Strip Foundation Depth, Width & Concrete Volume Estimator — 2026 Specifications & Pricing
The design and construction of building foundations in the United Kingdom is governed primarily by Approved Document A — Structure of the Building Regulations, supported by BS 8103-1 (Structural design of low-rise buildings — Code of practice for stability, site investigation, foundations and ground floor slabs for housing), Eurocode 7 (BS EN 1997 — Geotechnical design), and NHBC Standards Chapter 4.2 for new residential construction. Together, these documents set out the minimum acceptable foundation depths, widths, concrete specifications, and construction standards for all building work requiring building control approval in England and Wales.
In 2026, the requirements for building regulations foundations in England and Wales are set out in the 2013 edition of Approved Document A (Structure), which remains current, supplemented by local planning authority and NHBC requirements where applicable. Scotland operates under the Technical Handbooks, and Northern Ireland under its own Building Regulations — the fundamental engineering principles are the same, but specific dimensional requirements may differ. All new buildings, extensions, conversions, and significant material alterations require building control approval, and foundation design is one of the first elements scrutinised by building control surveyors.
The purpose of a foundation is to transmit the loads from the structure above — dead loads from the building's own weight, imposed loads from occupants and contents, and in some cases wind and snow loads — to competent bearing strata in the ground below, without causing settlement that could damage the structure or its services. Getting the foundation design right is critical: an over-designed foundation wastes money through unnecessary excavation and concrete; an under-designed foundation risks differential settlement, cracking, and structural failure — all of which are extremely expensive and disruptive to rectify after construction is complete.
The minimum depth for strip foundations under UK Building Regulations is set by several factors acting simultaneously. Approved Document A specifies that foundations must be taken down to a depth at which the ground has adequate bearing capacity to support the structural loads, and that this depth must also be sufficient to avoid frost heave — the expansion of the ground during freezing weather that can lift and crack a shallow foundation.
The minimum foundation depth for frost protection is 450mm below finished ground level for most of the UK — sufficient to place the base of the foundation below the depth at which ground freezing can cause significant expansion. In exposed or cold locations, 600mm is sometimes required. However, in practice, bearing capacity requirements almost always dictate a greater depth than frost protection alone, so the 450mm minimum is rarely the governing criterion except on rock or very dense gravel where bearing capacity is achieved at shallow depth.
For the most common UK ground condition — stiff clay — Approved Document A Table 10 specifies a minimum strip foundation depth of 1,000mm for two-storey buildings in the absence of trees, increasing to 900mm for single-storey buildings. These are minimum values under Table 10 which applies only to domestic buildings of not more than three storeys using conventional cavity wall construction, and they represent the minimum building regulations requirement — not necessarily the optimum engineering solution for a specific site.
Where shrinkable clay soils are present and trees are within influence distance, NHBC Standards Chapter 4.2 requires significantly deeper foundations — potentially 1,500mm, 2,000mm, 2,500mm, or even greater depending on the tree species, height, proximity, and the plasticity index of the clay. These enhanced depths are required because tree roots extract moisture from clay soils, causing them to shrink seasonally — and when trees are removed, the clay swells back, causing heave that can push up and crack shallow foundations. The NHBC Zones of Influence for different tree species are one of the most complex aspects of UK domestic foundation design, and a structural engineer should always be involved where significant trees are present or were recently removed.
The minimum width of a strip foundation under UK Building Regulations is determined by the wall loading and the bearing capacity of the ground. Approved Document A Table 10 sets out minimum foundation widths for standard domestic construction based on the total load from the structure above and the type of ground. For standard two-storey domestic cavity wall construction on stiff clay, the minimum strip width is typically 450mm — but wider strips are required on softer or more compressible ground.
The structural principle governing strip foundation width is that the foundation must spread the wall load over a sufficient area of ground that the bearing pressure on the soil does not exceed the allowable bearing capacity. The allowable bearing capacity of the ground — expressed in kilonewtons per square metre (kN/m²) — varies enormously between soil types: from over 600 kN/m² for rock to as little as 25–50 kN/m² for soft clay or loose fill. This is why foundation widths must increase significantly on poorer ground — more base area is needed to spread the same load over soil with lower bearing capacity.
