BS3621 Explained: What It Means for Your Front Door Lock

BS3621 is the British Standard for thief-resistant lock assemblies, mainly mortice locks. Many insurers mean this standard when a policy asks for a “five-lever mortice lock” on final exit doors, but your policy wording decides. A genuine BS3621 lock has a small kitemark on its forend, because the maker had it tested against picking, drilling and forced attack.

A lock without that mark is not certified, whatever the seller called it. Checking for the kitemark takes thirty seconds, yet it is the most useful thing to do before you assume your policy rules are met.

What BS3621 actually tests

The standard covers the whole mortice lock, not just the visible cylinder cap. Tests include picking, drilling and physical attacks such as levering or snapping. The standard also sets a minimum number of key differs, which are unique key combinations. As a result, keys are hard to copy or guess.

A lock that passes carries the kitemark and the standard number. An outside body checks the maker under an accreditation scheme, so the maker cannot simply claim it.

How to check whether your lock is BS3621

Open the door and look at the forend. This is the metal edge plate on the door edge, and it shows where the lock body sits. A genuine BS3621 lock shows a small kitemark with “BS3621” and a year stamped nearby.

If there is nothing there, treat the lock as “not confirmed” because you cannot prove it. Do not assume the best. A locksmith can check this on a visit and tell you whether to keep the lock body or replace it.

Why insurers ask for it

Many home insurance policies set a minimum lock standard for final exit doors. BS3621 is a standard they often name, but the policy wording decides. This matters because an insurer can dispute or cut a claim after a break-in if the lock did not meet the policy wording, even where the burglary itself is not in doubt.

The policy wording is the only way to know what your insurer requires. Do not rely on what “standard” locks usually mean. Since wording differs between insurers, ask yours in writing if the policy is unclear.

BS3621 vs TS007 vs anti-snap

BS3621 covers thief-resistant lock assemblies such as mortice deadlocks and sashlocks, and some nightlatches that deadlock, as the BSI standard describes. It does not apply to euro cylinders. Most modern uPVC and composite doors work differently because they use a multipoint lock with a euro cylinder, the round barrel behind the handle. A separate standard, TS007, rates that cylinder as 1 star or 3 star for its resistance to snapping. Two stars applies to security handles, not cylinders.

Snapping is a known weakness in older, unrated euro cylinders. In particular, unrated cylinders are the weak point. In practice, a timber front door with a mortice lock needs BS3621. A uPVC or composite door needs a TS007-rated or anti-snap cylinder instead. Our page on anti-snap euro cylinders covers that side, and our guide to how anti-snap cylinder ratings work explains the stars.

Upgrading to BS3621

If your mortice lock lacks the kitemark, the fix is usually a new lock case, not just a new cylinder. The standard applies to the whole mechanism. Therefore a new cylinder in an old case does not make it compliant.

This job fits well with a routine lock change. While the old lock is out, fitting a certified case adds little time. It also closes the insurance gap in one visit. For fitting and product options, see our BS3621 locks service page.

Flats, communal doors and mansion blocks

In a block with a shared entrance, the front door of your own flat is usually your job. The communal door is typically the freeholder’s or the managing agent’s. Both matter for insurance, however, so know which is which.

Your own final exit doors need to meet your policy’s lock standard. A weak communal entrance is a shared issue, so raise it with the managing agent. Period doors need extra care, since original ironmongery adds value. Our period property security guide shows how to upgrade without losing original features. If you want to keep a period look, see whether you can fit a smart lock to a period door while you keep a BS3621 mortice lock.

Getting a BS3621 lock fitted

Tell us the door type and how many locks need checking, and you get a clear price before anyone travels. We cover the W2 area, and you can see our Bayswater locksmith page for local detail. Request a call back or use the enquiry form to arrange a BS3621 check or upgrade.