Domestic Concrete Suppliers
Concrete for Home Extensions
Concrete plays a critical role in the success of any house extension. As the primary material used for foundations, concrete must safely transfer the loads of the new structure into the ground while resisting movement, moisture, frost, and long-term settlement. Choosing the correct concrete type, foundation depth, and placement method is essential to meet Building Regulations and ensure the extension remains structurally sound for decades.
This guide explains the key considerations when using concrete for house extensions, including recommended concrete strengths, foundation depths, soil conditions, reinforcement, pumping options, costs, and practical construction tips.
Domestic Concrete Suppliers
Concrete for Home Extensions
Concrete plays a critical role in the success of any house extension. As the primary material used for foundations, concrete must safely transfer the loads of the new structure into the ground while resisting movement, moisture, frost, and long-term settlement. Choosing the correct concrete type, foundation depth, and placement method is essential to meet Building Regulations and ensure the extension remains structurally sound for decades.
This guide explains the key considerations when using concrete for house extensions, including recommended concrete strengths, foundation depths, soil conditions, reinforcement, pumping options, costs, and practical construction tips.
The Role of Concrete in House Extensions
In domestic construction, concrete is most commonly used for strip foundations, trench fill foundations, ground-bearing slabs, and oversite floors. For extensions, foundations are particularly important because they must accommodate both the load of the new structure and the interface with the existing building.
Concrete offers several advantages:
- High compressive strength to support structural loads
- Durability under ground conditions
- Resistance to water and frost when correctly specified
- Compatibility with reinforcement for added tensile strength
The exact concrete specification required will depend on factors such as the size of the extension, number of storeys, soil type, proximity to trees, and guidance from Building Control or a structural engineer.
Work out how much concrete you need
Extension Concrete Calculator
Use our free extension concrete calculator to work out how much concrete you need, and order it.
Start by entering your measurements for depth, length, and width.
Choose if you need a concrete pump or additional time to transport the concrete from the delivery truck to where you need it by wheelbarrow.
Then Click the button at the bottom of the calculator to check your delivery postcode, and follow the instructions.
Concrete for Extensions
Recommended Concrete Types
Concrete is specified in the UK using strength classes such as C25/30 or C35/45. These figures refer to the characteristic compressive strength of the concrete measured in Newtons per square millimetre.
Concrete for Extensions
Foundation Depth Requirements
Foundation depth is influenced by soil type, ground stability, frost protection, and the presence of trees or nearby structures. Building Control will typically inspect foundation trenches before concrete is poured to confirm compliance.
Standard Ground Conditions
For most domestic extensions on stable ground, foundations are typically excavated to a depth of 600–900 mm. This depth protects the foundation from frost damage and provides an adequate bearing surface for the structure.
Clay Soils
In clay conditions, foundation trenches usually need to be deeper—often 900–1,200 mm. Clay expands and contracts significantly depending on moisture levels, so deeper foundations reduce the risk of movement and cracking.
Near Trees
Where extensions are built close to trees or hedgerows, foundation depths may increase substantially. Trees extract moisture from the soil, increasing shrinkage in clay ground. In these cases, depths of 1,000–2,500 mm may be required, depending on tree species and distance from the foundations.
Concrete for Extensions
Foundation Width and Trench Geometry
In addition to depth, the width of foundations is also regulated. Typical strip or trench fill foundations for domestic extensions range between 600–1,200 mm wide, depending on load, wall thickness, and soil bearing capacity.
Excavated trenches should be level, free from loose material, and not left exposed for long periods, especially in poor weather. Any soft spots identified at the base of the trench should be removed and made good before concrete placement.
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Concrete for Extensions
Cost Considerations
Concrete costs vary depending on location, supplier, specification, and delivery method. As a general guide, ready-mix concrete for domestic extensions typically costs between £100 and £130 per cubic metre, excluding installation.
Additional costs may include:
- Concrete pump hire
- Reinforcement materials
- Excavation and disposal of spoil
- Labour for placing and finishing
Where a builder provides an installed price, this will vary based on the complexity of the works and overall project scope.
Good Practice and DIY Considerations
Although concrete placement is usually carried out by professionals, homeowners and DIY self-builders should still understand best practice:
- Follow all Building Control guidance and approved drawings
- Ensure trenches are clean, dry, and correctly formed before pouring
- Place concrete promptly after delivery to prevent segregation
- Protect freshly poured concrete during curing, especially in cold or hot weather
Proper curing allows concrete to achieve its designed strength and reduces the risk of surface cracking or long-term durability issues.
Concrete is one of the most important materials in any house extension project. Selecting the correct concrete strength, foundation depth, and construction method ensures compliance with regulations and protects the long-term stability of the building. By understanding soil conditions, reinforcement needs, access constraints, and costs, homeowners and builders can avoid common pitfalls and deliver a durable, high-quality extension.
Above all, specifications should always be confirmed with Building Control and relevant professionals before ordering and placing concrete, as early decisions at foundation stage are critical and difficult to correct later.
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Domestic DIY Project FAQs
Frequently Asked Questions about Extensions
Yes. Building Control normally inspects foundation trenches before concrete is poured to confirm correct depth, width, and ground conditions. Pouring concrete without approval can lead to compliance issues and may require costly remedial work later.
As a general guide, ready-mix concrete for domestic extensions typically costs between £125 and £170 +vat per cubic metre, excluding labour and groundwork. Additional costs may include pump hire, reinforcement, excavation, and spoil removal, depending on site conditions and access.
Yes. Concrete pumping, particularly using a line pump, is very common for house extensions where access is limited. Pumping allows faster placement, reduced manual labour, improved safety, and a cleaner pour. It is especially useful for rear extensions where wheelbarrow access is difficult.
Not always. Many domestic extensions use mass concrete foundations without reinforcement. However, reinforcement may be required where ground conditions are uncertain, loads are higher, or Building Control or a structural engineer specifies it. Common forms include rebar cages in trench fill foundations and steel mesh in ground-bearing slabs.
Yes. Clay soils are prone to shrinkage and expansion, which increases the risk of ground movement. As a result, higher-strength concrete such as C35 is often required, along with deeper foundations, to improve durability and reduce the likelihood of cracking or settlement.
Foundation depth depends on soil type, ground conditions, and nearby trees. On stable ground, foundations are typically 600–900 mm deep. In clay soils, this often increases to 900–1,200 mm. Where trees are present, depths may extend beyond 1,000 mm and in some cases up to 2,500 mm, depending on tree species and distance.
For most domestic house extensions, C25 concrete is commonly used for single-storey extensions on good ground conditions. C35 concrete is often specified for two-storey extensions or where foundations are formed in clay soils. In deeper or more demanding situations, C40 concrete may be required, particularly for trench fill foundations or where specified by a structural engineer or Building Control.
