What Is a BS3621 Lock?
BS3621 is a British Standard for thief-resistant locks. A lock carrying this standard has been independently tested and certified to resist common attack methods including picking, drilling, and forced entry. The standard is maintained by the British Standards Institution (BSI) and is the benchmark used by most UK insurance companies when assessing home security.
Why Do Insurance Companies Require BS3621 Locks?
Most home insurance policies in the UK require BS3621-compliant locks on all external doors as a condition of cover. If your home is burgled and your locks don't meet this standard, your insurer may refuse to pay out — even if the burglar didn't enter through the non-compliant door. It's one of the most common reasons for rejected insurance claims. Check your policy wording carefully.
How to Check if Your Lock Is BS3621 Compliant
Look for the BS3621 kitemark on the lock faceplate (the metal strip visible on the door edge when the door is open). It should show the BSI kitemark symbol and the number BS3621. If there's no marking, the lock likely doesn't meet the standard. Older 5-lever mortice locks may meet the standard but lack the marking — a locksmith can assess this for you.
Types of BS3621 Locks
The most common BS3621 lock is the 5-lever mortice deadlock — the large lock fitted into the body of the door that requires a key to lock and unlock from both sides. There are also BS3621-compliant cylinder locks (nightlatches) available from brands like Yale and ERA. For full insurance compliance, you typically need a BS3621 mortice lock plus a BS3621 nightlatch on your front door.
Recommended BS3621 Lock Brands
For mortice deadlocks, Union (2134), ERA Fortress, and British Standard 5-lever locks are reliable and widely available. For cylinders, look at Ultion, ABS (Avocet), and Mul-T-Lock — these exceed BS3621 requirements and also carry the Sold Secure Diamond rating. Any reputable locksmith should stock these brands and be able to advise on the best option for your door type and budget.
How Much Does It Cost to Upgrade to BS3621?
A BS3621 mortice lock supply and fit across London typically costs £160–£300 before VAT, depending on the brand and locksmith. If your door doesn't already have a mortice lock cutout, the locksmith will need to cut one, which adds to the labour time. A BS3621-compliant nightlatch upgrade usually runs £200–£350 all-in. Most locksmiths can complete the work in under an hour. It's one of the most cost-effective security improvements you can make.