BS3621 Explained: What the Kitemark Means on a Door Lock

BS3621 is a British Standard for thief-resistant lock assemblies operated by a key from both the inside and outside of the door. If you have a traditional timber entrance door, you may see BS3621 mentioned by locksmiths, insurers or police security guidance.

BSI lists BS 3621:2017+A1:2024 as the current version of the standard. The easiest way for a homeowner to identify a certified product is usually to look for the BSI Kitemark and BS3621 marking on the lock itself.

What Is BS3621?

BS3621 sets requirements for thief-resistant lock assemblies designed to be operated by key from both sides of the door. It has historically been associated particularly with mortice locks fitted into timber entrance doors.

A certified product is assessed against defined security requirements rather than simply being described by a manufacturer as “high security”. BSI currently lists BS 3621:2017+A1:2024 as the active publication.

The standard should not be confused with a general description such as “5 lever”. A lock can have five levers without necessarily holding BS3621 certification.

What Does the Kitemark Mean?

The BSI Kitemark indicates that the specific product has been independently certified against the applicable standard under BSI’s certification scheme. For BS3621 locks, the Kitemark is commonly accompanied by the BS3621 number on the lock faceplate.

Open the door and inspect the metal faceplate on the door edge. On an applicable certified lock, you should normally be able to identify the Kitemark and the relevant standard marking without dismantling the lock.

Do not rely only on the appearance of the key or the number of visible levers. Certification should relate to the exact lock model.

What Does BS3621:2017+A1:2024 Mean?

The numbers describe the edition of the standard rather than a different security grade.

BS 3621 is the standard number, 2017 identifies the main edition, and A1:2024 means Amendment 1 was published in 2024.

Older locks may show earlier BS3621 editions. The existence of a newer edition does not by itself tell you whether an older installed lock must immediately be replaced, because insurance and property requirements depend on the individual circumstances.

Is Every BS3621 Lock a 5-Lever Mortice Lock?

No. Five-lever mortice deadlocks and sashlocks are strongly associated with BS3621, but the standard is about the certified lock assembly, not simply a generic lock style.

The BSI Product Directory currently includes certified five-lever mortice deadlocks and sashlocks as well as certain cylinder mortice locks and rim/night-latch products. This is why checking certification is more reliable than assuming a particular lock automatically meets the standard.

For many traditional timber doors, however, the familiar BS3621 lock remains a Kitemarked mortice deadlock or sashlock, as in this BSI Kitemark directory entry for a BS 3621 sashlock.

Is a 5-Lever Lock Automatically BS3621?

No. “5 lever” describes part of the lock’s mechanism, while BS3621 confirms that the lock assembly has been assessed against the relevant British Standard.

Two locks can both be described as five-lever mortice locks while only one carries appropriate BS3621 certification. If the requirement is specifically for BS3621, look for the Kitemark and standard number rather than relying on the lever count alone.

This distinction matters when replacing a lock for a stated security or insurance requirement.

BS3621 vs BS8621: What Is the Difference?

The most important difference for householders is how the lock operates from inside.

A BS3621 lock is key-operated from both the outside and inside. A BS8621 lock uses key operation from outside but allows non-key operation from inside, such as a thumbturn, which can be important where rapid escape is required.

This means a higher-looking or more familiar standard number is not automatically the right choice for every door. The escape needs of the occupants and the role of the door should be considered alongside security.

What Is BS10621?

BS10621 is another thief-resistant lock standard with a different internal operating arrangement. Secured by Design's guide to lock standards describes it as a dual-mode lock assembly with non-key operation from inside plus an additional facility to deadlock the door from outside through a deliberate action.

Because this arrangement can affect escape when deadlocked, it should not simply be substituted for BS3621 or BS8621 without considering the property and its escape routes. The correct product depends on how the door is used.

Does BS3621 Apply to uPVC Doors?

BS3621 is most commonly relevant to appropriate lock assemblies on timber doors. Modern uPVC and composite doors usually rely on a multipoint mechanism and euro-profile cylinder, so different standards and security ratings are commonly relevant.

