Types of Door Locks Explained: A UK Homeowner’s Guide

Most UK homes use one of a handful of lock types. Timber doors usually have a mortice lock, often with a night latch above it. uPVC and composite doors have a euro cylinder driving a multipoint mechanism down the edge of the door. Internal doors have privacy locks or bathroom bolts. Which one you have decides how secure your home actually is, what your insurer expects to see, and whether a problem is a ten minute fix or a new mechanism.

The phone call I get every week

“How much to change a lock?”

I get that one constantly, and it is a perfectly fair question. The trouble is I often cannot answer it, because the person ringing does not know what they are changing. Not the lock, and every so often not the door either. Plenty of people are not sure whether their front door is timber, uPVC or composite, and honestly, that is hard to tell if nobody has ever pointed it out to you.

None of that is a criticism. You did not choose your locks. A developer chose them, or the previous owner did, or a locksmith fitted whatever was in the van fifteen years ago. It is the one part of your house whose entire job is keeping people out, and most people have never really looked at it.

But that gap is exactly what overcharging feeds on. If you cannot name the thing in your door, you cannot tell whether the person quoting you £355 for a “high security lock” has just fitted a £30 cylinder. So this is not about turning you into a locksmith. It is about being able to look at your own door, know roughly what you are looking at, and get a straight price on the phone.

The main types of door locks

Mortice locks: deadlocks and sashlocks

A mortice lock sits inside a pocket, a mortice, cut into the edge of the door. All you see is the metal faceplate on the door edge and the keyhole on the face. This is the standard on timber front and back doors across the UK.

The lever count tells you the security level. Two, three and five lever locks belong on internal or low risk doors. A five lever BS mortice deadlock or sashlock is what an external timber door wants. The mark to look for is BS3621. It is the British Standard most insurers ask for on a final exit door, and a genuine one is stamped on the faceplate alongside the BSI kitemark. No stamp, not a BS3621, whatever anyone tells you.

A deadlock throws a bolt and nothing else, with a key needed on both sides. A mortice sashlock is the same lock with a latch added, so the handles work the door and the key throws the deadbolt. You will find sashlocks on back doors, and on doors into a garage or utility room where you want a handle rather than a push.

Security comes down to the same two things: lever count and the BS3621 stamp. Worth knowing the difference when you ring a locksmith, because a sashlock and a deadlock are different parts in different sizes, and getting it right on the phone saves a second visit.

Other mortice locks: detainer, metal door and DIN

Not every mortice lock is a British five lever. Detainer locks work off detainers rather than levers. Metal door mortice locks are built narrow to fit the thin stile of an aluminium or steel door. DIN locks come from the German standards system, which fixes the case size and the fixing positions, so any DIN lock drops straight into any door prepared for one. They take a euro cylinder, and they turn up constantly on commercial, metal and factory built doors.

You will find all three all over London, on flats, shops and metal doors. BS3621 generally does not apply to them, so do not go looking for the stamp.

Euro cylinder locks

The euro cylinder is the barrel behind the keyhole on a uPVC or composite door. It has a rounded top cylinder where you put the key and a smaller oval bottom section. It does not lock the door itself, it drives the mechanism inside.

This is the part that gets snapped. An unrated cylinder can be broken in seconds by someone who has done it before, and it is behind a large share of UK break-ins. What you want is TS007 3 star or Sold Secure Diamond. You can also reach three star protection with a 1 star cylinder paired with a 2 star handle.

The good news: on most uPVC and composite doors the cylinder can be upgraded on its own. Same door, same handles, and nothing else needs touching.

Other cylinders: oval, screw-in and Scandinavian

Euro is the common one, not the only one. Oval cylinders do the same job with an oval profile instead of the euro shape. Screw-in cylinders thread into the lock case rather than being held by a screw through the door edge. Scandinavian cylinders are a different oval again, paired with Scandinavian style mechanisms.

None of them are interchangeable. Get the profile wrong and the part simply will not go in, which is why a photo of the actual cylinder beats any description over the phone.

Multipoint locking mechanisms

This is the metal strip running down the edge of a uPVC or composite door, with hooks, rollers or bolts spaced along it. Lift the handle and the points engage, turn the key and it locks.

A lot of “my key will not turn” calls end up here rather than at the cylinder. If the gearbox inside the mechanism has failed, no amount of new barrels will fix it, and it is a bigger job than a cylinder swap. It is also why a good cylinder in a worn out mechanism is not the win people think it is. Both parts have to be right, and anyone who only looks at one of them is guessing.

Night latches and rim locks

The night latch is the lock fitted to the surface of the inside face of the door, usually with a knob or a turn, latching shut behind you as it closes. Yale is a brand name that stuck to the whole category.

Two things worth knowing. A standard night latch is not a deadlock, and on its own it will usually not satisfy an insurer. And some can be opened without a key by anyone who knows the trick. There are auto deadlocking night latches made to BS3621, and those are a different animal. If yours carries no stamp, treat it as convenience rather than security and back it up with a proper deadlock below.

Internal door locks: bathrooms and bedrooms

People assume this one does not matter. It is a good chunk of my week.

Bathroom locks are privacy locks, not security locks. Usually a bathroom bolt or a bathroom sashlock, with a thumbturn on the inside and a slot on the outside that turns with a coin or a screwdriver. That outside release is not optional. It is what gets a child, an elderly parent, or anyone who has had a fall out of a locked bathroom without taking the door off its hinges.

