Do I actually need underpinning, or is it just settlement?

Settlement or subsidence?

Every house settles. In the first few years after construction the ground compresses slightly under the new load, the frame dries out and the building beds in, usually evenly, usually by a few millimetres, and then it stops. That is settlement: normal, uniform and finished business. Subsidence is different. Part of the footing keeps moving while the rest stays put, and that ongoing differential movement is what cracks brickwork, jams doors and tilts floors, because the structure is being bent rather than gently lowered.

So the practical question is never "is there a crack?" It is "is the movement uniform and finished, or uneven and ongoing?" A stable old crack from initial settlement needs paint. Progressive differential movement needs a diagnosis.

Seasonal clay movement

Much of urban Australia, including large parts of Melbourne, Adelaide and western Sydney, sits on reactive clay that swells when wet and shrinks when dry. Houses on these soils breathe with the seasons: cracks open in late summer once the clay has dried and shrunk, then close again over winter as it rehydrates. If your cracks follow that annual rhythm and return to roughly the same width each year, you are probably looking at seasonal movement. That is usually managed rather than underpinned, with drainage repairs, sensible garden watering near the footings and attention to thirsty trees.

Why engineers say monitor first

Ask an independent structural engineer about modest cracking and there is a good chance they will recommend six to twelve months of monitoring before any structural work. That is not fence-sitting. It is the only way to capture a full seasonal cycle and separate clay breathing from genuine progressive subsidence. Our guide to reading and monitoring cracks in brick walls covers the routine: a photo with a coin for scale, dated pencil marks, repeated monthly. As a rule, cracks under 1 mm are cosmetic, 1–5 mm justify the monitoring, and anything over 5 mm or clearly worsening justifies bringing an engineer in sooner.

The overselling problem

Underpinning is an industry where the person diagnosing your house is often the person selling the cure. A free inspection from a company that only installs resin injection will, with remarkable reliability, conclude that you need resin injection. The same goes for screw-pile specialists and mass-concrete crews. None of this makes them dishonest; a hammer sees nails. But it does make a free diagnosis worth roughly what you paid for it, and it is why the independent report exists as a step in its own right.

The decision sequence

What actually justifies underpinning is narrow: progressive differential movement, with an identified cause, that cannot be fixed any cheaper way. Getting to that answer follows a sequence.

• Monitor. Track the cracks monthly across a season or two, unless movement is obviously rapid or a crack has passed 5 mm.
• Get the engineer. An independent report identifies the movement pattern, the cause and the severity, and says whether action is needed at all.
• Fix the cause. Leaking stormwater or sewer pipes, poor site drainage and large trees near footings drive a large share of movement. Fixing them costs a fraction of structural work, and movement often stops once soil moisture stabilises.
• Underpin only if it keeps moving. If movement continues after the cause is addressed, choose a method on the engineer's advice and budget from our costs guide; most jobs land between $10,000 and $40,000.

Skipping straight from "I found a crack" to "sign here" is how people spend $25,000 underpinning a house whose real problem was a $2,000 drainage fix. Start with the full list of subsidence signs, and let the evidence lead.