BS3621 Locks Explained — Why Your Home Insurance May Require One

What Is BS3621?

BS 3621:2017+A1:2024 is the current British Standard for thief-resistant lock assemblies operated by key from both the inside and outside. It is published by the British Standards Institution (BSI) and is widely referenced for residential door security in the UK.

A BS3621 lock has been independently tested to resist common attack methods including:

Many familiar BS3621 mortice deadlocks use a five-lever mechanism, but the certification is the important evidence: look for the BSI Kitemark and standard number rather than relying on the lever count alone.

This distinction matters more than it sounds. "Five lever" describes how a lock is built. A British Standard describes what a sample of that lock survived in a laboratory. Locks are sold with the first claim and no second one, and those are not the same product even when they look identical on a shelf.

The BS3621 Family, and Why There Is More Than One

BS3621 has relatives that are frequently confused with it, and the difference between them is about escape rather than strength.

BS3621 covers locks operated by key from both inside and outside. Nothing opens the door without a key, which is why it appears so often in insurance wording.

BS8621 covers locks that can be opened from the inside without a key, usually by a thumbturn. That matters where people need to get out quickly and reliably — flats above ground level, houses in multiple occupation, and any door forming part of an escape route.

BS10621 covers locks that can be locked from outside so that the inside thumbturn is disabled, which suits a property being left empty.

PAS3621 and PAS8621 are publicly available specifications covering multipoint locking systems, which is the relevant family for most uPVC and composite doors, since those doors have no mortice pocket to put a traditional deadlock into.

Choosing the wrong one of these is not a small error. Fitting a key-both-sides lock on a door that people need to escape through in the dark creates a hazard, and fitting a thumbturn lock where a policy specifically requires key-both-sides may not satisfy the policy. The right answer depends on the door, the building, and what your own paperwork says.

Why Does Your Home Insurance Care?

Some UK home insurance policies specify BS3621 or another minimum lock standard on final exit doors; others use different wording. A claim is not automatically rejected because a door lacks one particular lock. Check the security conditions in your own policy or ask your insurer to confirm them in writing.

Because policy conditions vary, check the wording before relying on a particular lock standard for cover.

A "final exit door" is generally the door you leave by and secure behind you — the one that cannot be locked from inside once everyone has gone. Policies use the term because it identifies the doors that actually matter for security, but they do not all define it identically, and a property with two doors in regular use can raise a genuine question about which is which. If your policy is not clear, that is a question for the insurer rather than for a locksmith.

The good news: a BS3621 lock costs from £79 fitted (my price, no VAT, no call-out fee). If your policy names that standard, upgrading gives you clear evidence that the fitted lock meets it.

How to Check If You Have One

Look at your front door. If it is a wooden or composite door, look for a separate keyhole below the main door handle. This is typically where a mortice lock sits.

Turn the key. If the bolt extends from the edge of the door, you have a mortice lock. If there is a BS3621 mark on the lock faceplate or keyhole escutcheon, it meets the standard.

The mark to look for is the BSI Kitemark — a heart-shaped symbol — usually accompanied by the standard number, stamped into the faceplate on the edge of the door. You may need to open the door and look at the strip of metal the bolt comes out of, and it may need a wipe; decades of paint is the usual reason people conclude they have no marking.

Two things that are not evidence: the words "British Standard" printed on the box the lock came in, and a lever count stamped on the key. Neither is a certification mark.

If you are not sure, call me and I will check it for you.

uPVC Doors and BS3621

uPVC doors almost never have a traditional mortice lock. They use a multipoint locking system — multiple bolts that lock into the door frame simultaneously.

Policies describe uPVC and composite door security in different ways. Do not assume that a multipoint lock, TS007 rating, Sold Secure rating, or BS3621 product automatically satisfies your policy; ask the insurer to confirm its requirement for the actual door in writing.

If you want the cylinder assessed or your policy names a particular standard, I can identify the fitted products and explain the suitable certified options. Anti-snap upgrades start from £59; see the lock upgrade service and current prices.

Common Mistakes Worth Avoiding

Replacing like for like without checking. A door fitted with a certified lock twenty years ago may have had it swapped since for whatever was in the van that day. The lock on the door now is the only one that counts.

Confusing a sashlock with a deadlock. A mortice sashlock has a latch and a bolt and works with handles; a mortice deadlock has only the bolt. Both exist in certified versions, but they are not interchangeable in a door prepared for the other.

Ignoring the rest of the doorset. A certified lock in a frame with short screws, tired hinges, or a keep that no longer aligns is doing less than the certificate suggests. The lock is tested; the installation is not.

Overlooking escape. In flats and shared houses in particular, the ability to get out without hunting for a key can matter more than the resistance rating on the way in.

Getting a BS3621 Lock Fitted

I can assess suitable wooden and composite doors for a BS3621 mortice deadlock and explain compatible options. The service starts from £79 including the stated lock and fitting scope; no VAT or separate call-out fee is added. Check the areas page and call with the full postcode to confirm coverage.

Fitting one is not simply a swap. The pocket cut into the door edge has to suit the new case, the keep in the frame has to align with the bolt, and a door that has moved since the original lock went in may need attention before anything engages cleanly. Where a policy names a standard, it is also worth asking the insurer whether it expects any installation evidence, because some do.

Call 024 7522 4730 to book.

Authoritative Sources