BS 3621 Thief resistant locks operated by key from both sides
The lock standard most UK home insurers name. Applies to 5-lever mortice deadlocks and rim nightlatches on timber doors.
What it actually decides
- Minimum five levers — a three-lever lock can never comply
- At least 1,000 key differs
- Minimum 20 mm bolt throw into the frame
- Hardened anti-drill plates and anti-pick features
- Key required on both sides (no thumb-turn)
Applies to these lock types
5-lever mortice deadlock (BS 3621) · Rim nightlatch (surface-mounted)
Why a retrofit designer cares
The Kitemark is stamped on the faceplate, so a photo of the door edge answers the question. That makes this the easiest compliance check to automate from a photo.