The best concrete for most foundations is C25 or C30 ready mix. These two grades cover the vast majority of domestic and small commercial foundation work in the UK, from house extensions to new build ploT
C25 is generally used for standard strip and trench fill foundations on firm, stable ground. C30 is used where the ground is less predictable, where the foundation needs to carry more load, or where an engineer has specified a higher strength for peace of mind. Both are widely available from ready mix suppliers and both meet the requirements set out in building regulations for typical domestic foundations.
Choosing the correct concrete grade matters because foundations carry the entire weight of the building above them. Get the grade wrong, or skimp on quantity, and the result can be cracking, uneven settlement, or a foundation that simply is not strong enough for the loads placed on it. Concrete that is too weak for the ground conditions can fail over time, while concrete that is unnecessarily strong adds cost without adding benefit.
Durability matters just as much as strength. Foundations sit below ground, exposed to moisture, ground movement and, in some cases, sulfates in the soil. The right grade, mixed and poured correctly, gives a foundation that will perform for the lifetime of the building. This guide explains the grades used, how they are chosen for different foundation types, and what affects depth, quantity and cost.
Foundation concrete is graded by strength, with the number after the C indicating the compressive strength in megapascals once the concrete has cured. The higher the number, the stronger and more load-bearing the mix. For most foundation work, three grades cover almost every situation.
C25 is the standard grade for straightforward foundations on good, stable ground. If you are building a single-storey extension on a site with no unusual ground conditions, no nearby trees and no engineer’s report flagging anything out of the ordinary, C25 is usually the specification you will see on the drawings.
A typical example is a small rear extension to a semi-detached house, where the strip foundation sits on firm clay or sandy ground at a standard depth. C25 gives more than enough strength for the loads involved and is the most cost-effective choice.
C30 is used where slightly more strength or durability is needed. This might be because the ground is less consistent, because the foundation is taking a heavier load, or because the building inspector or structural engineer has specified it as a precaution. C30 is also common in trench fill foundations, where the concrete is in the ground for longer and benefits from the extra strength margin.
A typical example is a two-storey extension where the additional floor adds load, or a garage built on ground that has shown some signs of softness during excavation. Many ready mix suppliers will recommend C30 as a safe default when there is any doubt about ground conditions, since the cost difference between C25 and C30 is usually small relative to the benefit.
C35 is a higher strength mix used less often in straightforward domestic work, but it becomes necessary in a few specific situations. The most common is sulfate-resisting concrete for sites where soil testing has found high sulfate levels, since sulfates can attack standard concrete over time. C35 is also used for raft foundations and pad foundations carrying heavier point loads, such as steel structural posts or a garage built to take a car lift or heavy machinery.
A typical example is a new build on a site where a soil report has identified sulfates in the ground, or a self-build project using a raft foundation across the full footprint of the house. In these cases the extra strength is not optional, it is what the engineer’s specification calls for.
In every case, the right grade depends on ground conditions, the load the foundation will carry and any specific risks identified by a soil survey or structural engineer. If you are working from approved drawings, the concrete grade will already be specified. If you are unsure, it is always worth checking with a structural engineer or your ready mix supplier before ordering.
A strip foundation is a continuous strip of concrete laid in a trench, following the line of the walls above. It is the most common foundation type for domestic extensions and low-rise housing on firm, stable ground. The trench is excavated to the required depth, often with a layer of brickwork or blockwork built up from the concrete strip to ground level.
C25 concrete is the standard choice for strip foundations on good ground. Where ground conditions are mixed or the building inspector wants extra assurance, C30 is used instead. Strip foundations work well because they use less concrete than filling the whole trench, while still giving a solid, level base to build from.
Trench fill foundations are used when the trench needs to be filled with concrete almost to ground level, rather than leaving room for brickwork below ground. This is common when foundations need to be deeper than usual, for example near trees with thirsty root systems, on clay soils prone to shrinking and swelling, or where the ground simply needs more depth to reach stable bearing ground.
Because trench fill foundations use significantly more concrete than strip foundations, getting the grade right matters for cost as well as strength. C25 is used on stable ground, while C30 is more common where the depth has increased specifically because of poor or variable ground conditions. Trench fill is often quicker to build than strip foundations, since there is no blockwork to lay below ground, but it does mean ordering a larger volume of concrete in one pour.
