Guide
When do foundations need piles? Clay, trees and bad ground
Updated
Nobody piles for fun, you pile because the ground within reach of a digger cannot be trusted to hold the building still. Here are the situations that put piles on your drawings, and who decides.
The usual suspects
- Shrinkable clay with trees nearby. Clay swells and shrinks with moisture, and tree roots dry it out seasonally. Foundations near trees in clay must go deep enough to sit below the zone the roots influence, on heavy clay with a thirsty species close by, that can be deeper than a trench can sensibly go, and piles take over.
- Made-up or filled ground. Old ponds, infilled cellars, demolition fill, former tips: fill settles under load and a strip footing settles with it. Piles push through to natural stratum.
- Soft ground and high water tables. Peat, soft silts and waterlogged ground either can't carry the load or make open trenches collapse; a piled foundation bypasses both problems.
- Drains, sewers and voids. Building over or near a public sewer often restricts traditional excavation, and piles either side with a bridging beam are the standard answer, agreed with the water company through a build-over agreement.
- Sloping or restricted sites where stepping a trench down a slope or getting deep excavation plant in simply doesn't work.
Who actually decides
- The soil investigation: trial pits or boreholes establish what the ground actually is and how deep stable stratum sits. On small jobs this is often a digger-dug trial pit inspected by building control or an engineer.
- The structural engineer designs the foundation for what the investigation found, trench fill if the numbers work shallow, piles if they don't. Pile design is engineering, not guesswork: depths come from ground bearing data, not habit.
- Building control signs off the foundation design and will condition deeper or piled foundations if a trench inspection shows bad ground, which is exactly the mid-dig surprise that piling rescues.
The cost consequence
Piles cost more than trench fill on good ground, mini piling runs £250–£450 per metre of depth per pile plus £1,500–£3,000 mobilisation (typical 2026 quotes), so a typical piled extension lands at £5,000–£15,000 for the piling package. But on bad ground the comparison flips: a trench chasing stable clay past 2.5 m deep eats concrete, spoil and shoring fast, see piling vs traditional foundations. If a tree survey or trial pit has put piles on your drawings, price them properly via the form rather than hoping the trench gets lucky.