Extensions
Extension subsidence
A crack where an extension meets the house usually means the two are moving differently, most often because the extension sits on shallower or weaker foundations than the house. A strip foundation is typically about three times as wide as the wall it carries, and a trial hole beside the wall shows what's really there.
Extensions are often built over old drains, backfilled ground or near trees, on footings that were never as deep as the main house.
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Why does an extension crack away from the house?
Shallower foundations than the house
The house and the extension were built at different times, often by different builders, to different depths. If the extension's footings sit in softer ground near the surface, they move with the seasons while the house stays put, and the joint between them opens.
Old drains under or beside it
Back extensions are often built over or next to the house's original clay drains. A leaking joint washes out the ground under the new footing. See leaking drains and subsidence.
Backfilled or made ground
Ground that was dug out and filled back, or built up, can compress over time. Building on made-up or filled-in ground is also something home policies exclude.
Trees
Soil shrinks as it dries, and a tree close to the extension draws water from it. Cutting down a large tree can make the ground swell instead.
Extra weight
A new roof, or a second storey added later, puts more load on walls whose footings were sized for less.
Is a crack between the extension and the house serious?
It depends on its width and whether it's growing. On the crack scale engineers use, cracks up to about 5 mm are aesthetic, 5 to 25 mm affect weather-tightness and doors and windows, and over 25 mm is a stability issue. A joint that keeps opening matters more than its width today. See subsidence cracks.
- A tapering crack at the joint, wider at the top or the bottom
- Doors or windows in the extension sticking
- The extension floor sloping away from the house
- Gaps opening under skirting boards or along the ceiling line
- Cracks running at about 45 degrees from the corners of windows and doors
How do you find out what's under it?
- Photograph and date the cracks.With a ruler in the shot, every few weeks, from the same spot.
- Get a chartered structural engineer.They read the crack pattern and decide what to investigate.
- Dig a trial hole beside the wall.It shows how deep and wide the footing is and what it sits on.
- Survey the drains.A camera survey shows whether a drain under or beside the extension is leaking.
- Monitor if asked.A crack monitor for six to 12 months shows whether it's still moving.
How is a subsiding extension fixed?
The engineer designs it. The cause gets fixed first, usually the drains. Then the extension's footings are taken down to firm ground:
- Mass concrete: bays dug under the footing and filled, where firm ground isn't far down. See mass concrete underpinning.
- Mini-piles: where firm ground is deep, or access is tight. See mini-piled underpinning.
- Injection: strengthening the ground through small holes, where it suits. See resin injection.
- The joint: once the extension stops moving, the crack is repaired, and the engineer decides how the extension and house are tied together or left to move separately.
Does insurance cover an extension that's subsiding?
Sometimes. Policies cover subsidence as an insured event, less a subsidence excess, but exclude defective design or construction, defective or inappropriate foundations, and building on made-up or filled ground. An extension built on footings that were too shallow can fall under those exclusions. Floor slabs are usually only covered if the foundations under the walls were damaged at the same time. See subsidence insurance.
A neighbour's extension cracked my wall. What now?
Record the damage with dates and talk to them early. For a shared wall, the Land and Conveyancing Law Reform Act 2009 says whoever does works to a party structure must make good any damage they cause. Have an engineer record the wall's condition before work starts next door.
Start with a chartered structural engineer. Nobody can tell from a photo or a phone call whether a house needs underpinning. An engineer looks at the cracks, the ground and the drains, then decides. This site never tells anyone their house is safe or unsafe.
FAQ
Extension subsidence questions
Is it normal for an extension to crack where it joins the house?
A fine crack at the joint can be the two parts settling differently and then stopping. A crack that keeps opening, or is over about 5 mm, needs an engineer.
Can an extension be underpinned without touching the house?
Often, yes. The engineer may design underpinning for the extension's walls only, with mass concrete or mini-piles, once the cause is fixed.
Is it cheaper to knock the extension and rebuild?
Sometimes it's an option the engineer considers. Compare both on the same design and get prices in writing. See our price guide for underpinning.
Will my insurer pay for my extension's foundations?
Only if the damage comes from an insured cause. Defective or inappropriate foundations and made-up ground are common exclusions.
How long before I can repair the crack?
Not until the cause is fixed and movement has stopped. Cracks are usually repaired six months to a year after the works.
Who will ring me?
We pass your request to one experienced groundworks and underpinning contractor who covers your county. Your number isn't sent to a list. They ring you, ask what's happening and about any engineer's report, and price the work from the engineer's design. Nothing is booked until you say yes.
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