Not an alarm, not a camera: the £10 device security experts say every front door should have

The knock comes just as you’ve sat down. Someone shuffles behind the door, the letterbox lifts a fraction, the bell goes again. You freeze in the hallway, squinting at frosted glass that shows nothing but a vague shape. The porch light is on, your phone pings with a “motion detected” alert, and yet you still don’t actually know who is standing 30 centimetres from your lock.

A former burglary detective stands beside you, watching the little drama with professional boredom. He glances at the smart doorbell by the frame, the motion sensor on the wall, the laminated “no cold callers” sticker. Then he points at the dead centre of the door. “How do you look out before you open?” he asks.

You mumble something about checking the app, peering round the curtain, asking who it is. He shrugs. “If I wanted to talk my way in, I’d stand where the camera can’t see me, or get you to come closer to the letterbox. The tech’s fine. But you still end up opening this.” He taps the timber.

From his tool bag he pulls something unremarkable: a short metal tube with glass at both ends. A door viewer. A peephole. About a tenner in most DIY shops. No subscription, no batteries, no Wi‑Fi. He measures, drills a neat tunnel through the door, threads the viewer in, and twists it tight with his fingers.

You lean in. The drab communal landing suddenly stretches into a wide, slightly fish‑eyed panorama. The neighbour’s mat, the lift, the stairwell, all in one glance. The silhouette at your door becomes a clear face, a lanyard, a jacket logo. You step back. He smiles. “That’s the one bit of kit I fit on every front door,” he says. “Not an alarm. Not a camera. Just the thing that decides whether you open at all.”

Why this tiny lens beats another camera

Most burglars still start with a knock. They want to know if anyone’s in, what kind of voice answers, how cautious you sound. Distraction thieves and doorstep scammers rely on something simpler: social pressure. It is surprisingly hard to keep a door closed on a person you can hear but not see.

A door viewer breaks that script. You see them; they can’t see you. No chime, no glow, no delay while an app wakes up. You’re not fumbling for your phone with wet hands or walking away from the door to find it. You simply move your eye 5 centimetres, take in the scene, and decide.

Security pros like it for three quiet reasons:

• It removes guesswork. You’re less likely to open “just in case” it’s a parcel or a neighbour.
• It buys time. You can think and compose yourself before speaking, instead of reacting to a voice right by the letterbox.
• It narrows opportunity. Many doorstep cons rely on momentum – they talk fast, hold something towards you, ask you to “just sign here”. A closed door stops that run‑up.

There’s another layer. The best smart cameras record for after the fact. A door viewer changes what happens in the moment. It’s not dramatic, and no company gets to put a logo on it, but it quietly flips the power balance in your favour.

What professionals notice at a front door

Walk a locksmith or crime prevention officer up to your home and they run an invisible checklist. Lock type. Frame condition. Letterbox position. Side windows. Escape routes. Then, almost always: How does the resident see a caller before opening?

On new-build houses with solid composite doors, and flats opening straight onto a communal corridor, the answer is often: they don’t. No side window, no clear glazing, no line of sight. Just a solid slab between you and whoever is asking to come closer.

This is where a £10 viewer earns its keep. It matters most when:

• Your front door opens straight onto a public or communal area.
• You live alone, or someone vulnerable answers the door.
• You already have a smart bell, but callers keep standing just out of shot.
• You’ve had cold callers, charity collectors or “utility staff” turn up unannounced.

Professionals also like door viewers because they are boring in the best way. They don’t break in a storm, can’t be hacked, don’t need a firmware update, and will still work in ten years’ time if the rest of your gadgets are in a drawer.

“I’ve been to plenty of homes with £300 cameras,” one Met officer said. “And no way for the person inside to quietly have a proper look before they unlock. The criminal only needs you to get this bit wrong once.”

How to choose and fit a £10 door viewer

Fitting a viewer is well within the comfort zone of anyone who can use a drill. If you’d rather not, most locksmiths will add one while servicing your locks for the price of the part and a few extra minutes. Either way, the steps are simple.

1. Choose the right viewer

Look for:

• Wide angle: 160–200° field of view. Narrow viewers show you a tunnel, not the landing.
• Correct door thickness: Most standard units cover 35–55 mm. Thicker composite or insulated doors need an “extra long” model.
• Privacy cover: A tiny sliding shutter on the inside stops anyone using gadgets to peek in.
• Finish that matches your hardware: Brass, chrome, black – purely aesthetic, but it helps it look like it’s always been there.

Spend the extra couple of pounds on a branded or well‑reviewed unit. Cheaper plastic lenses scratch and fog, and you’ll use this multiple times a week.

2. Mark the height properly

Stand where you usually answer the door and note your natural eye level. Measure from the floor up and mark it lightly with a pencil on the inside of the door. If members of the household differ a lot in height, err on the lower side; taller people can bend, small children should not be answering unaided anyway.

