Screw piles have an image problem. Ask most people what they’re used for and you’ll hear garden rooms, decking, lightweight timber frames. The perception is that helical pile foundations are fine for small structures but not up to the demands of serious masonry construction, and certainly not a two-storey brick and block extension.
That perception is wrong and usually based on a confusion with ground screws and it’s costing homeowners on projects where screw piles would perform just as well.
Where the Assumption Comes From
The confusion is understandable. Screw piles are associated with speed and low disruption. In construction, those qualities often get conflated with lightness or compromise. Many people also confuse screw/helical piles with ground screws 0 a smaller, lighter load fastening usually used for sheds/decking.
The reality is that load capacity is a function of pile design, not installation method. A two-storey extension screw pile foundation is engineered to a specific working load before a single pile goes in the ground. Diameter, shaft length, helix plates, and depth are all calculated by a structural engineer based on what the structure needs to carry. The installation process then verifies that capacity through torque monitoring on every pile.
What a Two-Storey Extension Actually Weighs
This is where the numbers matter. A typical single-storey rear extension in brick and block might impose a load of 30 to 50 kN per pile position, depending on span and wall thickness. Add a second storey: masonry walls, floor joists, ceiling, roof structure, and that figure can rise to 80 to 150 kN per pile position. On longer spans or beneath loadbearing walls, it can go higher.
The GoliathTech system installed by Preformed Substructures is rated to working loads of up to 300 kN per pile. That exceeds what most two-storey domestic extensions demand at any single pile position.
How the Screw Pile Foundation Design Changes for Two Storeys
The pile spec for a two-storey extension screw pile foundation differs from a single-storey. The structural engineer will typically:
Increase pile diameter where needed. Larger shafts carry higher compression loads, particularly beneath loadbearing walls or point loads from steel beams.
Add helix plates. More plates increase the bearing area and raise the working load per pile without needing extra depth.
Tighten pile spacing. A closer grid spreads the load more evenly and reduces the span the ground beam has to bridge.
Upsize the ground beam. The beams spanning between pile heads must be designed for the loads from two storeys above. This is standard structural design. The beams are sized to suit.
What the Structural Engineer Needs
The engineer needs:
Full architectural drawings. Floor plans, elevations, and a section showing wall build-up, floor construction, and roof structure. Without these, loads cannot be calculated.
Ground information. A ground investigation report, or at least trial pit data. Depth and pile diameter cannot be fixed without knowing what is beneath the surface. This applies on London Clay sites, chalk, made ground, and most other conditions across South East England.
Construction details. The loads depend on whether you are building in traditional masonry, timber frame, or a hybrid system. Each imposes different forces on the foundation.
Once the engineer has these inputs, they produce a pile layout drawing: positions, diameters, target depths, and ground beam sizes.
Restricted Access: A Major Advantage of Screw Piling
Most homeowners choose screw piles for single-storey extensions partly because of access. Concrete foundations need excavators, trucks, and pump hire: none of which fit through a standard side gate.
For a two-storey extension the foundation load increases. The access problem does not. The piles still go in through an 800mm gap. The equipment is the same. Installation still takes one to three days.
That matters because two-storey extensions are often planned on exactly the properties where access is tightest: terraced houses, urban infill plots, homes in London, Surrey, and Kent where a side gate is the only way to the rear garden. Screw piles remain the only piled foundation that works on those sites without road closures or heavy logistics.
What Screw Piles Cannot Do
There are two-storey projects where screw piles are not the right call.
Very high point loads. Where a steel frame puts very large loads at individual column bases, a larger or deeper pile may not be practical for the site. This is rare on domestic extensions but can occur on commercial projects.
No competent bearing stratum at a workable depth. If ground investigation shows stable strata at 8m or more, the economics shift. This is uncommon across most of SE England in the UK, but can occur on sites with deep-made ground or complex geology.
Unknown existing foundations. Where the extension adjoins a building with unknown foundations, the interaction between old and new structure needs careful engineering regardless of the foundation system chosen.
Planning a Two-Storey Extension in the South East of England? Get a Free Quote
No excavation, no concrete, no 28-day curing period. Installation takes one to three days and the foundation is ready to build off the morning the team leaves. For most two-storey extension screw pile foundation projects in London and the South East, that is the proposition.
The GoliathTech system is NHBC accepted and ISO9001 compliant. Preformed Substructures has over 18 years of specialist groundworks experience and covers London, Surrey, Kent, Hertfordshire, East Sussex, and West Sussex.
If you’re planning a two-storey extension and have been told, or assumed, that screw piles are not suitable, get in touch. We’ll tell you whether they’re right for your site before you commit to anything.
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