The concrete projection beyond the wall face on each side of a strip foundation must be at least equal to the concrete thickness — this is the 45-degree rule from Approved Document A, which ensures that the load disperses through the concrete at an angle no steeper than 45 degrees to avoid tensile cracking in the concrete. For a standard 300mm wall on a 600mm wide foundation, each projection is 150mm, and the minimum concrete thickness must also be at least 150mm. In practice, a minimum concrete thickness of 225mm is standard for domestic strip foundations to provide adequate robustness and workability during pouring.
The concrete used in strip foundations must meet the requirements of BS 8500-1 and the structural design specification. For standard domestic strip foundations not subject to aggressive ground conditions, a minimum concrete designation of GEN3 (C20/25 in Eurocode notation) is required — equivalent to a 1:2:4 site-mixed concrete or a proprietary GEN3 ready-mix. For foundations in aggressive ground containing sulfates at ACEC class AC-1 or above, sulfate-resisting Portland cement (SRPC) or Portland Limestone cement with GGBS additions must be used to prevent long-term sulfate attack on the concrete.
In 2026, most domestic strip foundations in the UK use ready-mixed concrete rather than site-batched mixes, for consistency of quality and ease of compliance with building control requirements. Ready-mix is ordered by truck (typically 6m³ per load) and poured directly into the excavated trench. For small volume pours — typically under 2m³ — bagged dry-mix concrete may be more economical despite the higher per-m³ cost. In 2026, ready-mix GEN3 concrete costs approximately £125–£160 per m³ delivered, depending on location and volume. Mini-mix deliveries (less than 1m³) cost significantly more per m³ — typically £175–£250 — but save the cost of unused ready-mix on very small pours.
The concrete must be placed within 2 hours of leaving the batching plant and must not be allowed to segregate during pouring. Foundations must not be poured in standing water — pump out any groundwater before pouring. In cold weather (below 5°C), anti-freeze admixtures or insulated formwork may be required to protect the fresh concrete from frost damage during curing. Building control inspectors typically inspect the open foundation trench before the concrete is poured — never pour without an inspection if your building control requires it, as this will result in a requirement to expose the foundation again at significant cost and delay.
Professional Resources 2026: Approved Document A at GOV.UK Approved Documents | NHBC Standards at NHBC.co.uk | Building control at LABC Building Control | Planning at Planning Portal UK
Understanding the ground conditions on your site is the single most important factor in foundation design. The UK has an exceptionally varied geology — within the space of a few kilometres, ground conditions can vary from dense gravel and chalk to soft alluvial clay and made ground, each requiring a completely different foundation approach. The following ground types are most commonly encountered in UK domestic construction:
Stiff clay is the most prevalent ground condition across much of lowland England, particularly in the Midlands, South East, and East Anglia. Stiff clay has adequate bearing capacity for most domestic construction (75–150 kN/m²) but is highly shrinkable — it loses moisture in dry summers and to tree roots, causing significant ground movement. This shrinkability makes tree proximity the dominant factor in foundation depth design on clay sites, and the combination of stiff shrinkable clay with established trees is the most common cause of significant foundation design challenges on UK domestic projects.
Sand and gravel are generally excellent foundation materials with high bearing capacity (100–600 kN/m² depending on density and grading), low compressibility, and no shrinkability. Foundations on dense sand and gravel can often be shallower than on clay — typically 600–750mm — and tree proximity has little effect on foundation depth because sandy soils do not shrink when dried. The main risks on sandy soils are loose or waterlogged conditions and the risk of undermining by groundwater movement.
Rock — including chalk, limestone, sandstone, and granite — offers the highest bearing capacities (200 kN/m² to over 1,000 kN/m²) and the shallowest foundation depths. Where competent rock is encountered at shallow depth, foundations can sometimes be reduced to 300–450mm — but the rock must be sound and not weathered, fractured, or dissolved (as chalk can be in certain locations). Careful inspection of the rock condition by a geotechnical engineer is advisable before specifying very shallow foundations on rock.
