Works with Matter
What is a door lock monitor? Locked is not closed.
Every smart-home device can tell you something about your front door - except the thing you actually lie awake wondering: is it locked? Contact sensors see open or closed. Cameras see the doorstep. Alarms see entry. Only two kinds of device know the state of the lock itself: a motorised smart lock, and a door lock monitor.
Definition: a door lock monitor is a sensor that reads the state of the lock mechanism itself - locked or unlocked - rather than the position of the door. A closed door is not always a locked one, and nothing that only watches the door can tell the difference.
See the Locksure monitor - from £44.99
Every way to watch your door, compared
Each of these is good at its own job. Only the first two know whether the bolt is thrown - and only one of them does it without replacing or motorising your lock.
| Lock monitor (Locksure) | Smart lock | Contact sensor | Alarm system | Video doorbell | Telecare sensor | Checklist app | |
|---|---|---|---|---|---|---|---|
| Knows the door is LOCKED | Yes | Yes (it turns the lock) | No - open/closed only | No | No - cannot see the bolt | No - open/closed only | No - records that you tapped |
| Knows open / closed | Yes | Some models | Yes | Yes | Visually | Yes | No |
| Keeps your existing lock | Yes - fits beside it | Replaces or motorises it | Yes | Yes | Yes | Yes | Yes |
| Insurance-rated hardware untouched | Yes - nothing changes | Depends on model and fitting | Yes | Yes | Yes | Yes | Yes |
| Subscription needed | No - core alerts included | Sometimes, for remote features | Often, via the alarm plan | Commonly | Commonly, for recording | Yes - monthly monitoring fees | Usually free |
| Typical battery | Over 2 years (standard); 18 months (Matter) | Months, then recharge or swap | 1-2 years | 1-2 years | Wired or months | 1-2 years | - |
| Could it be made to UNLOCK your door | Never - it has no motor | Yes - that is its job, so secure it well | No | No | No | No | No |
| Fits any UK door | Thumb-turn or euro-cylinder key-turn, incl. multipoint | Depends on cylinder and geometry | Yes | Yes | Yes | Yes | - |
| Typical cost | £44.99 (monitor) / £62.99 (starter pack) | £150-300+ | £15-35 | £200+ and/or plans | £80-180 | £60-120 per month privately | Free |
Category costs are typical UK ranges, checked August 2026; individual products vary. Every product above is good at its own job - the table shows which job each one does.
When do you want a monitor instead of a smart lock?
A motorised smart lock is the right buy if you want keyless entry, remote unlocking or auto-lock. A door lock monitor is the right buy if your question is simply "did I lock it?" - because it answers that for a fraction of the price, fits the lock you already have (including UK multipoint doors that motorised units often cannot), never runs the risk of unlocking anything, and leaves your insurance-rated cylinder exactly as it was.
Dig into a specific comparison
Smart lock vs lock monitorThe full buyer's-guide comparison
Matter door sensor vs contact sensorWhy closed is not locked, in depth
Did I lock the door?The 2am question, answered properly
Locksure vs KubuLock monitoring without a compatible door
Locksure vs NukiLock state without buying the smart lock
"Alexa, is my front door locked?"When Alexa actually knows
Can Ring tell you it's locked?What contact sensors can and cannot see
Can a doorbell camera tell?Why no camera can see the bolt
Does SwitchBot tell you?Yes - but it motorises your lock
Does Aqara tell you?Sensors vs the U-series locks
Can SmartThings tell you?Filling the lock-state gap over Matter
Common questions
How do I know if my door is locked when I am not home?
Fit a door lock monitor beside your existing lock. It senses whether the mechanism is locked and shows the state in an app, from anywhere. You do not need to replace your lock or fit a motorised smart lock.
Why can't my door sensor tell me the door is locked?
Contact sensors detect a magnet's proximity - whether the door is open or closed. The bolt inside the door is invisible to them, so a shut-but-unlocked door reads the same as a locked one.
Does a door lock monitor affect my home insurance locks?
Locksure attaches beside the lock and does not replace or modify the cylinder or mechanism, so the door's existing insurance-rated hardware stays exactly as it was.
What does the Matter version add?
The Matter-over-Thread version appears in Apple Home, Google Home, SmartThings and Home Assistant as a lock-state and door-state sensor - local control, no hub and no cloud lock-in.
From £44.99 · fits your existing lock · no subscription for core alerts · UK-made