Tying an Extension Into an Existing House
The join between a new extension and the old house is where cracks most often appear. Here is why that happens, and the one decision that prevents it: whether to match the foundations and tie in, or keep the new structure independent.
In short: an old house has long since settled and stopped moving, while a new extension beds in and settles a little as it takes up its load. If the two are rigidly joined but sit on different foundations or different ground, that small difference in movement, called differential movement, concentrates at the junction and cracks it. The fix is a design decision made before you build: either match the existing foundation type and depth and tie the new work in so the two move as one, or make the extension fully independent on its own foundations with a movement joint at the junction. The one thing to avoid is bonding new to old across two different foundation types.
Why the join is where cracks appear
A building is always moving a little. It settles when it is first built, and it swells and shrinks slightly with the seasons as the ground beneath it gains and loses moisture. An old house has been through decades of that and has come to rest. A new extension has not: as it is built and starts carrying its own weight, it settles a small amount, and if it sits on shrinkable clay it will also respond to the seasons in its own way.
On its own, none of that is a problem. The problem is the difference between the two. Where the new and old are locked rigidly together but are settling by different amounts or on different ground, the strain has to go somewhere, and it goes into a crack right along the line where they meet. This is one of the most common patterns we are called to look at on extension projects, and it is nearly always a question of how the junction and the foundations were handled, not a sign that the extension is falling down.
Start with the existing foundations
Before any of that can be designed, you have to know what the existing house is standing on, and you cannot assume it. Older London houses often have surprisingly shallow foundations by modern standards. So the first job is to expose and assess the existing foundations, usually by digging a small inspection pit against the wall, and to confirm they can carry any extra load the extension puts on them. The existing foundations are also protected during the work, because carelessly digging alongside or below them can undermine the very wall you are building onto. Where the existing foundations turn out to be inadequate, they may need underpinning, which is engineer-designed work in its own right.
The key decision: match, or separate
Once the ground and the existing foundations are understood, the extension foundation is designed to do one of two things. This is the decision that makes or breaks the junction.
The principle behind the diagram is simple. To stop the two parts cracking apart, you either make them move together or let them move separately, but you do not do a bit of both:
- Match and tie. Where the ground suits it, the extension is built on the same foundation type and depth as the existing house and bonded into it, so the whole thing behaves as one structure and there is very little difference in movement to crack the junction.
- Separate completely. Where the extension needs a different foundation, for example piles or a much deeper foundation because of clay and trees, the new structure is made completely independent of the house and a movement joint is detailed at the junction. The two are then free to move by different amounts without pulling on each other.
The mistake that causes most junction cracking is rigidly bonding a new extension to the old house when the two sit on different foundation types, so they are tied together but want to move apart.
How much movement is acceptable
Some settlement of new work is normal and unavoidable. The design aim is to keep the difference in movement between old and new small, generally within a few millimetres, of the order of 2 to 3 mm at the junction. Beyond that, finishes, plaster and roof coverings across the join start to suffer. This is why the junction is designed and detailed deliberately rather than simply built tight, and why fine cracking that appears and then stays put in the first year is usually the structure settling in, whereas a crack that keeps widening deserves a look. Our guide to types of cracks in walls explains how to read the difference.
The junction details that matter
Beyond the foundations, a good tie-in gets the smaller details right. New masonry is either properly bonded or tied to the existing wall, or deliberately separated with a joint, to suit the chosen approach. Just as importantly, the damp-proofing is linked up: the new damp-proof course laps the existing one, typically by around 100 mm, so that damp cannot track back from the new work into the old house or the other way round. Getting this wrong trades a structural problem for a damp one.
Extensions, boundaries and the Party Wall Act
On a London terrace or semi, an extension often runs along the boundary with a neighbour, and that brings in the Party Wall etc. Act 1996. If you build up to or along the boundary, tie into a shared party wall, or dig foundations close to a neighbour's building, the work is usually notifiable and you serve notice on the neighbour and agree the works before starting. It applies only where the neighbour owns the adjoining property, so if you own both it does not arise. It is separate from planning permission and Building Regulations, and, as with a chimney breast removal, it is worth starting early because the notice periods take time.
The clay and trees factor
One reason extension foundations so often end up deeper than the existing house is the ground itself. Much of London sits on shrinkable clay, and where there is a tree nearby, a new foundation may need to go well below the depth of the old house's footings to reach stable ground. That is exactly the situation where the extension ends up on a different, deeper foundation and is kept independent. Our guide to building foundations near trees on clay explains how that depth is worked out.
What EMA Structures does
Designing the tie-in is core structural engineering work, and it is exactly what saves an extension from cracking at the join a year later. We inspect the house, expose and assess the existing foundations, check the ground, and design the extension foundation and junction to either match and tie in or stay independent, whichever the site calls for. We produce the calculations and drawings Building Control needs, advise on the party wall steps, and, with our own groundworks team, can carry out the below-ground work as well as design it, so the whole junction is handled by one team.
Planning an extension?
Send us your drawings, or even just a sketch and a photo of where the extension meets the house, along with a note of any nearby trees. Send us the details and a structural engineer will advise on how the foundations and junction should be handled. An inspection with a written report costs £495 + VAT, see our fees guide, correct as of July 2026.
Extension junction questions
Why does my extension crack where it joins the house?
Almost always because the extension and the original house move by slightly different amounts. The old house settled long ago and has stopped, while a new extension beds in and settles a little as it takes up load. If the two are rigidly joined but sit on different foundations or different ground, that small difference in movement concentrates at the junction and opens a crack there. It is usually a detailing issue rather than a sign the extension is failing, but it should be looked at to confirm the cause.
Do extension foundations have to match the existing house?
Not necessarily, but the two have to be thought about together. The reliable approaches are either to use the same foundation type and depth as the existing house and tie the new work in, so the two move as one, or to make the extension structurally independent on its own foundations with a movement joint at the junction, so the two are free to move separately without cracking. What you must avoid is rigidly bonding new work to old on a different foundation type, which is what causes junction cracking.
How deep do extension foundations need to be?
It depends on the ground, not on the depth of the existing house. Older houses often have shallow foundations by modern standards, and on shrinkable clay, especially near trees, a new extension may need to go considerably deeper to reach stable ground. The existing foundations are exposed and assessed, the ground is checked, and the new foundation is designed to suit both. Simply matching the old shallow depth can be the wrong answer on clay.
Do I need a party wall agreement for an extension?
Often yes on a terraced or semi-detached house. If you build up to or along the boundary, tie into a shared party wall, or excavate for foundations close to a neighbour's building, the work is usually notifiable under the Party Wall etc. Act 1996 and you serve notice on the neighbour and agree the works. If you own both properties it does not apply. It is separate from planning permission and Building Regulations approval, and it is worth starting early because the notice periods can add weeks.
How much movement between an extension and the house is normal?
Some small settlement of new work is expected as it beds in. As a general guide, differential settlement at the junction is usually kept within a few millimetres, of the order of 2 to 3 mm, beyond which finishes, roof coverings and the junction detailing can start to suffer. The aim of the design is to keep any difference in movement small and to detail the junction so that the little that does occur does not crack the wall.
Extending, or seeing cracks at the join?
Send us your plans or a photo and we will advise on how the foundations and junction should be handled, before or after the work.