Made ground — any ground that has been artificially deposited, including demolition rubble, ash, domestic waste, or general fill — is the most challenging foundation condition. Made ground has highly variable and often very poor bearing capacity, is frequently contaminated, and can compress significantly under load as it consolidates over time. Foundation design on made ground almost always requires a structural engineer and may involve piled foundations, ground improvement, or deep excavation through the made ground to competent natural strata below.
The influence of trees on foundation depth on clay soils is one of the most important and often underestimated factors in UK domestic foundation design. Tree roots extract moisture from clay soils, causing them to shrink and settle — and when trees are removed or die, the clay progressively re-absorbs moisture and swells, potentially pushing up foundations from below. The NHBC Standards Chapter 4.2 provides detailed tables of minimum foundation depths for different tree species, heights, and proximity to the foundation.
For high water demand species such as poplar, willow, and oak, the zone of influence extends to distances of up to 1.5 times the mature tree height from the trunk — for a mature oak of 20m height, this gives a zone of influence of up to 30m. Within this zone, on shrinkable clay, the minimum foundation depth may need to be increased to 1,500mm, 2,000mm, or even greater depending on the specific species, tree condition, and soil plasticity. Where trees have recently been removed, the ground may be in a process of re-wetting and swelling — in such cases, NHBC Chapter 4.2 requires even deeper foundations or a period of monitoring before construction.
In 2026, the most common approach to tree-influenced foundation design is either to increase the strip foundation depth to the required NHBC depth, or to use an engineered solution such as trench fill foundations (concrete filled to within 150mm of the surface), bored pile foundations with a reinforced ground beam, or raft foundations designed to accommodate the anticipated ground movement without cracking. The choice between these solutions depends on the severity of the tree influence, the structural loading, and the relative cost of deep excavation versus piled alternatives.
The two main types of concrete foundation used in UK domestic construction are the traditional strip foundation and the trench fill foundation. Both create a continuous strip of concrete under load-bearing walls, but they differ significantly in their construction method, cost profile, and suitability for different ground conditions.
A traditional strip foundation uses a relatively wide, relatively shallow concrete pad — typically 450–600mm wide and 225–300mm thick — at the base of the excavation, with blockwork or brickwork built up from the top of the concrete to the DPC level. This approach uses less concrete than trench fill but requires more labour for the brickwork below ground and is more vulnerable to frost and ground movement during construction.
A trench fill foundation fills the excavated trench with concrete from the base to within 150mm of the finished ground level — eliminating the need for below-ground brickwork entirely. Trench fill uses significantly more concrete (typically 2–3 times the volume of a traditional strip) but reduces labour costs, speeds up construction, and is particularly well-suited to unstable or waterlogged trenches where bricklaying below ground is impractical. In shrinkable clay soils, the smooth concrete sides of a trench fill foundation also reduce the surface area available for clay adhesion, minimising the uplift force from swelling clay on the sides of the foundation.
The cost of building foundations in the UK in 2026 depends on the foundation type, depth, width, soil conditions, and regional labour rates. For standard strip foundations on good ground conditions, expect the following approximate costs in 2026:
Excavation by machine (mini-digger for residential) costs approximately £35–£45 per m³ of excavated material including loading and disposal to skip. Skip hire for soil disposal costs approximately £250–£380 per 8-yard skip in 2026, with each skip holding approximately 4–5 tonnes of compacted soil. For deeper foundations or larger footprints, a larger tracked excavator will be required, costing approximately £350–£600 per day including operator.
Ready-mix concrete (GEN3 / C20/25) costs approximately £125–£160 per m³ delivered in 2026. A standard two-storey house with 40m perimeter foundation, 600mm wide and 300mm deep, requires approximately 7.2m³ of concrete — costing approximately £900–£1,150. The same footprint on clay at 1.0m depth with 600mm width requires approximately 24m³ — costing approximately £3,000–£3,840. Depth has a dramatic effect on both concrete volume and cost.
