Replace a two-way light switch

Turn off the power, swap the switch terminals like-for-like, then test both switches operate the light safely.

LS-01Buy the matching switchTake the old switch or a photo of it to the shop so the new one matches: same plate size, same number of gangs, and 2-way not 1-way. Check the amp rating on the old switch (usually 10A or 20A for lighting) and match it. If either switch is a dimmer, make sure it is rated for 2-way use — not all are.

1 each · 2-way light switch (matching plate)

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LS-02Gather tools and protect the areaLay a dust sheet under the switch and set out your tools within reach: insulated screwdrivers, a voltage tester, and a torch if the room is dim. Working with the power off, good light matters more than usual. Have a pen and paper or your phone ready to note down or photograph the wiring before you disturb it.

1 each · Dust sheet

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1 set · Insulated screwdriver set

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1 each · Torch

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LS-03Isolate the circuit and prove it's deadTurn off the lighting circuit at the consumer unit, not just the switch on the wall — the switch itself can be live even when it's off. Prove the switch is dead with your voltage tester before touching any wires, checking your tester works on a known live source first and again after testing. Cables run in places you don't expect, so even though this is a straightforward switch swap, never assume — test it yourself.

RULES

KNOW THE RULES

Replacing a light switch like-for-like is not notifiable work under Part P of the Building Regulations. It must still meet the UK Wiring Regulations (BS 7671). If the switch is inside a bathroom or near a bath or shower, that's a special location under Part P — get a qualified electrician instead.

LS-04Remove the old switch and note the wiringUndo the two screws holding the faceplate and gently pull it away from the box. Photograph the terminals before disconnecting anything — note which wire sits in COM, L1 and L2 on each switch, since 2-way wiring depends on getting this right. Loosen each terminal screw and free the wires one at a time, keeping them separate so they don't touch.

LS-05Check the wall type behind the boxIf the existing box is loose, damaged, or you're fitting a deeper one, you'll need to work out what's behind the plasterboard or plaster before drilling new fixing holes. Run a stud detector or a magnet over the wall to find timber studs or metal stud tracks — a line of regularly spaced screw heads under a magnet usually marks a metal stud. Tap either side of the box: a dull thud means a solid stud or masonry behind, a hollow sound means you're between studs. A small pilot hole tells you for certain — fine timber dust means a wood stud, grey metal swarf means a steel one, and if the drill just breaks through plaster and keeps going, you're likely on lath and plaster or a stud bay.

1 each · Stud detector

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LS-06Secure or replace the mounting box for your wall typeOn solid brick or block, drill a 6mm hole 40mm deep, fit a wall plug, and use a 32mm No.8 wood screw. On plasterboard with a TIMBER stud behind, screw straight into the centre of the stud with a wood screw — never a plastic plug in bare plasterboard, it will pull out. On plasterboard with a METAL stud, use a self-drilling metal-stud fixing or a hollow-wall toggle rated for steel — an ordinary wood screw or plastic plug strips straight through 0.5mm steel and won't hold. On dot-and-dab plasterboard, the box needs a fixing long enough to reach the masonry behind the adhesive dabs — typically 60-75mm — or a cavity fixing rated for the combined depth. On lath and plaster, find a lath or the timber behind it with a bradawl first, then use a short screw into solid timber; the surrounding plaster is brittle, so don't overtighten. A metal stud fixing alone is fine here since a light switch is light, but it will not hold anything heavier added later without a noggin behind it.

1 pack of 10 · Wall plugs (red, 6mm)

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1 pack of 10 · No.8 wood screws 32mm

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4 each · Self-drilling metal-stud fixings

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4 each · Cavity toggle fixings

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1 pack of 4 · Long fixing screws 75mm (dot-and-dab)

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1 each · New mounting box (35mm)

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LS-07Wire the new switchMatch each wire to the same terminal it came from on the old switch — COM to COM, L1 to L1, L2 to L2 — using your photo as a reference. Strip back only as much insulation as needed so no bare copper shows outside the terminal, and tighten each screw firmly. Give each wire a gentle tug to check it's held before tucking them back into the box.

1 roll · Insulation tape

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LS-08Fix the faceplate and tidy the boxFold the wires neatly into the box so nothing is squashed or strained, then offer the switch faceplate up and screw it to the box lugs. Don't overtighten — cracked plastic plates are a common mistake. Check the plate sits flush and level against the wall.

LS-09Restore power and testSwitch the lighting circuit back on at the consumer unit. Operate both 2-way switches in turn and check the light responds correctly from each one. If nothing happens, or the light flickers or trips the circuit, turn the power off again at the consumer unit and recheck your terminal connections against your photo.

LS-10Tidy upFold away the dust sheet, sweep up any dust or debris, and check no offcuts of wire or screws are left on the floor. Dispose of the old switch responsibly — many DIY shops and recycling centres accept small electrical items. Keep any spare fixings and the switch packaging in case you need the model details later.

KNOW THE RULES

Before you start: Which electrical jobs can I do myself? Part P explained

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