A raft foundation is a single slab of concrete that spreads the load of the whole building across its full footprint, rather than concentrating load along strip lines. Rafts are used where the ground is too soft or inconsistent for strip or trench fill foundations to perform reliably, such as on made-up ground, soft clay, or sites with a history of subsidence.
Raft foundations require stronger concrete, typically C30 or C35, because the slab needs to resist bending and cracking across its full span. They are almost always reinforced with steel mesh or rebar, since the concrete alone cannot resist the tension forces involved when ground conditions vary beneath the slab. A structural engineer will specify both the grade and the reinforcement layout, and this is not a foundation type to build without engineer input.
Pad foundations are isolated blocks of concrete, usually square or rectangular, used to support a single point load rather than a continuous wall. They are common under steel posts, conservatory corner posts, garage support columns and extensions built using a steel frame.
Grade depends on the load. A small pad supporting a lightweight conservatory post might use C25, while a pad supporting a structural steel column in a larger extension could need C30 or C35. Because pad foundations carry concentrated loads in a small area, the size and depth of the pad matter just as much as the concrete grade, and both should come from the engineer’s calculations rather than a standard rule of thumb.
Foundation Type | Typical Concrete Grade | Typical Application |
|---|---|---|
Strip foundations | C25 | Domestic extensions, garden walls, low-rise housing on stable ground |
Trench fill foundations | C25 or C30 | Deeper trenches, clay soils, sites near trees |
Raft foundations | C30 or C35 | Soft or unstable ground, new builds spreading load over a wide area |
Pad foundations | C25 to C35 | Point loads such as steel posts, conservatories, garages, extensions |
Foundations with sulfate-rich ground | C35 (sulfate-resisting mix) | Sites where soil testing shows high sulfate content |
Concrete strength matters because foundations are not just sitting under load, they are also resisting ground movement, moisture and, in some cases, chemical attack from the soil. A foundation that is too weak for its job can crack, settle unevenly, or fail to spread load properly, which shows up later as cracks in walls or doors and windows that stick.
Higher grades are needed when ground conditions are poor or variable, when the building carries more load than a typical single-storey extension, when soil testing shows sulfates present, or when the foundation type itself demands it, as with rafts and heavily loaded pads. In most straightforward domestic projects, though, C25 or C30 will be exactly what is specified on the drawings, and there is rarely a need to go higher unless an engineer says so.
It is also worth saying that more strength is not automatically better. Specifying C35 for a foundation that only needs C25 adds cost without adding any real benefit, since the extra strength is never used. The right approach is to match the grade to the ground conditions and load, not to over-specify for the sake of caution.
Foundation concrete volume is calculated using three measurements: length, width and depth, all converted to the same unit (usually metres) and multiplied together to give a volume in cubic metres.
Length × Width × Depth = Volume in m³
For strip and trench fill foundations, length is the total run of trench, width is the trench width (matching the wall thickness plus working space, typically 450mm to 600mm), and depth is how deep the trench has been dug or how deep the concrete fill needs to be.
A typical single-storey rear extension might have 18 metres of trench at 0.45m wide and 0.6m deep for the foundation strip. That works out to 18 × 0.45 × 0.6, which comes to roughly 4.9m³ of concrete. Most suppliers will recommend ordering slightly more than the calculated figure to allow for uneven trench bases and minor over-excavation.
A detached single garage with a footprint of 6m by 3m might need around 18 metres of strip foundation trench at 0.45m wide and 0.5m deep, giving roughly 4m³. If the garage is built using trench fill due to softer ground, the same trench filled to a greater depth, say 0.9m, would need closer to 7.3m³.
Getting falls right at the laying stage is easy. Correcting them after the mortar has set means taking everything up and starting again.
A new build house with a more complex footprint, say 40 metres of trench fill foundation at 0.6m wide and 1m deep, would need around 24m³ of concrete. Larger projects like this are usually broken into separate pours and ordered in stages to match the build programme, rather than delivered all at once.
Because trench depths and widths often vary slightly across a real site, most builders and homeowners use an online concrete calculator to get a more accurate estimate once the trenches have actually been dug. Suppliers will also generally accept a phone measurement and confirm a recommended order quantity, including a reasonable allowance for spillage and uneven ground.
There is no single depth that applies to every foundation. Depth is determined by ground conditions, local building control requirements and, in many cases, a structural engineer’s calculations based on a site-specific soil report.