Total installed costs — excavation, concrete, and blinding — for standard domestic strip foundations in 2026 typically range from £180–£380 per linear metre of foundation run depending on depth, width, and ground conditions. A standard two-storey detached house with a 40m foundation perimeter would therefore have a total foundation cost in the range of £7,200–£15,200 before the below-DPC brickwork and oversite concrete are added.
The following table sets out the minimum foundation depths from Approved Document A Table 10 for standard domestic construction (not more than three storeys, conventional cavity wall). These are absolute minimums — site conditions, trees, and engineering judgement will frequently require greater depths.
| Ground Type | Bearing Capacity | Min. Depth (1–2 storey) | Min. Depth (3 storey) | Min. Width (2 storey, typical) | Notes |
|---|---|---|---|---|---|
| Rock / very dense gravel | 600+ kN/m² | 450mm | 450mm | 450mm | Frost protection governs — check rock quality |
| Dense gravel / compact sand | 200–600 kN/m² | 500mm | 600mm | 450mm | Check for loose zones / water table |
| Medium dense sand | 100–200 kN/m² | 600mm | 750mm | 500mm | Water table and loose zones are key risks |
| Stiff clay | 75–150 kN/m² | 900mm–1,000mm | 1,000mm–1,200mm | 450–600mm | Most common UK soil — increase for trees |
| Firm clay | 50–75 kN/m² | 1,000mm | 1,200mm | 600–750mm | Greater depth / width may be needed |
| Soft clay | 25–50 kN/m² | 1,200mm+ | 1,500mm+ | 750mm–900mm | Structural engineer required — consider piling |
| Made ground / fill / peat | <25 kN/m² | Specialist design | Specialist design | Specialist design | Structural engineer and SI essential |
Source: Approved Document A Table 10 (2026). These are minimum values only. Site investigation and structural engineer confirmation required for all projects. Tree proximity on clay soils will increase required depths significantly — see NHBC Chapter 4.2.
On shrinkable clay soils, the presence of trees within the zone of influence is the most common reason for increased foundation depths above the Approved Document A minimums. The table below gives indicative minimum foundation depths for common UK tree species at various distances from the foundation. Always use the full NHBC Chapter 4.2 tables and confirm with a structural engineer on real projects.
| Tree Species | Water Demand | Zone of Influence | Min. Depth at 3m | Min. Depth at 6m | Min. Depth at 10m |
|---|---|---|---|---|---|
| Poplar / Willow | Very high | Up to 35m | 2,500mm+ | 2,000mm | 1,500mm |
| Oak / Elm | High | Up to 30m | 2,500mm+ | 1,800mm | 1,500mm |
| Ash / Sycamore | High | Up to 20m | 2,000mm | 1,500mm | 1,000mm |
| Lime / Beech / Plane | Medium–High | Up to 15m | 1,500mm | 1,200mm | 1,000mm |
| Cherry / Apple / Pear | Medium | Up to 10m | 1,200mm | 1,000mm | N/A (outside zone) |
| Birch / Holly | Low–Medium | Up to 7.5m | 1,000mm | N/A | N/A |
| Conifers / Pine | Low | Up to 6m | 1,000mm | N/A | N/A |
Source: Indicative values from NHBC Standards Chapter 4.2. Depths assume high plasticity clay (PI > 40%). Lower plasticity clays require less depth. Confirm with the full NHBC tables and a structural engineer. If trees have been recently removed, additional depth or monitoring may be required for clay rehydration / heave.
