What is a five-lever mortice lock?

It is a deadlock fitted into the edge of a timber door (morticed into the wood) that uses five separate levers inside the lock body. The correct key has to lift all five to the exact height before the bolt will move. Fewer levers, fewer height combinations, easier to pick.

What is the difference between a 3-lever and 5-lever mortice lock?

A 3-lever lock has three internal levers and far fewer possible key combinations, which makes it quicker to pick. A 5-lever lock has more combinations and is the minimum most UK insurers will accept on an external door. 3-lever locks are still fine for internal doors, sheds, and outbuildings.

Do I need a BS3621 lock for my home insurance?

Most UK home insurance policies specify a BS3621-rated 5-lever mortice deadlock on external timber doors. Check the security conditions section of your policy schedule. If you only have a Yale-style night latch or a 3-lever lock, you may not be compliant.

Can a 5-lever mortice lock be picked?

Any mechanical lock can be picked given enough time and skill, but a BS3621 5-lever lock takes considerably longer than a 3-lever one and is built to resist drilling too — hardened steel rollers sit behind the keyhole specifically to stop a drill bit. That is most of the security benefit: time. Most opportunist burglars move on.

How much does it cost to fit a 5-lever mortice lock in London?

Labour for a lock change starts at £109. A BS3621 5-lever mortice lock itself costs £50–£90 depending on finish and bolt throw. Fitting one into a door that has never had a mortice lock requires chiselling a new mortice, which adds time and cost — ask for a quote that covers both.

Do UPVC doors need a 5-lever mortice lock?

No. UPVC and composite doors use euro cylinders and multi-point locking systems, not mortice locks. The equivalent security standard there is TS007 3-star, which applies to the cylinder. BS3621 mortice locks are specifically a timber door fitting.

How do I know if my current lock is BS3621 rated?

Open the door and look at the edge of the lock — the faceplate, where the bolt comes out — for the words 'BS3621' or a kite mark stamp. It is sometimes on the lock body itself rather than the faceplate. No markings usually means no rating, though very old locks occasionally had the mark stamped somewhere less obvious.

Photo by Ksenia Chernaya / Pexels

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What Is a 5-Lever Mortice Lock? BS3621 Explained

Q: Can I fit a mortice lock myself?

Replacing an existing mortice lock with one of the same size is a job some confident DIYers manage. Cutting a brand new mortice into a door that has never had one is a different job — get the depth or position wrong and you weaken the door or the lock will not align with the keep. We see both outcomes regularly.

A five-lever mortice lock is the deadlock most UK insurers require on front doors. Here is how it works, what BS3621 means, and when a 3-lever lock is fine instead.

James Mitchell

29 June 2026 · 6 min read

The short answer

A five-lever mortice lock is a deadlock fitted into the edge of a timber door, operated by a key that has to lift five separate levers to the exact right height before the bolt will move. It is the lock most UK home insurers mean when their policy says "British Standard lock" or "BS3621" — and it is a different thing entirely from the euro cylinder in a UPVC door.

If your front door is timber and your only lock is a Yale-style latch, you almost certainly do not have one of these fitted, and your insurance may not cover you for it.

How the levers actually work

Inside the lock body, each lever is a small flat piece of metal that has to be lifted clear of a fixed post before the bolt can slide across. The key's serrated edge lifts every lever simultaneously but to different, specific heights — one wrong height on any single lever and the bolt stays locked.

A 3-lever lock has three of these gates to satisfy. A 5-lever lock has five. Each extra lever does not just add a little security — it multiplies the number of possible height combinations, which is why picking a 5-lever lock takes a genuinely different amount of time and skill to picking a 3-lever one.

5-lever vs 3-lever — what's the actual difference

Both look almost identical from the outside. The difference is entirely inside the lock body and in what each one is rated for.

- 3-lever mortice lock: fewer combinations, quicker to pick, no BSI kite mark. Fine for internal doors, sheds, and garden gates where the consequence of a forced lock is low. - 5-lever mortice lock (BS3621): more combinations, hardened steel rollers behind the keyhole to resist drilling, a bolt that throws at least 14mm into the frame. This is the standard most insurers specify for the front door.

A 3-lever lock on your front door is not "half the security" of a 5-lever one. The jump in pick-resistance between the two is large enough that insurers treat them as different categories, not points on the same scale.

Where you will actually find one

Mortice locks are fitted into timber and some composite doors — the lock body sits inside a cavity cut into the door's edge, which is what "morticed" means. You will not find a mortice lock on a standard UPVC door; those use euro cylinders and multi-point locking systems instead, which are a different mechanism entirely and covered by the TS007 standard rather than BS3621.

Most London period properties — the Victorian terraces across Islington, Camden, and Hackney, the mansion blocks in Kensington and Marylebone — were built with timber front doors and still rely on a mortice deadlock as the primary lock, often alongside a Yale-style latch for everyday use.

BS3621 and your home insurance

BS3621 is the British Standard a 5-lever mortice lock has to pass to carry the kite mark: a minimum bolt throw, hardened steel protection against drilling, and a 5-lever (or equivalent) mechanism resistant to picking.

Most UK home insurers specify BS3621 on the security conditions page of the policy. We covered this in more detail in our [guide to home insurance lock requirements](/blog/what-locks-does-home-insurance-require-uk/) — the short version is that fitting anything below this standard can leave a burglary claim reduced or refused, even if the rest of the door and frame are solid.

Check the lock's faceplate, on the edge of the door, for "BS3621" stamped or a kite mark symbol. No mark usually means no rating.

Fitting one yourself

Swapping an existing mortice lock for a new one of the same case size is within reach for a careful DIYer — the mortice is already cut, the keep is already in the frame, and you are mostly matching like for like.

Cutting a brand new mortice into a door that has never had one is a different job. Get the depth wrong and you weaken the door stile. Get the position wrong relative to the keep and the bolt will not align, so the door will not lock properly under load — which is a bigger problem than the lock you started with. Pay a locksmith once or pay a carpenter twice.

When you do not need one

Not every door needs a BS3621 5-lever lock. Internal doors, garden sheds, and side gates are usually fine with a basic 2 or 3-lever lock — there is no insurance requirement on them and no reason to pay £50–£90 for a lock rated for a front door. Save the BS3621 spec for the doors your insurer actually names in the policy.

What it costs to fit in London

Labour for a lock change starts at £109. A BS3621 5-lever mortice lock itself costs £50–£90 depending on finish and bolt throw. If the door has never had a mortice cut into it, factor in extra time for chiselling the mortice and aligning a new keep in the frame — ask for a price that covers the whole job, not just the lock.