Insurance-Approved Locks: BS3621 Explained (UK 2026 Guide)

Almost every UK home insurance policy sold today references British Standard BS3621 in its locks clause. Most homeowners have never heard of it. Most also have at least one external door that doesn't comply. This guide explains what the standard means, how to check your locks in 30 seconds, and what to upgrade if you find a gap.

BS3621 is the British Standard for thief-resistant locks. In practical terms it means a 5-lever mortice deadlock that has been independently tested against drilling, picking, sawing, and brute-force attack. The standard has been around since 2007 and is the baseline insurers use to define an 'acceptable' lock on a final exit door. The Kitemark for BS3621 is a tiny stylised K-shape stamped into the brass faceplate of the lock — usually visible on the door edge when the door is open.

Insurers require BS3621 on every 'final exit door' — meaning every external door that locks the property when nobody is home. For most UK houses that's the front door and the back door. Side doors, conservatory doors, and patio sliders count too if they're external. Internal doors don't need it. Garage doors don't need it for the house insurance (separate cover usually). UPVC and composite doors don't typically have a BS3621 mortice — they have multipoint locking systems that are dealt with separately under TS007.

Open your front door. Look at the edge of the door, at the metal faceplate where the deadbolt slides out. There should be a small Kitemark stamped into the brass — looks like a stylised letter K with curves. Below or beside it should be a marking that includes 'BS3621' and a year (e.g. BS3621:2007 or BS3621:2017). If you see the Kitemark and BS3621 marking, you're compliant. If the faceplate is bare, or the marking says BS3621 with no Kitemark, the lock is not compliant.

Aviva, Direct Line, AXA, LV, More Than, and most comparison-site brands all reference BS3621 in their standard wording for wooden and composite doors with mortice locks. For UPVC and composite doors with multipoint locking, they additionally require TS007 3-star anti-snap cylinders (the 3-star Kitemark). After a break-in claim, insurers commonly request a written security upgrade certificate from a qualified locksmith before settling. Some insurers offer a premium discount of 5-10 percent for documented BS3621 + TS007 3-star upgrades.

Fitting a new BS3621 mortice lock to an existing wooden door costs £150-180 fitted, including parts. The job takes 1-2 hours per door because the existing mortice needs to be enlarged or re-cut to fit the new lock body. TS007 3-star anti-snap cylinder fitting on a UPVC door takes 30 minutes per cylinder and costs £120-180 fitted. A typical 3-bedroom home with two external doors costs £270-540 for a full insurance-grade upgrade.

When booking the upgrade, ask for the lock brand and Kitemark number to be specified in the invoice. Ask for a written security upgrade certificate that lists each lock fitted, the standard it meets, and the date of fitting. Insurers want this paperwork. A reputable locksmith will provide it as standard at no extra cost. Keep the certificate with your insurance documents — you'll need it at next renewal and at any future claim.

Mistake one: thinking the Yale rim latch on the front door is enough. It isn't — most insurers require both a Yale rim AND a BS3621 mortice on a wooden front door. Mistake two: assuming a heavy-looking lock is BS3621. Many cheap mortice locks look the part but lack the Kitemark. Mistake three: upgrading the front door but ignoring the back door. Insurers care equally about every external door. Mistake four: not keeping the certificate. After a claim, no certificate often means no payout.