| Foundation Type | Typical Depth | Best Ground Condition | 2026 Cost (supply + install) | Key Advantage | Limitation |
|---|---|---|---|---|---|
| Strip foundation (traditional) | 600–1,200mm | Stiff clay, gravel, sand | £180–£280/m run | Simple, familiar, well understood | Labour intensive below ground; slower |
| Trench fill foundation | 600–1,500mm | Clay, poor ground, waterlogged | £220–£380/m run | Fast, no below-ground brickwork | More concrete — higher material cost |
| Wide strip / pad foundation | 450–900mm | Soft ground, large column loads | £250–£450/m² pad | Spreads load over large area | Requires reinforcement — SE needed |
| Raft foundation | 300–600mm slab depth | Variable / poor ground | £85–£160/m² slab area | Works on variable ground; stiff and robust | Requires structural engineer; complex |
| Bored pile + ground beam | 3,000–12,000mm pile | Trees / very poor / made ground | £800–£2,500/pile | Bypasses poor ground; tree influence | Most expensive; specialist contractor |
| Screw pile + ground beam | 2,000–8,000mm | Soft / waterlogged / restricted access | £600–£1,800/pile | Fast installation; no spoil; low vibration | Not suitable for all soil types |
| Ground Condition | Aggressivity Class | Concrete Designation | Min. Cement Content | Max. w/c Ratio | Notes |
|---|---|---|---|---|---|
| Normal (low sulfate) | AC-1 | GEN3 / C20/25 | 240 kg/m³ | 0.65 | Standard domestic — most common |
| Slightly aggressive | AC-2 | FND2 / C25/30 | 280 kg/m³ | 0.60 | SRPC or PC+GGBS blend required |
| Moderately aggressive | AC-3 | FND3 / C28/35 | 320 kg/m³ | 0.55 | SRPC + GGBS or PC+FA blend |
| Highly aggressive | AC-4 | FND4 / C32/40 | 360 kg/m³ | 0.45 | Specialist advice — contaminated land |
| Very highly aggressive | AC-5 | Special mix | 400+ kg/m³ | 0.40 | Structural engineer and chemist required |
Source: BS 8500-1:2023 and BS 8103-1. Ground aggressivity class must be confirmed by a chemical analysis of ground and groundwater samples. GEN3 is suitable for the majority of UK domestic strip foundations on normal ground. Ready-mix concrete suppliers can advise on the correct designation for your ground conditions once aggressivity class is known.
Building Regulations require foundations to be a minimum 450mm deep to protect against frost heave across most of the UK. In practice, bearing capacity and tree influence almost always govern — requiring greater depths.
On stiff clay — the most common UK soil — Approved Document A Table 10 requires a minimum foundation depth of 900–1,000mm for two-storey domestic buildings in the absence of trees. This is the starting point, not the final answer.
Foundation trenches must be inspected by a building control surveyor before concrete is poured. Never pour concrete before the inspection — failure to comply can result in a requirement to expose the foundation again, causing major delay and cost.
The minimum concrete specification for standard domestic strip foundations is GEN3 (C20/25) under BS 8500. Aggressive ground conditions — sulfates, acids, made ground — require stronger designated mixes (FND2–FND4) with SRPC or blended cements.
The concrete projection beyond the wall on each side must be at least equal to the concrete thickness — the 45-degree rule. For a 600mm wide strip under a 200mm wall, each projection is 200mm and the concrete must be at least 200mm thick.
High water demand trees (oak, poplar, willow) within 10m of a foundation on clay can require depths of 1,500–2,500mm — potentially more than double the Approved Document A minimum. NHBC Chapter 4.2 tables govern.
Trench fill foundations use 2–3× more concrete than traditional strip foundations but eliminate all below-ground brickwork, speeding construction significantly and reducing labour costs. The total installed cost is often comparable to traditional strip.
No calculator can replace a site investigation. Trial pits or boreholes to confirm soil type, depth to bearing strata, groundwater level, and sulfate content are essential for any project. SI costs typically £500–£3,000 for domestic projects.
Ready-mix GEN3 concrete costs approximately £125–£160 per m³ delivered in 2026 — up around 5–8% from 2024. Mini-mix (under 1m³) costs £175–£250/m³. Always budget for a 10% over-order to avoid running short mid-pour.
