with brown sleeving or tape at every termination (BS 7671 Table 51).
4. Metal-faceplate switches must be earthed via the earth terminal on the switch plate, not just the back box.

Full safe-isolation detail is in the 2-way light switch guide.

What you will need

• Two 2-way switches – one to replace the existing 1-way switch, one for the new position
• 3-core and earth cable for the new run between the two switches (1.0 mm² or 1.5 mm² to match the existing circuit)
• Brown identification sleeving or PVC tape for the black and grey strappers
• Green and yellow earth sleeving
• Wago lever connector or approved screw terminal block for jumpering the two browns inside the existing back box
• 35 mm deep back box at the new switch position (the existing back box should already be 35 mm or deeper; check before you start)
• GS38-compliant two-pole voltage tester
• Insulated terminal screwdriver, side cutters, wire strippers
• Wall-chasing tool, cable clips, capping, or surface-mount trunking depending on how you are routing the new cable

How the conversion works

In a standard UK loop-at-the-rose lighting circuit, the existing 1-way switch has a single twin and earth cable running up to the ceiling rose. The brown core of that cable brings permanent live down to the switch; the blue core (sleeved brown at the terminals) takes switched live back up to the rose and on to the lamp.

To convert to 2-way switching without touching the ceiling rose, the existing switch position becomes the load end of the 2-way chain (Switch 2). Inside that back box, the brown of the existing twin-and-earth is jumpered to the brown of a new 3-core and earth cable, which carries permanent live across to the second switch position. The blue (switched live) of the existing twin-and-earth lands on the COM of the new 2-way switch. The black and grey cores of the new 3-core (both sleeved brown) act as the two strappers between the switches.

At the new switch position, the brown of the new 3-core lands on COM (permanent live in); black and grey on L1 and L2. The ceiling rose wiring is completely unchanged.

Wiring diagram (UK retrofit)

The diagram shows the standard UK retrofit arrangement after conversion. The existing twin and earth between the ceiling rose and the existing switch position is unchanged – brown carries permanent live down, blue (sleeved brown) carries switched live back up. The existing 1-way switch has been replaced with a 2-way switch and now sits at the load end of the chain (Switch 2). Inside its back box, the brown of the existing cable is jumpered with a Wago connector to the brown of the new 3-core and earth cable that runs to the new switch position. Switched live (blue of existing cable, sleeved brown) lands on Switch 2 COM. Black and grey of the new 3-core, both sleeved brown per BS 7671 Table 51, run as strappers between L1 and L2 of the two switches. At the new switch position, the brown of the new 3-core lands on COM, black on L1, grey on L2. Earth is bonded to both back boxes and the light fitting. Neutral remains at the ceiling rose only and does not enter either switch – standard UK loop-in topology.

Step-by-step procedure

1. Isolate the lighting circuit at the consumer unit and prove dead at the existing switch.
2. Remove the existing 1-way switch. Photograph the wiring first and note which conductor was on which terminal before disconnecting anything.
3. Route the new 3-core and earth cable from the existing switch position to where the new switch will go. Chase into the wall plaster, route through the ceiling void or floor void, or use surface-mount trunking if the cable run is exposed.
4. Fit a 35 mm deep back box at the new switch position, with the new cable routed in.
5. At the existing switch position (becomes Switch 2, load end), connect:
   • Blue of existing twin-and-earth (sleeved brown for switched live) → COM
   • Brown of existing twin-and-earth (permanent live) and brown of new 3-core both into a Wago lever connector or approved screw terminal block – this jumpers permanent live across to the new switch position
   • Black of new 3-core (sleeved brown at termination) → L1
   • Grey of new 3-core (sleeved brown at termination) → L2
   • All earth conductors joined at the back box earth terminal, sleeved green/yellow where bare
6. At the new switch position (Switch 1, supply end), connect:
   • Brown of new 3-core (permanent live) → COM
   • Black of new 3-core (sleeved brown) → L1
   • Grey of new 3-core (sleeved brown) → L2
   • Earth conductor → back box earth terminal
7. Confirm brown sleeving on every black and grey conductor at every termination. The brown of the existing T+E that goes into the Wago does not need additional sleeving – it is already brown.
8. Gentle pull-test every termination. Loose conductors cause arcing and intermittent faults.
9. Reassemble both switches. Tuck conductors carefully into each back box; make sure no insulation is pinched under the screw clamps and that the Wago connector sits flat in the back box.
10. Restore power and test that the light can be switched on and off from both switch positions in any combination of switch states.

Common pitfalls

Trying to reuse the existing 1-way switch in the new position. A 1-way switch only has two terminals; you cannot wire 2-way switching with it. Both positions need a true 2-way switch (COM, L1, L2).

Connecting the existing brown directly to a switch terminal at the existing position. The brown of the existing T+E must be jumpered to the brown of the new 3-core inside the back box – not landed on a switch terminal at this end. Landing it on COM or L1 here breaks the circuit.

Using twin and earth instead of 3-core between the switches. Twin and earth has only two cores – enough for the two strappers but with no third core to carry permanent live across to the new switch position. Twin and earth only works if you have a separate route to bring permanent live to the new switch position, which is rare in domestic UK lighting circuits. Use 3-core and earth.

Forgetting to sleeve the black and grey strappers brown. BS 7671 Table 51 requires non-brown conductors carrying line voltage to be identified with brown sleeving at every termination.

Doing anything at the ceiling rose. You do not need to touch the rose wiring – it stays exactly as it was before the conversion.

Adding a dimmer at the same time

You can install a 2-way dimmer at the same time as the conversion – just substitute a 2-way dimmer for one of the two 2-way switches. Standard UK rule: only one dimmer per circuit. The dimmer typically replaces the new switch (Switch 1) since you can choose the back box depth more freely at a new position; some dimmers require a specific end-of-circuit installation, check the manufacturer’s instructions.

Smart control after conversion

Once converted to 2-way, the circuit also supports the SM323 smart dimmer range. Substitute the smart dimmer for one of the 2-way switches; the other position stays as a standard 2-way switch. The smart dimmer controls dimming level; toggling the standard switch turns the light on or off at the last set brightness.

• SM323 Zigbee Dimmer – Zigbee mesh; Home Assistant, Philips Hue Bridge, SmartThings, Hubitat
• SM323-WF WiFi Dimmer – direct WiFi; Tuya / Smart Life, Alexa, Google Home
• SM323-MT Matter Dimmer – Matter-over-Thread or WiFi; Apple Home, Alexa, Google Home

All three are no-neutral dimmers – suitable for UK switch positions where the neutral wire is not available at the back box (the standard UK loop-at-the-rose topology, which is exactly the situation this conversion guide assumes).