For most UK domestic work a 25mm back box takes a standard socket or switch, and 35mm or 47mm is what you fit when the accessory is deeper than the box you were about to buy.
Get the depth wrong and the faceplate will not sit flat. You will either force it, crush the conductors behind it, or drive back to the merchant. We see all three, usually in that order.
Back box, pattress box, knockout box, wall box — same object, different regional habit. Here is how to pick the right one first time.
1. Flush, surface or dry-lining — pick this before the depth
A flush box is sunk into the wall so the faceplate sits against the plaster. Standard for masonry.
A surface box, sometimes called a pattress, sits on top of the wall and the accessory mounts onto it. Right where you cannot or should not chase — solid concrete, listed buildings, garages, or an extra socket without redecorating.
A dry-lining box grips plasterboard with fold-out lugs, so it needs no fixings into anything behind. Right for stud walls and dot-and-dab.
Choose this first, because it decides which product family you are even looking at.
2. The depths, and what each is actually for
16mm — shallow work. Light switches on a solid wall where chasing deeper is awkward, and situations where there is simply nothing behind the plaster to cut into.
25mm — the workhorse. The great majority of domestic sockets and switches are fine in 25mm.
32mm — the common depth for surface boxes, giving the accessory somewhere to sit proud of the wall with room for the tails.
35mm — where the accessory is deeper, or the cable count is higher and 25mm has become a wrestling match.
47mm — dimmers, some smart accessories, cooker units, and anything with electronics behind the plate. If you are fitting a dimmer, start here and be pleasantly surprised, rather than starting at 25mm and being annoyed.
Trade Tip: the depth on the box is the depth of the box, not the space you get. Subtract the cable, the earth terminal, the fixing lugs and the accessory's own body. A 25mm box with three twin-and-earth tails in it has far less than 25mm of usable room, which is exactly why dimmers get their own rule.
Knightsbridge CU1400 — Curved Edge Single Pattress Box, 25mm, White Thermoset Resin, IP20, Pack of 10. The everyday single surface box in the standard depth, curved edges, and sold in tens because nobody ever needs exactly one.
Knightsbridge CU1500 — Double 25mm Pattress Box, Rounded Edges, White Thermoset Resin, IP20, 146mm x 86mm, Pack of 5. The double version, and note the stated 146mm x 86mm footprint — worth checking against your faceplate before you commit, particularly on a tiled or panelled wall.
Knightsbridge CU1610 — Curved Edge Single Pattress Box, 47mm, Earth Terminal, White, IP20, Thermoset Resin, Pack of 10. The deep one, with an earth terminal and cable strain relief built in. This is the box to reach for when there is electronics behind the plate.
3. Metal or plastic?
Plastic is fine for the majority of domestic flush and surface work, it does not need earthing in its own right, and it is cheaper.
Galvanised steel is the choice for anything taking mechanical abuse, for metal-clad accessories, and where an installation specification calls for it. Steel boxes need their earth terminal used, and they need a grommet in every knockout the cable passes through.
Knightsbridge GSK16 — 1-Gang Galvanised Steel Wiring Box, 16mm Depth, Earth Terminal, 73mm x 73mm, Pack of 20. The shallow steel box for switch work, with the earth terminal you are obliged to use. Twenty to a pack, which tells you how often the trade gets through them.
Trade Tip: a steel box without a grommet is a cable stripper with a five-year fuse. The knockout edge is sharp, the cable moves a fraction every time the plate is unscrewed, and eventually the insulation loses. Grommets cost pennies. Fit them.
The full metal back box range runs from that 16mm switch box up to 35mm and 47mm two-gang boxes, and the surface box range covers the plastic side in white, grey, black and chrome finishes.
4. The measuring mistake that costs a second trip
People measure the hole in the wall. The number that matters is the accessory.
Before you buy, turn the faceplate over and look at how far the body projects behind the plate, then add the cable you have to fold in behind it. That total is your minimum depth, and it is almost always more than the wall cavity suggested.
The other one: a surface box and a flush box of the same nominal size are not interchangeable in appearance. Swap a flush socket for a surface one and the plate now stands 32mm off the wall, which looks deliberate in a garage and looks like a mistake in a lounge.
Once the box is in, changing a light switch and deciding whether you need a fused spur are the next two questions, and they have their own guides.
Summary: back box depth quick reference
| Depth | Typical use | Notes |
|---|---|---|
| 16mm | Light switches, shallow chases | Tight for anything with bulk behind the plate |
| 25mm | Most sockets and switches | The default for domestic work |
| 32mm | Surface / pattress boxes | Sits proud of the wall by design |
| 35mm | Deeper accessories, more cables | Buys room without going to 47mm |
| 47mm | Dimmers, smart plates, cooker units | Start here for anything with electronics |
Expert Help in the West Midlands
Our Villa Road counter in Birmingham has been selling back boxes since 1975, and the fastest way to get the right one is to bring the accessory with you. Thirty seconds with the faceplate in hand beats any amount of guessing at depths, and it saves the second trip.
See the full back boxes and pattress boxes range, or browse the wider wiring accessories department for the sockets and plates that go on them.
Back box FAQs
What size back box do I need for a socket?
25mm suits most standard UK sockets. Go to 35mm or 47mm if the accessory body is deep, if several cables are landing in the box, or if there is any electronics behind the plate.
What is the difference between a back box and a pattress box?
In everyday UK usage they are used interchangeably, though "pattress" most often means a surface-mounted box that sits on the wall rather than sunk into it. If someone says pattress, picture the box standing proud.
Do I need a metal back box?
Not for most domestic flush work — plastic is fine. Use steel for metal-clad accessories, where mechanical strength matters, or where the specification calls for it. If you do fit steel, use the earth terminal and grommet every knockout.