The short answer
They can be, especially for a light timber, SIPs or modular extension on awkward ground. Steel screw piles are wound into the soil in a day, with little digging and no concrete wait, and they cope with clay and nearby trees. They need a structural engineer's design and building control approval, and on firm, easy ground ordinary strip foundations are usually cheaper.
On this page
Every extension needs foundations, including one built in a factory. The usual choices are a trench filled with concrete (strip foundations), a reinforced concrete slab (a raft) or piles. Screw piles are a type of pile: galvanised steel shafts with a helix near the tip, wound into the ground by a small machine until they reach a set resistance.
How screw piles work
Each pile is screwed down like a giant woodscrew. The torque needed to turn it tells the installer how much load the soil will carry, so the pile goes as deep as it has to, often two to six metres. A steel or timber ring beam then sits on the pile caps and the extension floor is built on that.
Because there is little digging, there is little spoil to take away. There is no concrete to pour and no curing time, so the frame or module can go on the next day.
When they make sense
| Situation | Why screw piles help |
|---|---|
| Shrinkable clay | They reach stable ground below the zone that swells and shrinks |
| Trees nearby | They pass below roots and reduce the depth problem of trench foundations near trees |
| Poor access | A compact rig fits through a side gate where a digger and skip cannot |
| Drains in the way | Piles can be placed either side of a drain run |
| Light structures | Timber frame, SIPs and modules are light enough to suit a pile-and-beam base |
| Tight programme | No concrete cure, so the base is ready when the factory delivers |
When strip foundations are better
On firm, level ground with no trees nearby, ordinary strip foundations are often the cheapest choice. Published UK builder price guides put them at about £1,000 to £3,000 for a single-storey extension, with rafts at £3,000 to £6,000 and piles typically from about £6,000. Screw piles can come in below conventional piles, but on good ground they rarely beat a simple trench.
Bear in mind that masonry walls, heavy brick slip cladding or a two-storey build put more load on the base, and the engineer may prefer concrete.
Building control and design
Screw piles are a structural design, not a product you pick from a brochure. A structural engineer should specify the pile type, spacing, minimum depth and the ring beam, using a soil investigation or at least trial holes. Building control will want to see the design and the installation records, which list the torque reached on each pile. Some building control bodies ask for a load test on one pile.
The joint between the new floor and the house needs care. The house sits on its old foundations and the extension on piles, so the two can move slightly differently. A properly detailed movement joint avoids cracks at the junction.
Neighbours and the Party Wall Act
Screw piles still count as excavation for the Party Wall etc. Act if they go within 3 m of a neighbour’s building and deeper than its foundations, or within 6 m and below a 45 degree line from the bottom of those foundations. Notice must be served at least one month before work starts, and the neighbour has 14 days to respond. The GOV.UK explanatory booklet sets out the process.
Installation is quieter than driven piles, with very little vibration, which neighbours tend to appreciate.
Questions to ask
- Who designed the piles, and can I see the calculations?
- Was a soil investigation done, and how deep were the trial holes?
- What torque must each pile reach, and will I get the records?
- How is the ring beam protected from corrosion and damp?
- How does the floor connect to the house, and what movement joint is used?
- Is the foundation covered by the same warranty as the extension?
Foundations are one of the biggest variables in an extension budget. The cost calculator lets you switch between strip, raft and piled ground to see the effect. When you want prices from a specialist, use the form below.