BS3621 Explained — The British Standard Your Home Insurer Quietly Requires
BS3621 is the most important four-character code in UK home security — and most homeowners have never heard of it. It's the British Standard for mortice deadlocks and night latches that resist forced entry, picking and drilling for a specified period. Almost every UK home insurer requires BS3621-rated locks on external timber doors as a condition of cover. If your locks don't meet it and you make a claim after a break-in, the insurer can reduce or refuse the payout.
What BS3621 actually tests. The current standard (BS3621:2017) tests resistance to: 5 minutes of manipulation attack (picking, bumping, decoding); 5 minutes of forced entry attack (drilling, levering, kicking); destructive entry tools commonly used in UK burglaries. A lock that fails any of these doesn't earn the kitemark.
How to spot a BS3621 lock. The kitemark — a heart-shaped logo with 'BS' inside — is stamped onto the faceplate (the metal strip running down the edge of the door where the lock sits). The full reference 'BS3621' is usually printed alongside. If you can't see it, the lock isn't BS3621-rated. Old Yale-style rim latches without the kitemark are no longer adequate for primary security.
Which doors need BS3621. Insurance policies vary in wording, but the typical requirement is: 5-lever BS3621 mortice deadlock on every external timber door (front, back, side); auto-deadlocking BS3621 night latch on the front door in addition to the mortice. UPVC and composite doors are covered separately — they need TS007 3-star anti-snap cylinders rather than BS3621 (because UPVC doors use multipoint locking, not mortice locks).
BS3621 vs BS8621 vs BS10621 — what's the difference? BS3621 is the standard 5-lever deadlock. BS8621 is the 'thumbturn' version — same security level but openable from inside without a key, which is required by fire safety regulations for HMOs and flats (you can't be locked in if you need to escape a fire). BS10621 is similar to BS3621 but allows external locking from outside with a key while remaining openable from inside without a key — useful for back doors where you want to lock up from outside.
Brands that fit the standard. The most-fitted BS3621 mortice locks in the UK are: Chubb 3G110 / 3G114 (the classic), Banham M2002 (high-security upgrade), Union 2C13 (mid-market workhorse), ERA Fortress, Yale Y-Series. All are van stock for any reputable locksmith and most are fitted in 30–45 minutes per door.
What it costs to upgrade.
How to check your own. Walk to your front door, open it and look at the metal faceplate on the door edge where the lock body sits. Look for the kitemark heart logo. If it's there, you're compliant. If it isn't — or you can't see one because the lock is old, painted over or recessed — a 15-minute visit from a working locksmith will confirm and quote any upgrade needed.
Why this matters more than the marketing copy suggests. After a burglary, insurance loss adjusters specifically inspect the lock for the BS3621 kitemark. If it's missing — even if the actual cause of loss was an unlocked window — the claim can be reduced because you breached a policy condition.