The foundation transfers building loads safely into the ground. Choosing the wrong type is one of the most expensive mistakes in construction — and one that Building Control will catch before the concrete is poured. In the UK, foundation design falls under Approved Document A of the Building Regulations, which references BS EN 1997-1 (Eurocode 7) and the NHBC Chapter 4 standards for domestic work.
Which type you need depends primarily on ground conditions (soil bearing capacity, presence of trees, made ground), building loads, and depth to firm strata. A soil investigation or at minimum a visual assessment of trial pits is essential before committing to a foundation type.
Strip vs Trench-Fill vs Raft vs Pile
These four types cover the vast majority of domestic and small-scale commercial work in the UK.
| Foundation Type | Typical Use | Min Concrete Grade | Approx Cost (2026) |
|---|---|---|---|
| Strip (traditional) | 2–3 storey domestic on firm ground | C25 | £150–£250/m run |
| Trench-fill | Domestic where labour is costly or clay is present | C25–C30 | £180–£320/m run |
| Raft | Poor/variable ground, extensions on soft fill | C30 | £80–£150/m² |
| Mini-piles + ground beam | Made ground, high trees, very poor bearing capacity | C30–C35 | £4,000–£9,000/pile + ground beam |
Costs include excavation and concrete but exclude any sub-base, DPM or insulation above foundation level.
Strip Foundations
The strip foundation is the UK default for domestic walls on reasonable ground. The strip is a continuous concrete pad running under every load-bearing wall, wider than the wall it carries, distributing load across a greater area of ground.
Typical dimensions:
- Width: wall thickness + 150 mm each side minimum; structural engineer or Building Control may require wider on weak ground
- Depth: minimum 150 mm, but usually 225–300 mm to BS 8004:2015
- Minimum burial depth (top of concrete to ground surface): 450 mm on non-frost-susceptible, non-shrinkable soils; 1,000 mm or more in shrinkable clay (NHBC Chapter 4.2 Table 1 based on tree proximity and water demand)
The standard formula for minimum strip width without calculation is given in Approved Document A Table 10 — for a two-storey house on dense gravel (150 kN/m²), the minimum width is 400 mm; on soft clay (75 kN/m²), it is 850 mm. These are conservative presumptive values; a structural engineer can justify narrower widths using Eurocode 7 calculations.
Concrete volume example: A 20 m perimeter house extension, 300 mm deep × 600 mm wide strip ≈ 3.6 m³ of concrete — a single ready-mix truck (minimum 6 m³ is common but many suppliers will do 3 m³) can handle this.
Trench-Fill Foundations
Trench-fill fills the excavated trench with concrete to within 150 mm of ground level, eliminating blockwork footings below DPC. This is now the more common approach in the UK because:
- Labour costs for bricklaying below ground are high
- It is faster — the trench is backfilled the same day, reducing open-trench time
- It performs better in shrinkable clay because the smooth concrete surface allows vertical movement without snagging
Typical dimensions:
- Width: 600–700 mm for domestic two-storey (trench machine width)
- Depth to bottom of concrete: 1,000 mm minimum in clay (NHBC); up to 2,500 mm near high-water-demand trees (e.g. oak, poplar, willow within 15 m)
Concrete volume example: The same 20 m perimeter at 700 mm wide, 1.0 m deep fill = 14 m³. At £105/m³ (C25 delivered), that is approximately £1,470 in concrete alone — more than double the strip, but the labour saving on below-ground blockwork typically offsets this on most sites.
Trench-fill is specified at C25 minimum; many engineers specify C30 because the large volume generates heat during curing (reducing cold-weather risk) and the mix needs to be workable enough to self-level without vibration.
Raft Foundations
A raft is a reinforced concrete slab covering the entire building footprint, distributing load across a wider area. It is the correct choice where:
- Ground bearing capacity is low or variable (soft silts, made ground, uneven demolition fill)
- Differential settlement risk is high
- A suspended ground floor is needed anyway and combining raft + floor makes structural and economic sense
Typical specification:
- Slab thickness: 250–400 mm for domestic; thickened edge beams (often 500–600 mm deep × 600 mm wide) under load-bearing walls
- Grade: C30 reinforced with A252 mesh as a minimum; often designed with T16 bars at 200 mm centres in both directions in the edge beams
- Insulation: 100 mm PIR or EPS under the slab to achieve Part L U-values (≤0.13 W/m²K for a new dwelling ground floor)
Raft design should always be checked by a structural engineer — the reinforcement layout is critical to preventing punching shear failure and differential cracking. Building Control will require calculations.
Piled Foundations
Piling is specified when the depth to firm strata exceeds the economic limit for trench-fill (roughly 2.5–3.0 m), or where trees make shallow foundations unworkable. Mini-piles (150–300 mm diameter CFA or driven steel tube) are common in domestic work.
Typical domestic piling:
- Pile diameter: 150–250 mm CFA (continuous flight auger)
- Pile spacing: 1.5–2.5 m centres depending on load
- Pile depth: 5–12 m to reach firm stratum (London Clay: often 8–10 m)
- Ground beam: 400–600 mm deep × 300 mm wide RC connecting pile heads
Piling cost varies enormously with depth and rig access. Budget £3,500–£8,000 per pile installed (2026), plus the RC ground beam at roughly £200–£350/m run. A six-pile scheme for a modest rear extension can therefore cost £25,000–£50,000 before the superstructure begins — making geotechnical investigation before design essential.
Depth and Building Regulations
Approved Document A (2004, amended 2013) and its supporting tables give presumptive foundation sizes for domestic work, but these are subject to:
- Frost depth — minimum 450 mm burial to avoid heave in frost (in practice most UK sites are fine at 600 mm if not shrinkable clay)
- Shrinkable clay depth — NHBC Table 4.2-1 cross-references plasticity index and tree proximity; depths range from 0.75 m (low plasticity, no trees) to 2.5 m (high plasticity, high-water-demand tree within 5 m)
- Adjoining structures — foundation bottom must be no higher than any adjoining foundation within 45° influence zone
Building Control will require a trial pit (usually 1.5 m deep, assessed by the SE or BC officer) or a geotechnical report for anything other than straightforward conditions. NHBC Chapter 4.1 specifies the investigation requirements for new homes.
Key Standards
- Approved Document A (Structure) — Sections 1A and 2E for domestic foundations; Tables 10–12 for presumptive widths
- BS EN 1997-1:2004 (Eurocode 7) — geotechnical design; used by structural engineers for calculated designs
- BS 8004:2015 — code of practice for foundations
- NHBC Standards Chapter 4.2 — foundation design for new homes; widely accepted by Building Control
- BRE Special Digest 1 (SD1) — concrete in aggressive ground; defines DS classes DS-1 to DS-4 and recommended concrete mixes
Always commission a structural engineer to design foundations for extensions of any significance — the cost (typically £500–£1,500 for a domestic scheme) is trivial against the risk of under-designed foundations.
