There are many reasons why you might want to fit a new door lock. It might be a security concern, you may have lost your keys, or you may just enjoy the satisfaction that comes from a successfully completed DIY project. Whatever the reason, we're here to help, with this handy, step-by-step guide to fitting a new mortice lock to your door.

1. Choose your lock

The first step is to make sure you choose the right lock for your needs. A mortice lock is a locking mechanism which is housed in a recess inside the body of the door. There are two types of mortice lock. A Mortice sash lock is a deadbolt, operated by a key, usually housed in the same mechanism as the door handle. A mortice deadlock, or Yale lock has a bolt with one rounded edge and one straight edge, meaning it will slide into its housing when the door is closed and be unable to be opened without using a key. This type of lock can also be deadlocked from the inside for additional security.

It is not appropriate to fit a mortice lock on a narrower door. For this type of door, you will need to use a rim lock which is mounted on the surface of the door. Whichever type of lock you choose, be sure to check it meets BS3621 or kitemark standards. This ensures the lock meets the required security standards and may even be insisted on by your insurer.

2. Make your mark

When installing a mortice lock, first mark the position where the lock will be inserted. The manufacturer may provide a template; if not, use the lock itself to mark the mortice’s position on the door. Don’t fit the lock at the rail joint, as this will weaken the door. Instead, place the lock about half way up the door.

Next, set a marking gauge to half the thickness of the door and score a line down the centre of the door edge.

3. Make the recess

Along the centre line, drill holes to the same width as the lock, creating an opening all the way up the middle of your marked area. Then chisel out the space, a little at a time to create a rectangular hole for the lock.

Insert the lock in the hole and mark out the face place. Then chisel out a shallow recess so that the face plate sits flush to the edge of the door, then mark and fix the screw holes.

Finally, use the manufacturer’s template or the lock itself to mark the position of the keyhole. Then, using a drill and small-bladed saw, cut the keyhole into the door. Check the keyhole for fit on both sides of the door, then install the lock and screw the face plate into place.

4. Install the striking plate

With the lock bolt out, close the door and use the bolt to mark the position and depth of the striking plate. Then repeat the above process to make a recess for the striking plate on the inside of the door frame. With the plate in place, check the door closes. If there is resistance, chisel out a slightly deeper recess until the face plates of both the lock and the bolt box are far enough apart to allow the door to close properly. Screw the strike plate into place.

5. Final checks

As with all DIY projects, you’ll want to check that it works before you dust your hands and walk away from it. If you are experiencing resistance in the bolt when locking the door, it may mean the bolt and bolt box are slightly out of line. Check the alignment and make any necessary adjustments.