Approved Document A specifies that strip foundation concrete must be at least 225mm thick (or equal to the projection, whichever is greater). In practice, 300mm is standard for two-storey domestic construction to provide adequate robustness.
Machine excavation and disposal of foundation soil costs approximately £35–£45 per m³ in 2026, including loading to skip. Skip hire for soil disposal costs £250–£380 per 8-yard skip. Deep foundations on clay generate large volumes of spoil.
All UK building foundations must comply with Approved Document A (Structure) of the Building Regulations, supported by BS 8103-1 for domestic buildings and Eurocode 7 (BS EN 1997) for engineered foundation designs requiring structural calculations.
The minimum foundation depth under UK Building Regulations depends on the soil type, the structural loading, and whether trees are present. The absolute minimum depth for frost protection is 450mm below finished ground level across most of the UK. However, on the most common UK ground condition — stiff clay — Approved Document A Table 10 requires a minimum depth of 900–1,000mm for two-storey domestic buildings even without trees. Where trees are present on clay soils, NHBC Chapter 4.2 may require depths of 1,500mm, 2,000mm, 2,500mm or more depending on tree species and proximity. Always confirm foundation depth with a structural engineer and your building control surveyor for your specific site.
Minimum strip foundation widths under Approved Document A depend on the wall loading and ground bearing capacity. For standard two-storey domestic cavity wall construction on stiff clay, a minimum width of 450–600mm is typically required. On firmer ground with higher bearing capacity, narrower widths may be acceptable; on softer ground, wider strips are needed to spread the load over a greater area. The concrete must project beyond the wall face by at least the concrete thickness on each side (the 45-degree rule). In practice, 600mm is the most common strip foundation width for two-storey domestic construction in the UK.
The minimum concrete specification for standard domestic strip foundations under UK Building Regulations is GEN3 (C20/25 in Eurocode notation, equivalent to a 1:2:4 mix by volume). This applies to normal ground conditions without aggressive sulfates or acids. Where a site investigation reveals aggressive ground conditions (sulfate classes AC-2 to AC-5), designated concrete mixes FND2–FND4 using sulfate-resisting Portland cement (SRPC) or blended cements must be used. Always tell your ready-mix supplier the ground aggressivity class — they will specify the correct concrete designation and cement type. In 2026, GEN3 ready-mix costs approximately £125–£160 per m³ delivered.
For standard domestic strip foundations within the parameters of Approved Document A Table 10 — standard cavity wall construction, up to three storeys, normal ground conditions, no significant trees — building control typically accepts the Approved Document A deemed-to-satisfy provisions without requiring separate structural calculations. However, a structural engineer should be engaged where: the ground conditions are unusual (soft clay, made ground, high water table, peat, contaminated land); significant trees are present on clay soils; the building loading is unusual; a raft, piled, or other non-standard foundation is required; or the building control surveyor requests additional structural evidence. The cost of a structural engineer (typically £800–£3,000 for domestic foundation design) is always worthwhile compared to the risk and cost of an inadequate foundation.
Both are continuous concrete foundations running under load-bearing walls, but they differ in how the trench is filled. A traditional strip foundation uses a relatively wide, shallow concrete pad at the trench base — typically 600mm wide and 225–300mm deep — with blockwork or brickwork built up from the concrete surface to DPC level. A trench fill foundation fills the entire trench with concrete from the base to within 150mm of finished ground level, eliminating all below-ground masonry. Trench fill uses more concrete but reduces labour, speeds construction, and is better suited to unstable trenches and shrinkable clay soils where the smooth concrete sides minimise clay adhesion and upward heave forces.
On shrinkable clay soils — the most common condition in lowland England — tree roots extract moisture from the clay, causing it to shrink and settle. This seasonal shrinkage and swelling creates ground movement that can crack shallow foundations. NHBC Chapter 4.2 requires increased foundation depths within the zone of influence of trees, which extends to distances of up to 1–1.5 times the mature tree height from the trunk. High water demand species such as oak, elm, poplar, and willow have the greatest influence. If trees are present within 15–30m of a proposed foundation on clay, always consult a structural engineer and the full NHBC Chapter 4.2 tables before specifying foundation depth. Recently removed trees can also cause foundation heave as the clay re-absorbs moisture — in some cases requiring even deeper foundations than living trees.