Typical heights in UK homes fall between 1.45 m and 1.60 m from the floor. Avoid putting the viewer so high you have to stand on tiptoe – you’ll simply stop using it.

3. Drill, don’t splinter

You’ll need:

• A drill with a wood or metal bit sized to the manufacturer’s instructions (often 12–14 mm)
• A smaller pilot bit
• Masking tape
• A sharp pencil

Steps:

1. Stick a square of masking tape over your pencil mark, inside and out. This reduces chipping.
2. Drill a small pilot hole straight through from the inside to the outside. Keep the drill level.
3. From the outside now, enlarge the hole with the full-size bit until you meet the pilot hole.
4. Repeat from the inside if needed to clean the edges.

Drilling from both sides avoids blowing out the surface – especially important on veneered or uPVC‑faced doors.

4. Fit and test

Unscrew the viewer into its two halves. Push the lens half in from the outside, thread the inside half on, and twist until snug. You can usually tighten the final turn with a coin in the slot provided.

Look through from inside. You should see the whole approach, not just a small patch of corridor. Step back, approach again, and check for any glare from nearby lights. If needed, rotate slightly so the top and bottom of the lens align with the scene, not with any panel pattern on the door.

Common mistakes to avoid:

• Fitting a narrow‑angle viewer “because it was cheaper”
• Forgetting a privacy shutter
• Placing it low for a child – children should never open to unknown callers
• Drilling only from one side and tearing the outer surface

Use it well: habits that shut down doorstep scams

The device is only half the story. What makes a difference is how you use it on an ordinary Tuesday when you’re distracted and the door goes.

Security trainers suggest a simple routine:

1. Pause. Do not rush straight to the lock because the bell sounded urgent.
2. Look first, silently. Use the viewer before you say a word. Let you gather information.
3. Keep the barrier. Even if the caller looks genuine, keep the door locked or on a chain while you talk.
4. Verify, don’t apologise. Genuine staff expect checks. Call their company on a number you find yourself, not one they give you.
5. Say no, then step away. If you’re unsure, you are allowed to decline through the closed door. You don’t owe anyone an explanation on the doorstep.

At night, turn the hallway light down or off behind you before you look out. Back‑lighting makes it harder for your eye to adjust to the darker landing; you’ll see more detail if your own side is dimmer for a second.

Teach the household one shared rule: nobody opens if they can’t first see who it is. For children or anyone vulnerable, go further – nobody opens at all without a known adult present, no matter what the caller says.

The £10 upgrade that works with everything else

A door viewer doesn’t replace good locks, proper cylinders, or, if you want them, cameras and alarms. It sits quietly in the middle of that stack and makes each bit more effective, because you use it at the precise moment of decision.

If you want to build on it later, the cheap, low‑tech pairings that experts like are:

• A door chain or limiter you actually keep on when you’re home.
• A letterbox guard or cage to stop people fishing keys or handles.
• A decent anti‑snap euro cylinder, properly sized, for uPVC and composite doors.

But those can wait. The lens can go in this weekend, for less than the cost of the takeaway you might not open the door for anyway.

You’ll notice it in quiet ways. The charity collector you feel fine declining. The “parcel” that turns out to have no parcel at all. The teenager who can wait until you’ve clocked their friends on the path. Visibility is a form of calm. The viewer doesn’t beep, flash or send you a push notification. It simply gives you that one clear look before you choose what happens next.

Key point	Detail	Why it helps
Wide‑angle viewer	160–200° lens at natural eye height	Lets you see caller, surroundings and anyone standing just out of the way
Simple fitting	Single drilled hole, screw‑together body	Cheap, fast upgrade even on existing solid doors
New habit	Look first, talk through a closed or chained door	Cuts off most doorstep scams and impulsive “just open it” moments

FAQ:

• Is a door viewer still useful if I already have a video doorbell? Yes. Viewers work in power cuts, don’t depend on Wi‑Fi, and let you take a quick look without grabbing your phone. They also cover the space right up against the door, where many cameras have a blind spot.
• Can I fit one in a rented flat? Often, yes, but you should ask your landlord or managing agent first, especially if the door is a certified fire door. The hole is small and can usually be filled neatly if you move out.
• Will drilling the door affect its fire rating or warranty? On many domestic front doors it won’t, but on certified fire doors and some composite doors it might. Check the door’s documentation or ask the manufacturer or building manager before you drill.
• What about people with different heights in the household? Choose a height that suits the shortest adult who regularly answers the door. Taller people can easily bend; children should not be opening to unknown callers, even with a viewer.
• Can someone look in through the viewer from outside? A basic viewer makes this very difficult, but not impossible with specialist equipment. That’s why a model with an internal privacy shutter is worth having – just slide it closed when you’re not using it.