What Concrete Mix Do You Need for Foundations? (C20 vs C25 Explained)

Admin
Feb 24
3 min read

When building foundations for a structure, choosing the correct concrete mix is essential. The strength of the concrete plays a major role in supporting the weight of the building and ensuring the foundations remain stable for many years.

Whether you're planning a house extension, garage, garden room or structural footing, selecting the right concrete mix helps ensure the project meets building standards and performs as expected.

Builders and homeowners across East Sussex and South Kent, from Bexhill and Hastings to Battle, Robertsbridge, Rye, Tenterden and Paddock Wood, often ask which concrete strength should be used for foundations.

In this guide, we explain the common concrete mixes used for foundations and the differences between C20 and C25 concrete.

The Quick Answer

For most residential foundations in the UK:

C20 concrete is commonly used for standard house foundations.
C25 concrete provides additional strength and is often used for heavier loads or where extra durability is required.

The exact mix required will depend on the type of structure being built and the conditions of the ground.

What Do Concrete Strength Grades Mean?

Concrete is classified using strength grades, which indicate how strong the concrete will be once it has fully cured.

These grades are usually written as C followed by a number, such as:

The number refers to the compressive strength of the concrete in megapascals (MPa) after 28 days of curing.

For example:

C20 concrete has a compressive strength of around 20 MPa
c25 concrete has a compressive strength of around 25 MPa

Higher strength concrete is generally used for structures that must support heavier loads.

C20 Concrete for Foundations

C20 concrete is one of the most commonly used mixes for residential foundations.

It is typically suitable for:

house extension foundations
strip footings
trench fill foundations
garden buildings and garages

For many standard projects, C20 concrete provides sufficient strength while remaining cost-effective.

Builders carrying out extension work in areas such as Hastings, Bexhill, Battle and Robertsbridge often use C20 concrete for typical residential foundations.

C25 Concrete for Foundations

C25 concrete is slightly stronger and is often used when additional strength or durability is required. it may be used for:

foundations supporting heavier structures
reinforced concrete slabs
areas with poorer ground conditions
projects requiring higher structural strength

Because C25 has a higher compressive strength, it can provide extra reassurance where foundations need to support larger loads.

Which Concrete Mix Is Best for a House Extension?

For most domestic house extensions, C20 concrete is usually sufficient for foundations.

However, builders may sometimes specify C25 concrete depending on:

the size of the extension
soil conditions
structural engineering requirements
building control guidance

If you're planning a kitchen diner extension or rear extension, your builder or structural engineer will normally specify the correct concrete mix for the foundations.

Concrete Delivery for Foundations in East Sussex and South Kent

Whatever type of concrete you need, William The Concreter can supply you with fast and reliable delivery in our volumetric lorries, meaning low wastage as the concrete is mixed on site to your exact specification, and you only pay for what you use.

We regulaly supply homeowners, builders and developers with C20 and C25 concrete.

Whether you're based in Bexhill, Battle, Hastings, Uckfield, Hailsham, Wadhurst, Rye, Hawkhurst, Tunbridge Wells or anywhere in between, we would be pleased to provide your project with concrete.

We even have a concrete calculator so you can work out exactly how much concrete you need. Click here to use it!

Contact William The Concreter today for fast C20 or C25 concrete delivery.