Extension Foundations: Types, Selection, and What to Expect

Strip foundations are the most commonly used foundation type for domestic extensions in the UK — a continuous concrete strip running beneath all load-bearing walls, spreading the wall loads over a greater ground area.

• **How strip foundations work**:
• •A continuous trench is excavated to the required depth and width
• •The trench is filled with concrete (usually C25/30 ready-mixed) to the required depth
• •Masonry blockwork rises from the foundation to ground level, then above ground
• •The strip must be wide enough to spread the load without exceeding the soil bearing capacity

• **Required depth**:
• Building Regulations (Part A) require strip foundations to be taken to a depth below which seasonal ground movement (frost heave, clay shrink-swell) will not affect them:
• •Minimum depth for most soils: 450mm (the frost penetration depth in the UK)
• •For clay soils in London: typically 900mm–1,000mm minimum, to get below the zone of seasonal moisture change in London Clay (which has significant shrink-swell potential)
• •Where trees are within the influential zone: may need to be 2.5–3.0m deep (or deeper) to get below the root influence zone — see the guide on building near trees
• •Building Control inspects and approves the foundation depth before concrete is poured

• **Required width**:
• •For a single-storey extension on low-to-medium bearing capacity soil (such as stiff London Clay): typically 600mm wide for a two-leaf cavity wall
• •Structural engineer to calculate for abnormal loads or poor ground

• **London Clay specific considerations**:
• •London Clay has excellent bearing capacity (150–300 kN/m²) when undisturbed — strip foundations on London Clay generally do not need to be very wide
• •London Clay has high shrink-swell potential (Class H to Very High under NHBC Chapter 4.2) — particularly near trees. This is the primary driver of deep strip foundations in London
• •The critical depth for London Clay in tree-influence zones is determined by the tree species and height

• **London 2025 costs**:
• •Strip foundation excavation and concrete pour (900mm depth, standard conditions): £180–£280/m of wall length
• •Deep strip foundation (1.5–2.5m depth for tree proximity): £350–£600/m, increasing with depth and whether temporary shoring is needed

**Raft foundations**: A raft foundation is a reinforced concrete slab that extends over the full footprint of the extension (or building), spreading the loads over the maximum possible ground area.

• *When a raft is used*:
• •Where the ground has low and variable bearing capacity (made ground, soft fill, poorly consolidated ground)
• •Where differential settlement is a risk — the raft acts as a rigid plate, distributing differential settlement across the whole structure rather than allowing one part of the building to settle more than another
• •Where a continuous insulated ground floor is desired and the soil conditions preclude a suspended beam-and-block floor
• •Where the depth to suitable bearing strata is shallow and strip foundations would not be significantly cheaper than a raft

• *Construction*:
• •Typically a minimum 150–200mm reinforced concrete slab with a thickened toe beam around the perimeter (where the wall loads are greatest)
• •Reinforced with A393 steel mesh (or structurally designed reinforcement for higher loads)
• •DPM and PIR insulation below the slab for thermal performance

• *Limitations*:
• •Rafts spread loads across the full footprint — they are only effective if the ground beneath the full footprint is reasonably uniform. If the ground is soft only at one corner, a raft may not prevent differential settlement in that corner
• •A raft requires the soil to be capable of supporting the total structure weight evenly — on very soft or variable ground, piles may be necessary

*London 2025 costs*: Raft foundation to extension footprint: £150–£250/m² of raft area (including reinforcement, concrete, DPM, and blinding layer)

**Pad foundations**: Pad foundations are isolated square or rectangular concrete pads supporting individual structural columns or posts — used where a steel or timber frame structure (rather than masonry load-bearing walls) is the structural system.

• *When pad foundations are used*:
• •Steel portal frame extensions
• •Open-plan extensions where columns carry the roof loads directly to the ground
• •Garden rooms and outbuildings with a post-and-beam structure

• *Construction*:
• •Square or rectangular reinforced concrete pad (typically 750mm × 750mm × 300mm thick for light loads; larger and deeper for significant column loads)
• •Column base plate or timber post shoe cast in or attached to the pad

*London 2025 costs*: Pad foundation per column: £400–£1,200 depending on size, depth, and reinforcement requirement

Pile foundations are used when the soil near the surface is not capable of carrying the structural loads — the loads must be transferred deeper into the ground through piles to more competent strata, or by skin friction along the length of the pile.

• **When piles are needed for a London extension**:
• •Deep tree root influence zones where strip foundations would need to be 3m+ deep to be below root influence (at which depth piles become more economical than deep strip)
• •Made ground or fill of unknown composition where bearing capacity cannot be confirmed
• •Soft clay, silt, or peat near the surface where bearing capacity is insufficient for a strip foundation
• •Sites adjacent to existing buildings where underpinning risk from traditional excavation is too high (contiguous or secant pile walls are used for basement construction)

• **Mini-piles (the most common residential pile type)**:
• •Small-diameter piles (typically 150–300mm diameter) installed by a small rig that can access through a standard side gate
• •Drilled into the ground and filled with concrete reinforced with a steel cage or central bar
• •Capacity: 30–150 kN per pile depending on diameter and depth
• •Typical depth in London: 6–15m depending on the depth to London Clay beneath any fill or disturbed ground
• •Ground beams: after the piles are installed, reinforced concrete ground beams span between the pile heads, carrying the wall loads from pile to pile

• *London 2025 costs*:
• •Mini-piles (150mm diameter, 6–10m deep): £800–£1,500 per pile (mobilisation + per pile; plus ground beams)
• •A typical single-storey extension with 10–15 piles and associated ground beams: £20,000–£45,000 for the foundation structure alone (significantly more expensive than a standard strip foundation)

• **Screw piles**:
• A proprietary pile type where a steel tube with helical plates is screwed into the ground. Faster than drilled mini-piles and requiring no concrete cure time — the structure can be built immediately after installation. Popular for garden rooms and light structures.
• •Capacity: 30–80 kN per pile typically
• •Cost: £300–£600 per pile (installed), typically more economical than drilled mini-piles for light structures

**Underpinning — not a foundation type for a new extension**: Underpinning is the strengthening of an existing foundation (when it is settling, when new construction nearby increases the load on it, or when excavating adjacent to an existing footing). It is sometimes confused with foundation work for a new extension but serves a different purpose — see the guide on underpinning when it is needed.