Our High Security Locks service treats BS3621 mortice locks and TS007 or SS312-rated euro cylinders as separate upgrades. The door construction and locking system should therefore be identified before deciding which standard to look for.

Do not assume every external door should simply receive a BS3621 mortice lock.

Does Home Insurance Require BS3621?

Some insurance policies refer specifically to BS3621 or British Standard locks, particularly for certain external timber doors. However, there is no safe basis for saying every UK insurer requires exactly the same lock standard.

The Master Locksmiths Association's minimum security guidance says insurers will sometimes accept existing locks that conform to earlier versions of the standard, and recommends getting written confirmation from the insurer where there is any doubt.

We can identify the fitted lock through our lock replacement service, but always check your own policy wording rather than relying on a generic “insurance-approved” label.

Does BS3621 Mean the Entire Door Is Secure?

No. A certified lock is only one part of the doorset.

The lock depends on the door, frame, keep or strike area, hinges and installation remaining strong enough to support it. A high-security lock installed into a badly split timber frame does not automatically create a secure entrance.

Correct fitting also matters. The bolt needs to engage properly and the door should close and lock without excessive force.

Should You Replace an Older BS3621 Lock?

Not purely because a newer year appears in the current standard. First check the condition of the existing lock, which version it carries, the security requirements of the property and any specific insurer wording.

Replacement becomes more relevant when the lock is worn, damaged, unreliable or does not meet a requirement that applies to the property. An upgrade may also be sensible after a security review, but it should be based on the complete door rather than the standard year alone.

For an upgrade assessment, see our high security locks service.

How to Check Your Existing Lock

You can make a basic visual check without dismantling the door.

Open the door and inspect the faceplate along the edge. Look for:

• the BSI Kitemark;
• the marking BS3621;
• the edition or year where shown;
• manufacturer or model markings;
• signs of physical damage or excessive wear.

If the markings cannot be identified, take a clear photograph of the faceplate and lock area. A locksmith can then help determine what is fitted without relying only on the shape of the key.

Frequently Asked Questions

What does BS3621 mean on a door lock?

BS3621 identifies a British Standard for thief-resistant lock assemblies operated by key from both the inside and outside of the door. A genuinely certified product should carry appropriate certification markings, and the current BSI publication is BS 3621:2017+A1:2024.

How can I tell if my lock is BS3621?

Open the door and check the lock faceplate for the BSI Kitemark and the BS3621 standard number, which may also include the edition year. Do not assume a lock is BS3621 simply because it uses a long key or is described as a five-lever mortice lock.

Is a 5-lever mortice lock the same as BS3621?

No, because five lever describes the mechanism while BS3621 refers to compliance with a defined security standard. A five-lever mortice lock should therefore not be described as BS3621 unless the exact product carries the appropriate certification.

What is the difference between BS3621 and BS8621?

BS3621 uses key operation from both sides, while BS8621 is designed with key operation from outside and non-key operation from inside so occupants can exit without finding a key. The appropriate choice depends on the door, security requirement and escape considerations.

Does my home insurance require a BS3621 lock?

Some policies specify BS3621 or another defined lock standard, but requirements vary between insurers and properties. Check the exact policy wording or obtain confirmation from the insurer rather than assuming every home requires the same lock.

Final Thoughts

BS3621 is useful because it gives homeowners a recognised way to identify independently assessed thief-resistant lock hardware. The important identifiers are the actual certification markings, not simply phrases such as “5 lever”, “insurance lock” or “high security”.

The current publication is BS 3621:2017+A1:2024, but the correct lock for a property still depends on the door type, internal escape requirements and any specific insurer wording. BS8621 or another locking arrangement may be more appropriate where keyless escape from inside is important.

For timber door upgrades and British Standard locks, Kingdom Locksmith can help through our high security locks service. If the existing lock is worn or simply needs changing, see our lock replacement service.