Changing a lock yourself sounds intimidating but is genuinely a 10-minute job for the most common type (a euro cylinder on a uPVC or composite door). The risk isn't damaging the door — it's buying the wrong-sized replacement, ending up with a protruding cylinder that defeats the point of upgrading. This guide covers the three common lock types you might want to change and how to do each one (or recognise when to call a locksmith).

Should you do it yourself?

Reasonable DIY:

  • Euro cylinder swap on uPVC/composite (10-15 mins, one screwdriver)
  • Night latch (Yale-type) replacement on a wooden door (30 mins)
  • Adding a security chain or door restrictor (15 mins)

Call a locksmith:

  • BS3621 mortice lock replacement (needs chiselling, alignment work)
  • Multipoint gearbox replacement (specialist parts, complex strip-down)
  • Master-keyed system (must match existing differs)
  • Any lock where you're unsure of the current type

Method 1: Replacing a euro cylinder (uPVC / composite door)

Step 1: Measure the existing cylinder

This is the only step that matters. A euro cylinder is sized by the distance from the centre cam screw outwards in each direction. Common sizes: 35/35, 40/40, 40/50, 45/55. Measure with a ruler from the centre point of the cam screw hole to each face. Write both numbers down.

Why it matters: an over-long cylinder protrudes from the faceplate and is the easiest possible target for lock snapping. The visible cylinder should sit flush with the handle escutcheon — no more than 2-3 mm protrusion on either side.

Step 2: Buy a correctly-sized replacement

If you're upgrading for security, get a TS007 3-star + Sold Secure Diamond cylinder. See our anti-snap upgrade guide for brand recommendations. A correctly-sized Yale Platinum or Avocet ABS costs £35-£70 from most Cornwall builders' merchants or locksmiths.

Step 3: Remove the old cylinder

Open the door. On the door edge between the latch and the deadbolt, find the cam screw — a single long screw (60-100 mm). Unscrew it completely. Put the screw in a pocket.

Step 4: Pull the cylinder out

Insert the key and turn it about 5 degrees from horizontal. This aligns the cam (the wing inside the cylinder) with the cylinder body so it can slide out. With the key turned, gently pull the cylinder out from one side of the door. If it sticks, wiggle the key a little — the cam isn't quite aligned.

Step 5: Fit the new cylinder

Insert the new cylinder (with key in, cam aligned the same way) from the outside. It should slide smoothly into place. Refit the cam screw and tighten firmly — but not so hard you strip the threads.

Step 6: Test from both sides BEFORE closing the door

This is where DIYers get caught out. Lock and unlock from inside, then outside, with the door still open. If anything jams, take the cylinder back out and check the cam alignment. Only close the door once both keys work smoothly.

Method 2: Replacing a night latch (Yale-type rim lock)

A night latch is the rectangular surface-mounted lock you see on Victorian wooden front doors — Yale-branded most often. Replacing one is more involved than a euro cylinder but still DIY-able with basic skills.

  1. Remove the existing latch: Unscrew the back-plate (inside the door), then the latch body itself (3-4 screws).
  2. Note the centre point and the cylinder position on the door (the cylinder pokes through a circular hole, usually 32 mm diameter).
  3. Fit the new latch: Position to match the existing hole. Most night latches use a standard footprint, so swapping like-for-like is usually a direct replacement.
  4. Adjust the strike plate on the door frame so the latch engages cleanly.
  5. Test 10 times — close and open the door fully on both keys before considering it done.

If you're upgrading from a basic night latch to a BS3621-compliant one (e.g., Yale 89 or Union 1037), insurance compatibility improves. A BS3621-compliant night latch on its own usually isn't enough for insurance though — you'll also need a separate BS3621 mortice deadlock.

Method 3: Replacing a BS3621 mortice deadlock — call a locksmith

This is the recessed lock fitted into the edge of a wooden external door. Replacement involves:

  • Chiselling out the existing mortice if the replacement is fractionally different in size
  • Drilling/widening the spindle and key holes
  • Aligning the strike box in the door frame
  • Test-fitting multiple times to ensure smooth operation

Goes wrong easily for inexperienced DIYers — common failures include split door faceplates, misaligned strikes that prevent the door closing, and bolts that bind. A Cornwall locksmith does the job in 45-60 minutes for £120-£200 fitted including the BS3621 lock. Worth the cost.

Common DIY mistakes

  1. Wrong cylinder length. Buying a 40/50 when you needed 40/45 — leaves the cylinder protruding 5 mm, snappable, and defeating the security upgrade.
  2. Not testing from both sides BEFORE closing the door. Causes locked-out-yourself emergencies.
  3. Over-tightening the cam screw. Strips the threads — now the cylinder won't sit straight.
  4. Forgetting to turn the key during removal. The cam blocks removal; people resort to forcing and damage the cylinder body.
  5. Buying a non-certified "anti-snap" cylinder. If it doesn't have TS007 stars stamped on the body, it doesn't have the protection. Don't trust marketing copy alone.

Tools you'll need

  • Long Phillips or Pozidriv screwdriver (for the cam screw — usually PZ2 or PZ3)
  • Ruler or short tape measure
  • For mortice/night latch work: also a wood chisel, pencil, possibly a small drill with the right bit

When it's not worth doing yourself

If you're changing locks because you've lost keys or moved house, you'll be doing 2-5 locks at once. The total time for a full house is 60-90 minutes for an experienced DIYer; about 45 minutes for a locksmith. The price difference between a full DIY job (£100-£200 in parts) and a full locksmith job (£300-£600 fitted) is mostly the labour. For most people, the locksmith's certainty and warranty is worth it on a full house.

Want a Cornwall locksmith to do it properly? Submit your postcode for an itemised quote — no call-out fees if we can't help, no surprise add-ons.

Frequently asked questions

How hard is it to change a euro cylinder yourself?

Genuinely easy — one screw, slide out, slide in. 10-15 minutes. The hard part is buying the correctly-sized replacement. Measure carefully (centre of cam screw to each face) and match the dimensions exactly. An over-long cylinder is a security downgrade.

What size euro cylinder do I need?

Measure your existing cylinder from the centre of the cam screw outwards in each direction. Common sizes: 35/35, 40/40, 40/50, 45/55. The total length must match your door thickness exactly; the visible portion should sit flush with the handle escutcheon — no more than 2-3 mm protrusion on either side.

Should I change a BS3621 mortice lock myself?

Usually no — the chiselling, drilling, and strike alignment work goes wrong easily for inexperienced DIYers. Common failures: split faceplates, misaligned strikes, bolts that bind. A Cornwall locksmith does it in 45-60 minutes for £120-£200 fitted, including the lock and a warranty.

Can I change a uPVC door lock without changing the handles?

Yes — euro cylinders are independent of the handle. You can upgrade the cylinder (TS007 3-star anti-snap) without touching the handle set. Worth doing if your cylinder is unrated but your handles are decent quality.

How much does a locksmith charge to change a lock in Cornwall?

Single euro cylinder: £80-£140 fitted. BS3621 mortice: £120-£200. Night latch: £100-£170. Full external locks for a 3-bed home: £300-£600. Includes labour, parts, and travel within standard coverage areas.