As a general guide, most domestic strip foundations in the UK are dug to a minimum of 1 metre deep, though this varies considerably depending on the factors below.
Foundations need to reach a layer of ground that can bear the load without excessive settlement. On firm, well-drained ground this might be reached at a shallow depth. On soft, made-up or variable ground, foundations often need to go significantly deeper, or a different foundation type such as a raft may be more appropriate altogether.
Clay is particularly sensitive to moisture. It shrinks in dry conditions and swells when wet, which can cause shallow foundations to move seasonally. On clay sites, foundations are typically dug deeper than the standard minimum, often 1m to 1.5m or more, to get below the zone where seasonal moisture changes have the biggest effect.
Trees draw large amounts of moisture from the ground through their roots, which can dry out clay soils for a significant distance around the trunk. The closer a foundation is to a mature tree, particularly species like oak, willow or poplar, the deeper it generally needs to go. Building control guidance includes standard tables relating tree species, distance and soil type to minimum foundation depth, and a soil report will usually confirm what applies on a specific site.
Building control will not approve foundations based on guesswork. Depth needs to be sufficient to reach safe bearing ground, to get below the frost line, and to account for any nearby trees or drainage. In practice, this means the building inspector or a structural engineer signs off the foundation depth before work proceeds, based on what is found when trial holes are dug.
Heavier buildings, multi-storey extensions, or foundations supporting significant point loads may need to go deeper, or wider, regardless of soil type, simply to spread the load adequately. This is where an engineer’s design takes over from general guidance, since the calculation depends on the specific loads involved.
In short, foundation depth should be confirmed on site, not assumed from a generic figure. If you are unsure, a soil survey and a chat with a structural engineer before digging will save problems later.
Most slab failures are preventable. These are the measures that separate a job that lasts 20 years from one that needs relaying in three.
Using a lower grade than the ground conditions or load require is one of the most serious mistakes, since it can lead to cracking or settlement that only becomes visible once the building is finished. Equally, over-specifying a higher grade than needed wastes money without adding any benefit. The grade should match the engineer’s specification or the building control requirement, not a guess.
Trenches are rarely perfectly uniform. Running short partway through a pour means a delay while more concrete is ordered, and a cold joint can form where the first batch starts to set before the second arrives. This weakens the foundation at that point. Most experienced builders add a reasonable allowance on top of the calculated volume to avoid this, and confirm quantities with the supplier before the pour date.
If a trench base is not level, has loose soil at the bottom, or has not been properly compacted, the concrete is not bearing on stable ground as intended. This can lead to uneven settlement later. Trenches should be cleared of loose material and checked for level before any concrete is ordered, since fixing this after the pour is far harder than before.
Where a structural engineer has specified a particular grade, depth or reinforcement detail, that specification exists because the site has something about it that needs addressing, whether that is poor ground, a heavy load or a sulfate risk. Substituting a different grade or skipping reinforcement to save cost removes the safety margin the engineer built in, and can also cause problems with building control sign off.
Concrete begins setting from the moment it is mixed. If a pour is interrupted for too long, whether due to access problems, a breakdown, or simply misjudging how long the pour will take, the concrete can start to set unevenly, creating weak points or visible joints in the finished foundation. Planning access, confirming delivery times and having enough labour on site to keep the pour moving all help avoid this.
Ready mix concrete is batched at a plant to an exact specification and delivered to site ready to pour, rather than being mixed by hand or in a small mixer on site. For foundation work, this brings several practical advantages over site-mixed concrete.
Consistent strength: every batch is mixed to the same specification, so the grade ordered is the grade delivered, without the variation that comes from manual mixing ratios on site.
Reduced waste: ready mix is ordered to the volume needed for the pour, rather than mixing in small batches that can lead to over- or under-ordering materials.
Faster installation: a ready mix lorry can discharge several cubic metres in minutes, allowing a foundation to be poured in one continuous operation rather than across a full day of site mixing.
Accurate quantities: suppliers calculate the order based on the measurements provided, removing the guesswork involved in estimating bags of cement, sand and aggregate.
Better quality control: batching plants test and record the mix design, giving a paper trail of the concrete’s specification, which can matter for building control or warranty purposes.
