In short: a new extension is joined to the existing house either by toothing in (cutting new bricks into the old wall) or, more commonly today, with a stainless-steel wall-starter and tie system fixed to the existing wall. A vertical damp-proof course, and a cavity tray where the new roof meets the house, keep water out at the join. It is building-control work.
Extending a house means the new brickwork has to connect to the wall that is already there. Get it right and you cannot tell where the old stops and the new begins. Get it wrong and you get cracks, damp, or a join that looks like an afterthought. Here is how it is done, in plain terms.
Two ways to make the connection
Toothing in
The traditional method: bricks are cut out of the existing wall in a stepped pattern and the new bricks are bonded into the gaps, so old and new are physically interlocked. It gives a strong, seamless bond and is often the right choice on period and heritage brickwork. The downsides are that it is slow, dusty, and any mistake shows on the existing wall.
Wall-starter (tie) systems
The modern standard for most extensions. A stainless-steel vertical channel is fixed to the existing wall and special ties clip into it as each course of new brickwork is laid. It is quicker, far less disruptive to the existing wall, and many systems are designed to allow a little vertical movement so the join does not crack as the new foundations settle.
Wall ties and spacing
The two leaves of a cavity wall, and the join to the existing house, are held together with wall ties. Under UK building regulations these are normally spaced at 900mm horizontally and 450mm vertically in a staggered pattern, and closed up to every 300mm vertically around openings and at the junction with the existing wall, where the loads concentrate. Ties should be stainless steel to BS EN 845 and embedded at least 50mm into each leaf.
Keeping water out at the join
The join is the most likely place for damp to get in, so two details matter:
- A vertical damp-proof course where the new wall meets the existing one, stopping water tracking across the join.
- A cavity tray with weep holes where the new (usually lower) roof abuts the existing house wall. The tray catches any water in the cavity above the roofline and directs it back out.
The main damp-proof course, like the rest of the house, sits at least 150mm above the finished ground level.
Allowing for movement
New foundations always settle a fraction, and new mortar shrinks slightly as it cures. A good tie-in either allows for that small vertical movement or includes a movement joint, so the new work can move independently of the old without pulling cracks through the join. This is the single most common reason DIY or rushed extensions crack at the corner.
Foundations at the junction
The new foundation must not undermine the existing one, and where they meet it may need to be stepped or deepened. Your structural engineer and building control decide the detail. See our guide on what an extension costs for how the groundworks fit into the wider budget.
Who checks it
Joining an extension to your home is notifiable work. Building control (your council or an approved inspector) inspects the tie-in, the damp-proofing and the cavity trays as the work goes up — so it is worth using a bricklayer who builds to that standard as a matter of course.
Matching the brick at the join
Because the eye goes straight to the corner where new meets old, brick matching matters most right at the join. Selecting the closest brick, bond and mortar colour is what makes an extension look original rather than bolted on.
Frequently asked questions
Do you tooth in or use wall ties to join an extension?
Both work, but most extensions today are joined with a stainless-steel wall-starter system: a vertical channel is fixed to the existing wall and ties clip into it as the new brickwork rises. Toothing in (cutting bricks into the existing wall) is still used, especially on period brickwork, but it is slower and more disruptive.
Will the join between the extension and the house crack?
A small amount of hairline movement is normal as new foundations settle. It is minimised by using ties that allow slight vertical movement, a proper vertical damp-proof course, and getting the foundations right. Poor tying-in or skipping a movement allowance is what causes visible cracks.
Do I need building regulations approval to join an extension to my house?
Yes. The connection to the existing wall, the damp-proofing and any cavity trays are all inspected under building regulations, whether or not you need planning permission.
Can you match the brick where the extension joins the house?
Yes, and it matters most right at the join. We select brick, bond and mortar to blend with the existing wall so the extension reads as part of the original house rather than an add-on.
Getting a price for your extension
Every extension is different, so the brickwork is priced after we have seen the job. If you are planning an extension in Hampshire, Berkshire or Surrey, see our extension work or send us the details and we will come and look.