Can I change a single socket to a double socket myself?

The one accessory swap that genuinely sits at owner level, the back box depth problem that stops most attempts, and the three findings that mean you must stop and call someone.

Difficulty: intermediate
Time: 45 min
Read: 5 min
Safety: restricted

Short answer

Yes, in an ordinary room this is a like-for-like accessory replacement and is not notifiable in England and Wales. The obstacles are practical: the existing back box is usually 25mm and too shallow, so it has to be swapped for a 35mm one, which means chasing masonry. If you find aluminium cable, old rubber insulation, no earth conductor, or scorching, stop.

Of all the electrical jobs householders attempt, this is the one that is genuinely on the right side of the line — provided you can isolate properly and provided the box behind is deep enough. The failures are nearly all mechanical rather than electrical: cables crushed into a shallow box, a box not fixed square, or terminals not tightened.

Safety

Isolate the circuit at the consumer unit, lock or tape the MCB off, tell everyone in the house, and prove the socket dead with an approved two-pole voltage indicator before removing it — not with a non-contact pen, which can read dead on a live circuit. If you are not equipped to prove dead properly, this job is not yours.

Step by step

Confirm the location is ordinary.Not a bathroom, not a shower room, not outdoors, not a kitchen position within 300mm of a sink where the accessory would be inappropriate. If it is any of those, stop — that is notifiable or prohibited work.
Identify and isolate the circuit.Find which MCB feeds it, switch it off, and lock it off or tape it with a note. Tell the household. Then prove the socket is dead at its terminals with an approved two-pole voltage indicator using the prove-test-prove sequence on a known live source before and after.
Photograph the existing connections before disturbing anything.Which cores are in which terminals, how many cables, and whether the earth cores are sleeved. Take more photos than you think you need.
Assess the back box depth.A single socket usually sits in a 25mm box. A double socket with two cables needs 35mm to fold the conductors without crushing them. Forcing a double into a 25mm box is the single most common failure of this job and it damages insulation.
Stop if you find any of three things.Cables with rubber or fabric insulation that crumbles when moved; a circuit with no separate earth conductor; or scorching, melted insulation or a smell at the terminals. Any of these and you refit what you can, leave the circuit off and call an electrician.
Enlarge the recess and fit the deeper box if masonry.Mark round the new box, chase with a bolster or a multi-tool, remove the old box, feed the cables through a grommet in the knockout, and fix the new box level and flush with the finished plaster. A grommet is not optional — a raw metal knockout edge will cut the cable sheath.
Use a dry-lining box if the wall is plasterboard.A dry-lining box with fold-out lugs clamps to the board. It must be the right depth and it must be fitted squarely, because it takes the strain of every plug that will ever be pushed in.
Terminate carefully.Every earth core sleeved green-and-yellow, cores cut to length so they fold without strain, correct core in each terminal, no copper visible outside the terminal, and every screw properly tight including any unused terminal. Loose terminals are what cause the hot sockets described elsewhere in this cluster.
Reassemble, restore and check.Screw the faceplate without trapping any conductor, restore the MCB, and confirm the socket works and the RCD still holds. Press the RCD test button afterwards to confirm the protection is functioning.
Record what you did.Note the date and the change with your house papers. If you are ever asked at an EICR or a sale, having a record is far better than not.

Tips

Buy the deeper back box before you start. Discovering mid-job that you need one, with the circuit off and the wall open, is the classic Sunday afternoon.
A socket with three cables in it is usually a junction point on the ring plus a spur — take extra photographs and be careful to reproduce exactly what was there.
Modern sockets with integral USB take up more depth behind the faceplate than a plain one, so the box depth question gets harder, not easier.

Common mistakes

Proving dead with a non-contact voltage pen — Pens give false negatives through screening, insulation and low battery, and false positives from adjacent cables. Only a two-pole approved voltage indicator, proved on a known source before and after, tells you anything you can rely on.
Cramming a double socket into a 25mm box — The conductors are crushed against the back of the accessory, insulation is damaged where you cannot see it, and the faceplate stands proud so it flexes every time a plug goes in.
Leaving an unused terminal screw loose — A loose screw in a terminal that carries no conductor is harmless; a loose screw on a conductor you thought was in a different terminal is not. Tighten every screw and tug-test every core.

If it doesn't work

Faceplate will not sit flush

Cause: Box too shallow, cables not folded neatly, or box not fitted square — Fix: Do not force the screws. Isolate again, fit a deeper box or redress the conductors.

Socket works but RCD trips when something is plugged in

Cause: Conductor damaged during installation, or a core trapped against the box — Fix: Isolate and inspect. If nothing is obvious, get it tested — a nicked cable inside a box is a real and progressive fault.

Only one half of the double socket works

Cause: A conductor not properly in its terminal on one side — Fix: Isolate, prove dead, and recheck every termination. Never diagnose this with the circuit live.

Old cable insulation cracks as you move it

Cause: Rubber or early PVC insulation at end of life — Fix: Stop. Do not disturb it further. Leave the circuit off and get an electrician to assess the installation.

Questions people ask

Do I need to notify anyone?

In England and Wales, replacing an accessory like for like on an existing circuit in an ordinary room is not notifiable. In a bathroom or outdoors it is. Scotland and Northern Ireland have their own rules and are worth checking before you start.

Can I fit a surface-mounted pattress instead of chasing the wall?

Yes, and it is a legitimate answer when a deeper flush box is impractical. A surface pattress over the existing box gives you the depth without chasing. It stands proud of the wall, which some people mind and some do not.

Does it matter which brand of socket?

Buy from a reputable supplier and check it is marked BS 1363. Counterfeit accessories with undersized terminals and poor contact material are a real problem in online marketplaces, and this is not a place to save two pounds.