For most foundation pours, particularly anything beyond a very small job, ready mix is both quicker and more reliable than mixing on site, and the cost difference is often smaller than people expect once labour and wastage are taken into account.
How concrete gets from the supplier to the trench depends on the size of the job, site access and how the pour is being managed.
Standard ready mix delivery uses a lorry that batches concrete at the depot and brings it to site ready to pour, usually within around two hours of leaving the plant. This works well where the lorry can get close enough to the trench to discharge directly or via chutes.
Volumetric lorries carry separate raw materials and mix the concrete on site as it is needed. This is useful for jobs where the exact quantity is uncertain until the trench is fully excavated, since the mixer can produce only what is required rather than a fixed pre-batched volume.
Where the trench is not accessible to a delivery lorry, for example behind a house with no rear access, a concrete pump can move concrete from the lorry on the road to the trench via a hose. This adds cost but solves access problems that would otherwise rule out ready mix delivery.
Access Considerations
Before ordering, it is worth checking whether a lorry can physically reach the pour location, whether the ground can support the weight of a loaded lorry, and whether there is space to manoeuvre. Tight site entrances, soft ground or overhead obstructions are common reasons pumps or smaller delivery vehicles end up being needed.
For substantial pours, such as a full new build raft or a long run of trench fill, deliveries are often split into multiple loads timed to arrive in sequence, keeping the pour continuous without one oversized delivery overwhelming the site team’s ability to place it before it begins to set.
C25 and C30 ready mix concrete are the best choices for most domestic foundations in the UK. C25 suits standard ground conditions, while C30 is used where ground is more variable or load is greater. The right choice ultimately depends on ground conditions and any engineer’s specification for the site.
Yes, C25 is suitable for most standard strip and trench fill foundations on firm, stable ground. It is the most common grade used for single-storey extensions and similar domestic projects. Where ground conditions are weaker or the load is higher, C30 or above may be specified instead.
Most house extensions use C25 concrete for strip foundations on good ground, or C30 where ground conditions are more variable or the extension has an additional storey. The exact specification will usually be confirmed on the approved building drawings.
Most domestic strip foundations are at least 1 metre deep, though this varies with ground conditions, nearby trees and soil type. Clay soils and proximity to trees often require greater depth. Building control will confirm the required depth once trial holes are dug on site.
Volume is calculated by multiplying trench length, width and depth in metres to get cubic metres. A typical single-storey extension might need around 4 to 5m³, while a new build could need 20m³ or more. A concrete calculator or supplier quote will give an accurate figure based on actual trench measurements.
Most domestic foundations can be poured in a single day, often in a few hours once the concrete lorry arrives. Larger or more complex foundations, such as a full raft slab, may need a longer pour or multiple deliveries timed to keep the pour continuous.
It depends on the foundation type. Strip and trench fill foundations on good ground often do not need reinforcement, while raft foundations and pad foundations carrying point loads usually do, in the form of steel mesh or rebar. An engineer’s specification will confirm what is required for a specific site.
Strip foundations fill only the base of the trench, with blockwork built up to ground level above. Trench fill foundations fill the trench with concrete almost to ground level. Trench fill is generally used for deeper trenches, such as those needed for clay soils or proximity to trees.
Concrete typically reaches enough strength to build on within about 7 days, though full curing strength takes around 28 days. Work can often continue on top of the foundation well before the 28 day mark, depending on the load being placed on it.
Light rain after pouring is not usually a problem, but heavy rain before or during a pour can wash out cement paste and weaken the mix, and standing water in the trench should be removed before concrete is placed. Pours are best avoided during heavy or prolonged rainfall.
Choosing the right concrete for a foundation comes down to matching the grade to the ground conditions, the load the building will place on it, and any specific risks identified on site, such as sulfates or nearby trees. For the large majority of domestic projects, C25 and C30 ready mix concrete remain the most commonly used grades, covering everything from small extensions to new build housing.
Getting the foundation right also depends on accurate quantities, proper excavation, the correct depth for the ground type, and a pour that runs without unnecessary delay. Ready mix concrete, delivered by lorry or pump depending on site access, remains the most reliable way to achieve consistent strength and quality across the whole foundation.
If you are unsure which grade or quantity applies to your project, it is always worth checking with a structural engineer or your ready mix supplier before ordering. Foundations are not the place to guess, and a short conversation before the pour is far cheaper than fixing a problem afterwards.
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