What Is a BS3621 Lock and Why Your Insurance Requires It
The Lock Standard That Governs Your Insurance
If you have ever read the small print of a home insurance policy, you will have come across BS3621. It is the British Standard for thief-resistant locks, and the majority of UK insurers require it on all external doors as a condition of cover.
Despite being referenced in millions of insurance documents, most homeowners have no idea what BS3621 actually means, how to tell whether their locks meet it, or what happens if they do not. This guide covers all of it.
What BS3621 Means in Practice
BS3621 is a specification published by the British Standards Institution (BSI). A lock that carries this standard has been independently tested and certified to resist common methods of forced entry. The tests cover:
- Drill resistance - the lock must withstand drilling for a minimum of 5 minutes using standard tools
- Pick resistance - the mechanism must resist manipulation by picking tools
- Force resistance - the bolt must withstand a specified level of lateral force without retracting
- Key security - the lock must have at least 1,000 key differs (unique key combinations) to prevent unauthorised duplication
- Bolt throw - the deadbolt must extend at least 20mm from the lock face when fully engaged
- Bolt composition - the bolt must be made of hardened steel or be otherwise resistant to sawing
A lock that passes all these tests is awarded the BS3621 Kitemark - a specific logo that indicates independent testing and compliance.
Which Types of Lock Can Be BS3621?
The standard applies primarily to mortice deadlocks - the type of lock that is recessed into the edge of a timber door, with a separate keyhole. These are the traditional “five-lever” locks found on most British front doors.
BS3621 mortice deadlocks are available in two main variants:
- 5-lever mortice deadlock - the classic design. Five internal levers must be correctly lifted by the key before the bolt will retract. Widely used on timber doors.
- 5-lever mortice sashlock - combines a deadbolt with a latch (the sprung bolt that holds the door closed when you push it shut). Operated by both a key and a door handle.
What About Euro Cylinders?
Euro cylinder locks - the ones found on uPVC and composite doors - are tested to a different standard: TS007. This is the BSI’s test for cylinder security, and it rates cylinders on a 1-star to 3-star scale based on their resistance to snapping, picking, bumping, and drilling.
A TS007 3-star cylinder (or a 1-star cylinder paired with a 2-star handle) provides an equivalent level of security to BS3621 for euro profile doors. Most insurers now accept TS007 3-star as meeting the spirit of BS3621 for doors that use euro cylinders rather than mortice locks.
If your front door is uPVC or composite, you need TS007 - not BS3621. For more on euro cylinder security, see our guide to lock snapping and how to prevent it.
How to Check Whether Your Locks Are BS3621
On the Lock Itself
Open the door and look at the metal faceplate on the edge of the door (the visible part of the lock body). A BS3621-compliant lock will have the BSI Kitemark stamped or engraved on the faceplate, along with the standard number “BS3621”.
Some manufacturers also print the standard on the key or on the lock casing visible from inside the door.
On the Key
Five-lever mortice keys for BS3621 locks are typically flat, double-sided keys with a complex bit pattern. If your front door key is a simple, single-sided key or a pin-tumbler key (the kind with a serrated edge), it is unlikely to be a BS3621 mortice lock.
If You Cannot Find a Marking
The absence of a Kitemark does not necessarily mean the lock is substandard - some older locks may meet the specification but predate the Kitemark scheme. However, without the marking, you cannot prove compliance to an insurer. If in doubt, have a locksmith inspect the lock.
What Happens If Your Locks Do Not Comply?
This is where the insurance question becomes serious. If your policy states that external doors must have BS3621 locks and yours do not comply, two things can happen:
Your claim may be rejected. If you are burgled and the locks do not meet the specified standard, your insurer can argue that you failed to meet the conditions of the policy. This applies even if the burglar entered through a window or a different door - insurers examine the overall security of the property, not just the entry point.
Your premium may increase. Some insurers check lock compliance during the claims process. If they discover non-compliant locks, future premiums may rise, or the insurer may add an excess specifically for theft claims.
The practical risk is especially high in cases where there is no sign of forced entry - for example, if someone used a copied key or an unlocked door. Without BS3621 locks, it is harder to demonstrate that reasonable security measures were in place.
Common Situations Where Locks Fall Short
- Older properties - homes built before the 1990s may have original mortice locks that predate the current BS3621 specification
- Rental properties - landlords are not always diligent about maintaining lock standards, and locks may have been changed with budget replacements over the years
- New builds - surprisingly, not all new-build developers fit BS3621 mortice locks or TS007 3-star cylinders as standard. Check before assuming.
- uPVC doors with standard cylinders - a uPVC door may have a multi-point locking system, but if the euro cylinder does not meet TS007 3-star, the door is not as secure as it appears
- Properties where locks have been replaced DIY - the replacement may not match the original specification
The Cost of Upgrading
Upgrading to a BS3621-compliant mortice deadlock is a relatively modest expense:
| Item | Typical Cost |
|---|---|
| BS3621 5-lever mortice deadlock (part only) | £25-£60 |
| Professional fitting (labour) | From £69 |
| Total (supply and fit) | From £94 |
If your door requires modifications to accept a mortice lock (for example, if the existing mortice pocket is the wrong size), the labour cost may be slightly higher.
For euro cylinder doors, upgrading to a TS007 3-star anti-snap cylinder costs between £80 and £150 fitted. See our locksmith price list for current rates.
Beyond BS3621: Other Standards Worth Knowing
| Standard | Applies To | What It Tests |
|---|---|---|
| BS3621 | Mortice deadlocks | Drill, pick, force, key security |
| TS007 | Euro cylinders | Snap, pick, bump, drill (1 to 3 stars) |
| SS312 | Euro cylinders | Sold Secure Diamond - highest physical security rating |
| PAS 24 | Complete door sets | Tests the door, frame, and lock as a system |
| Secured by Design | Various products | Police-endorsed standard for crime prevention |
If you are unsure which standard applies to your doors, a locksmith can advise based on your door type and insurance requirements.
City Locksmith London: Insurance-Compliant Lock Upgrades
We supply and fit BS3621 mortice deadlocks and TS007 3-star euro cylinders across London. If you are unsure whether your current locks meet your insurance requirements, we can inspect them and advise on what (if anything) needs upgrading. Our lock change service covers all lock types, and we carry stock in the van for same-day fitting. Get in touch.