Bedroom locks are normally a small mortice or a privacy latch, two or three lever, because they exist for privacy rather than burglars. If you are fitting one, the question is not how strong it is. It is how you get in from the outside when you need to.

Garage, gate, grille and roller shutter locks

Past the front door there is a whole other family. Up and over garage doors take T handle locks and garage bolts. Gates and security grilles take narrow mortice locks, hook locks, or a padlock and hasp. Roller shutters take bullet locks or a central shutter lock.

Different metal, different fixings, and different parts entirely from anything on your house door.

Padlocks

Worth a mention, because most people buy them on price alone. Look for a Sold Secure rating, and a closed shackle anywhere a pair of bolt croppers could reach.

And remember a padlock is only ever as strong as the hasp and staple it hangs off. A gold rated padlock on a hasp fixed with two short wood screws is not security, it is expensive decoration.

Smart and digital locks

Keypad, app or fingerprint. Some replace the whole lock, others retrofit onto the cylinder or the inside thumbturn so the outside of the door does not change.

Two things to check before you buy one. Whether your insurer accepts it on a final exit door, and what happens when the batteries die or the wifi drops. The decent ones keep a mechanical override. The cheap ones leave you standing on the step.

How to tell which lock you have, and how to measure it

Do this with the door open, looking at the edge.

Mortice lock. Look at the metal faceplate on the door edge. The lever count, the BS3621 mark and the kitemark are stamped there if they exist at all. For size, measure the faceplate and the depth of the lock case into the door. Common case sizes are 64mm (2.5 inch) and 76mm (3 inch).

Euro cylinder. Look at the door edge below the keyhole for one long screw. That single screw is all that holds the cylinder in. Measure from the centre of that screw hole out to the external face, then from the centre to the internal face. Two numbers, for example 35/45, and those are what a locksmith needs. Cylinders are deliberately not symmetrical. Get it wrong and you end up with one sticking proud of the handle, which is exactly what gets snapped, or one that does not reach.

Night latch. It is on the inside face of the door, so simply look at it. Any stamp will be on the case or faceplate.

Or skip the tape measure entirely. Take a photo of the door edge and the keyhole and send it to me. I will tell you what you have got and whether it needs changing. It costs nothing and takes me a minute.

Which door lock is the most secure, and what does my insurance need?

There is no single most secure lock, because it depends what the door is made of.

• Timber external door: a five lever BS3621 mortice deadlock or sashlock, ideally with a night latch above it. The BS3621 stamp is the bit that counts, not the lever count on its own.
• uPVC or composite door: a TS007 3 star or Sold Secure Diamond anti snap euro cylinder, in a multipoint mechanism that is actually working properly.
• Internal doors: privacy, not security. An emergency release matters more than strength.

On insurance, the honest answer is that most UK insurers ask for BS3621 on final exit doors, but policies vary more than people expect and some now specify anti snap cylinders on uPVC as well. Read your own policy wording rather than take any locksmith’s word for it, mine included. Send me the paragraph and I will tell you whether what is on your door meets it.

What you almost never need is a new door. On uPVC and composite, upgrading the cylinder usually is the whole job, and on a timber door a mortice swap is straightforward. It costs a lot less than most people brace themselves for, and my rates are all set out on my locksmith prices page if you want to measure a quote you have been given against them.

Common questions about door lock types

What is a mortice lock?

A mortice lock is fitted inside a pocket cut into the edge of the door, rather than on its surface. On an external timber door it should be a five lever BS3621 deadlock or sashlock, which is the standard most UK insurers ask for. A five lever lock without the BS3621 stamp is not the same thing.

What is a euro cylinder lock?

A euro cylinder is the barrel behind the keyhole on a uPVC or composite door. It drives the multipoint mechanism inside the door rather than locking the door itself, and it is the part that gets snapped if it is not rated.

What is the most secure door lock?

It depends on the door. A five lever BS3621 mortice deadlock or sashlock for timber doors, and a TS007 3 star or Sold Secure Diamond anti snap euro cylinder for uPVC and composite. The best lock on the wrong door is not secure.

Does my home insurance need a British Standard lock?

Most UK policies ask for BS3621 on final exit doors, and some now specify anti snap cylinders on uPVC too. Wording varies between insurers, so check your own policy rather than assume.

How do I know what lock I have?

Open the door and look at the faceplate on the edge. Mortice locks are stamped with their lever count and any British Standard mark. A euro cylinder shows as a figure of eight barrel with one long fixing screw below it.

Can I change a door lock myself?

Physically you can, since a euro cylinder is held in by a single screw. The trouble is what comes after. Measure it wrong and the cylinder sits proud of the handle, which is exactly what gets snapped. Buy the wrong rating and you have spent money without buying any security. A mortice lock that does not match the existing case size and hole positions means cutting the door. My honest advice is to get a locksmith to do it, whichever lock you have.

What this all comes down to

You do not need to memorise any of this. You need to be able to look at your own front door and say one sentence about it. It is a five lever mortice. It is a euro cylinder on a composite door.

That sentence is worth more than it sounds. It is the difference between “my lock is broken, do whatever you need to” and “my euro cylinder has failed, what would you charge to swap it”. The second person does not get sold a new door. Anyone solving your problem will happily tell you which of these you have got and why it matters. Anyone selling locks would rather you never found out.

Not sure what is on your door?

Send me a photo of your lock, or ring and describe it, and I will tell you straight what you have got, whether it meets what your insurer wants, and what it would cost to change.

Call Andrew: 073 7617 6366 Or contact me at: andrewthelocksmith.com/contact-andrew-the-locksmith