What Concrete Do I Need for a House Extension? (UK Guide)

Planning a house extension is a major project, and choosing the correct concrete is one of the most important early decisions. Whether you're building a kitchen diner extension, rear extension, garage conversion or garden room, the right concrete helps ensure the structure remains stable and long-lasting.

Builders and homeowners across East Sussex and South Kent, from Battle and Hastings to Eastbourne, Robertsbridge, Rye, Tunbridge Wells and Paddock Wood, regularly ask what type of concrete should be used for extension foundations and slabs.

In this guide, we explain the most common concrete mixes used for house extensions, what factors affect the choice of concrete, and why volumetric concrete is often the preferred option for extension projects.

The Quick Answer

For most domestic house extensions in the UK:

• C20 concrete is commonly used for foundations

• C25 concrete may be used where additional strength is required

• reinforced concrete may sometimes be specified for structural slabs or heavier loads

The exact mix depends on the size of the project, soil conditions, structural requirments and guidance from building control or engineers.

Why Choosing the Right Concrete Matters

The concrete used in an extension forms the foundation of the structure. Using the correct mix helps ensure:

• the extension remains stable

• the foundations can support the load safely

• the structure complies with building regulations

• long-term cracking or movement is avoided

For larger extensions or poor ground conditions, the wrong concrete specification can lead to expensive problems later.

Concrete for Extension Foundations

Most house extensions require concrete foundations to support the walls and roof structure.

For many domestic projects, C20 concrete is commonly used for:

• strip foundations

• trench fill foundations

• footings for single-storey extensions

Where heavier loads or more demanding ground conditions are involved, builders or engineers may specify C25 concrete for additional strength.

Concrete for Extension Slabs

Some extensions also require internal concrete slabs, particularly for:

• kitchen diner extensions

• utility rooms

• garages

• garden rooms

The slab forms the solid base of the floor and must be properly prepared and reinforced where necessary.

A suitable sub-base and insulation layer are also important parts of the construction process.

Do House Extensions Need Reinforced Concrete?

Not every extension requires reinforced concrete, but reinforcement may be used where:

• heavier structural loads are expected

• ground conditions are poor

• larger slab areas are being poured

• engineers specify additional support

Reinforcement is typically added using steel mesh within the concrete slab.

Why Volumetric Concrete Works Well for Extensions

Volumetric concrete is particularly well suited to extension projects because it allows the exact amount of concrete to be mixed on site.

This provides several advantages:

• reduces waste

• avoids over-ordering

• prevents running short mid-pour

• allows adjustments to the mix if needed

At William the Concreter, our modern volumetric lorries allow concrete to be mixed fresh on site, helping builders and homeowners keep projects running efficiently.

Concrete Delivery Across East Sussex and South Kent

We regularly supply concrete for house extensions across:

• Battle

• Hastings

• Bexhill

• Eastbourne

• Robertsbridge

• Haywards Heath

• Tunbridge Wells

• Paddock Wood

• Everywhere in between!

Our concrete is supplied for a wide range of extension projects, including kitchen diner extensions, rear extensions, side extensions, garage conversions and garden rooms and offices.

How Much Concrete Do You Need for an Extension?

The amount of concrete required depends on:

• foundation depth

• trench width

• slab thickness

• overall extension size

Before arranging delivery, it’s important to calculate the correct volume.

You can use our concrete calculator, which allows measurements in:

• metric units

• imperial units

This will help ensure you know how much concrete you need to order for your project.

Frequently Asked Questions

What concrete is used for extension foundations?

For most domestic extensions, C20 concrete is commonly used for foundations, although C25 may sometimes be specified for additional strength.

Do extensions need reinforced concrete?

Some extensions require reinforced concrete slabs or foundations depending on the structural design and ground conditions.

Is volumetric concrete good for extensions?

Yes. Volumetric concrete is ideal for many extension projects because it is mixed on site and allows the exact quantity to be produced as needed.

Need Concrete for a House Extension?

If you're planning a house extension in East Sussex or South Kent, William the Concreter supplies high-quality volumetric concrete for foundations, slabs and extension projects of all sizes.

Whether you're building a kitchen extension, garage conversion or larger structural addition, our team can help ensure you get the right concrete for the job.

Contact us today!