Strip foundation costs in 2026 range from approximately £180–£380 per linear metre of foundation run, including excavation, concrete, and disposal of spoil, but excluding below-DPC brickwork and oversite concrete. The main variables are depth (deeper foundations require more excavation and concrete), width (wider foundations use more concrete), and ground conditions (waterlogged or unstable trenches require additional temporary works). A standard two-storey detached house with 40m of foundation perimeter at 1.0m depth and 600mm width would generate approximately 24m³ of concrete (£3,000–£3,840) and approximately 24m³ of excavation (£840–£1,080), plus spoil disposal — giving a total material cost of approximately £5,000–£7,000 before labour.
Pouring concrete before a building control inspection of the foundation trenches is one of the most serious mistakes you can make on a construction project. Building control surveyors have the legal authority to require you to expose any covered work for inspection — including breaking out and removing freshly poured concrete from foundation trenches. This results in significant additional cost, delay, and waste. Always notify your building control body when the foundation trenches are ready for inspection and wait for written confirmation that they are satisfied before pouring. The inspection typically happens within 24–48 hours of notification for most local authority building control offices.
Yes — a raft foundation is a reinforced concrete slab spanning the entire footprint of the building, rather than running only under the load-bearing walls. Rafts are used where ground conditions are variable or poor — soft clay, made ground, mining areas, and sites where differential settlement is a risk — because the stiff slab can bridge over weaker spots and distribute loads more evenly than strip foundations. Rafts require structural engineer design and are more expensive than standard strip foundations, but they can be the most economical solution on challenging ground where deep strip or piled foundations would otherwise be necessary. In 2026, raft foundations cost approximately £85–£160 per m² of slab area installed, depending on thickness and reinforcement specification.
Yes — a site investigation is strongly recommended and frequently required by building control and structural engineers before confirming foundation design. Without site investigation data, foundation design relies on conservative assumptions that may result in unnecessarily expensive foundations, or on optimistic assumptions that result in inadequate foundations. A basic domestic site investigation — typically two or three trial pits excavated by machine — costs approximately £500–£1,500 and provides information on soil type, depth to bearing strata, groundwater level, and samples for laboratory sulfate analysis. This investment is always justified compared to the risk of specifying foundations that are too shallow, too narrow, or constructed in the wrong concrete grade for the ground conditions.
Approved Document A requires that the concrete in a strip foundation is at least 225mm thick, or equal to the projection of the concrete beyond the wall on each side — whichever is greater. In practice, 300mm is the standard minimum for two-storey domestic construction, providing adequate robustness for working in and pouring concrete without risk of the concrete being disturbed by foot traffic in the trench. For trench fill foundations, the concrete fills to within 150mm of finished ground level, so the concrete thickness equals the excavation depth minus 150mm.
Building control services in England and Wales are provided by either your local authority's building control department (LABC) or by a private approved inspector. You can find your local authority building control at LABC.co.uk. Approved inspectors are listed by the Construction Industry Council (CIC). Both local authority and approved inspectors have equal powers under the Building Regulations and both are authorised to inspect and certify foundation work. For most domestic projects, local authority building control is the most straightforward option — fees for a standard two-storey house extension are typically £600–£1,400 including the full inspection schedule.
Additional Resources 2026: Approved Document A at GOV.UK | NHBC Standards at NHBC.co.uk | Building control at LABC.co.uk | Structural engineers at IStructE | Geotechnical advice at Geological Society | Planning at Planning Portal UK
© 2026 Building Regs Foundation Calculator UK. All indicative depths and widths are based on Approved Document A (Structure) and BS 8103-1. Foundation design must be confirmed by a structural engineer and approved by your building control authority based on a site-specific ground investigation. Never use a calculator as a substitute for professional foundation design.