What Back Box Depth Do I Need? 25mm vs 35mm for Light Switches & Sockets

Most standard toggle light switches and plug sockets fit a 25mm back box. Dimmers, USB sockets and anything with electronics behind the plate usually need a deeper 35mm box. If you're not sure, a 35mm box is the safer choice: everything that fits a 25mm box will also fit a 35mm one, but not the other way round.

Choosing the wrong depth is one of the most common reasons a new switch or socket won't sit flat against the wall. It's an easy mistake to avoid if you check before you order. This guide explains what a back box does, which items need which depth, and what to do if the box in your wall is too shallow.

What is a back box?

A back box is the metal or plastic box set into the wall behind a switch or socket. It holds the wiring and gives the faceplate's screws something to fix into. The depth is measured from the front edge of the box to the back, and in the UK the most common depths are 25mm and 35mm.

The deeper the box, the more space there is for wiring and for the switch mechanism itself. Simple switches need very little room. Dimmers and USB sockets have electronics built in, so they sit further back into the wall.

25mm vs 35mm: what fits where

The exact depth varies by brand and range, so always check the product page. On our switch and socket pages, we list the back box each item needs. For example, in our black walnut range only the dimmers need 35mm, while in our antique brass knurled range most items need 35mm.

Tip for renovations: if you're having new back boxes fitted, ask your electrician to fit 35mm boxes throughout. It costs very little extra and lets you swap to dimmers or USB sockets later without opening up the wall again.

Metal or dry-lining box?

The type of wall decides the type of box:

• Metal flush boxes are set into brick or block walls and plastered in.
• Dry-lining boxes are plastic boxes with clips that grip the back of plasterboard. They're used on stud walls and dot-and-dab walls.

Both types come in 25mm and 35mm depths, so the depth rule above applies either way.

How to check the box you already have

1. Turn off the power at the consumer unit and check the circuit is dead.
2. Unscrew the existing faceplate and ease it forward.
3. Measure from the front edge of the box to the back wall of the box.
4. Count the boxes too: a double socket needs a 2 gang box, and a single switch a 1 gang box.

If you're not comfortable doing this, ask your electrician to check when they quote for the job.

What if my back box is too shallow?

You have three options:

1. Replace the box with a 35mm one. This is the neatest result. In a brick wall it means chasing out a little more masonry; in plasterboard it's usually a quick swap.
2. Fit a box extender or spacer. This brings the faceplate forward slightly from the wall. It works, but the plate won't sit fully flush.
3. Choose a slimmer product. A standard toggle switch instead of a dimmer, for example.

All electrical work in the home must meet UK Building Regulations (Part P), so we always recommend using a qualified electrician.

Frequently asked questions

Can I use a 35mm back box for a normal light switch? Yes. A standard switch works perfectly in a 35mm box; there's just a little more space behind it.

Why does a dimmer need a deeper box? A dimmer has electronic parts behind the plate that take up more room than a simple on/off switch. In a 25mm box it often won't sit flat.

Do USB sockets need a 35mm back box? In most cases, yes. The USB charging module sits behind the socket and needs the extra depth.

What size back box does a double socket need? A 2 gang box. Check the depth on the product page, as it depends on whether the socket has USB or other built-in features.

Do you sell matching back boxes? Yes. Many of our switch and socket ranges offer 25mm and 35mm back boxes as an option on the same page, so you can order everything together.

Shop switches and sockets

Find the right finish for your home in our light switches, dimmer switches, plug sockets and sockets with USB. Each product page tells you the back box depth it needs.

Still not sure? Call us on 0151 707 6858 or email support@lovehandles.uk.com and we'll help you choose.