"Locks must conform to BS3621" is the sentence in home insurance documents that sends more people to search engines than any other in our trade — and most of what they find explains the standard without answering the question that actually matters: which standard applies to your door? Because here is the part the guides skip: BS3621 is, in practice, a standard for locks on timber doors. If your home has uPVC or composite doors — as most built or re-doored since the 1990s do — a BS3621 mortice deadlock is not what your door takes, and the equivalent protection has a different name. This guide sorts it out door by door.
BS3621 is the British Standards Institution's benchmark for thief-resistant key-operated locks. To carry it, a lock must survive standardised attack tests — drilling, picking, sawing through the bolt, forcing — and meet build requirements such as (for lever mortice locks) at least five levers, a hardened bolt and anti-drill protection. Passing locks carry the BSI Kitemark plus the number BS3621 stamped on the faceplate — the metal plate on the door edge you can see with the door open.
Two siblings are worth knowing because they answer real situations: BS8621 covers locks that need a key outside but open from inside without one (a thumbturn) — the right standard for flats and HMOs where fire escape must never depend on finding a key. BS10621 covers locks that can only be deadlocked from outside, used in some commercial settings. If you live in a flat, 8621 is usually the standard your building actually needs.
No insurer approves locks. The BSI certifies them; insurers require certain certifications in their policy wording. The distinction matters because "insurance approved" on a box is marketing, not a guarantee your particular policy is satisfied. The only authority on what your policy needs is the policy document itself — usually under "endorsements", "security requirements" or "minimum standards". Two phrases dominate, and decoding them is the next section.
Timber doors. The classic insurer phrase is "a five-lever mortice deadlock conforming to BS3621". Note that five levers alone is not enough — plenty of five-lever locks are not Kitemarked, because levers are only one of the tested features. Open the door and read the faceplate: no Kitemark and "BS3621", no compliance. Common arrangement: a BS3621 mortice deadlock plus a nightlatch for daytime convenience — and if the nightlatch is the only lock, it should itself be a BS-rated one.
uPVC and composite doors. These use a multipoint locking strip driven by a euro cylinder, and a mortice deadlock cannot be fitted without ruining the door. Insurer wording usually reads "key-operated multi-point locking system" — which most modern doors satisfy as standard. The security (and increasingly the insurer interest) lives in the cylinder: a TS007 three-star or Sold Secure SS312 Diamond anti-snap cylinder is the uPVC-world equivalent of the BS3621 deadlock. We cover the ratings fully in our anti-snap guide; the short version is: look for the stars or the diamond stamped at the keyway.
Flats. Escape comes first: BS8621 (key outside, thumbturn inside) on the flat entrance door, so nobody hunts for a key in smoke. Many freeholders and fire risk assessments require exactly this — check before fitting anything keyed on both sides.
Why be fussy? Because security conditions in a policy are conditions. After a burglary, a loss adjuster can and does ask what locks were fitted and whether they were used. Discovering at claim time that the "five-lever lock" was never Kitemarked, or that the back door was on the latch, is the most expensive way to learn the difference.
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Timber doors: swapping a non-rated mortice lock for a Kitemarked equivalent of the same case size is usually a neat, same-visit job; where the old lock is an odd size, minor carpentry brings it to spec. uPVC doors: a cylinder upgrade takes minutes — the critical part is correct sizing so the new cylinder sits flush, which is precisely where off-the-shelf replacements go wrong. Costs in both cases sit far below what people expect for the word "insurance-grade", and every LocksmithLocal quote is a single fixed figure before work starts.
One last honesty note: standards resist attack; they do not abolish it. A Kitemark on the faceplate plus the habit of actually deadlocking every night — that combination is what the testing was for.
Built from what actually fails on British homes — not a camera sales pitch. A timed walk-round covering cylinders, deadlocking habits, letterboxes, windows and the side gate, with a score at the end and fixes ordered by cost.
Identify what's on your own door in thirty seconds, then learn what it means: how each lock family works, how it fails, what insurers expect of it, and how to upgrade every type without changing the door.
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