The crack usually runs vertically, sometimes with a slight step through the mortar joints, exactly where the new extension wall meets the original house. It often opens wider in summer and closes a little over winter, or the other way round, depending on the ground beneath.
Homeowners tend to assume the worst the first time they see it, but a crack at this specific location is common enough that it has a name among builders: the junction crack. The question that actually matters is whether it is stable or still moving.
What you're actually seeing
This is a crack at the specific line where two structures of different age meet, not a random crack in the middle of a wall. That location matters because two different buildings, built years apart, will always behave slightly differently underground.
The original house has foundations that settled decades ago and reached equilibrium. The extension's foundations are newer and may still be settling, even slightly, as the ground around them adjusts to the new load.
Symptom versus cause
The crack is the symptom. It tells you that something is moving relative to something else, but not what. The same visual symptom can come from several different underlying causes, and they call for very different responses.
Don't treat the crack itself as the problem to be fixed. Filling it and redecorating without understanding why it opened just buys you a repeat performance next year, usually along the same line.
Causes, in order of likelihood
Most junction cracks come down to one of a small number of causes, and they are worth ruling in or out in order.
- Differential settlement — the extension's newer foundation settling slightly against the older, already-settled original wall
- No movement joint provided where the two structures meet, so normal seasonal expansion and contraction has nowhere to go
- A rigid tie between the old and new brickwork that doesn't allow for the different ages moving independently
- Seasonal clay shrinkage and swelling beneath one or both foundations, particularly on Wealden clay
How to tell the causes apart
Measure the crack width at a couple of points and note the date. Check it again after a dry summer spell and after a wet winter. If it opens and closes with the seasons and returns close to its starting width, that points to seasonal movement rather than ongoing settlement.
Look at how the extension was tied into the original wall — whether there's a proper movement joint or whether the two skins were bonded solid. A solid tie with no joint is a common cause and is straightforward to identify from outside.
Ask what's known about the foundation depths on each side. An extension built to a shallower foundation than the original house is more likely to show ongoing settlement rather than settling and stopping.
What a survey should record
A useful record for this fault includes the crack's width measured at two or three points, the date, and a note of whether it runs dead vertical or has a stepped path through the mortar joints, since a stepped crack often follows differential settlement while a clean vertical line more often follows a missing movement joint.
It's also worth photographing the junction from outside at the same time each visit, so any change in width or extension of the crack's length is visible in a direct comparison rather than relying on memory.
Common questions
Should I fill the crack now or wait?+
Wait and monitor first if the crack has only just appeared. Filling it immediately hides the evidence you need to judge whether it's still moving, and a repair applied before the movement has been understood usually needs doing again.
Is this always a sign of subsidence?+
No. Subsidence describes a specific process where the ground beneath a foundation loses bearing capacity, and it's less common than ordinary differential settlement or a missing movement joint at a junction. The symptoms can look similar from the crack alone, which is exactly why monitoring matters.
How long should I monitor before deciding it's a problem?+
A full year is the useful benchmark, because it lets you see the crack through both a dry and a wet season. A crack that keeps widening past that point, rather than closing back up, needs proper investigation rather than more waiting.
Book a free site visit for a junction crack assessment
We'll look at the crack pattern, check how the extension was tied in, and give you a written, itemised view of what — if anything — needs doing.