How deep should foundations be in the UK?

For a standard single-storey extension, foundations are typically 1 metre (1000mm) deep. However, if you are building in clay soil near trees, you may be required to dig to 2.5 metres or more to avoid 'soil heave' or subsidence.

What concrete mix should I use for foundations?

GEN 1 (C8/10) is the minimum standard for foundations. However, for most modern residential extensions, Building Control usually recommends GEN 3 (C16/20) or RC20/25 if the ground is potentially aggressive (e.g., high sulphate content).

How much concrete does a standard ready-mix truck hold?

A standard UK 8-wheeler ready-mix truck can hold up to 8 cubic metres (m³) of concrete. Smaller 'minimix' trucks are available for restricted access sites, holding roughly 3 to 4 cubic metres.

When should I call Building Control for foundation inspection?

You must call your Building Control officer once the trenches have been dug and cleaned out, but *before* any concrete is poured. They need to verify the depth and the bearing capacity of the soil.

How deep should foundations be in the UK?

For a standard single-storey extension, foundations are typically 1 metre (1000mm) deep. However, if you are building in clay soil near trees, you may be required to dig to 2.5 metres or more to avoid 'soil heave' or subsidence.

What concrete mix should I use for foundations?

GEN 1 (C8/10) is the minimum standard for foundations. However, for most modern residential extensions, Building Control usually recommends GEN 3 (C16/20) or RC20/25 if the ground is potentially aggressive (e.g., high sulphate content).

How much concrete does a standard ready-mix truck hold?

A standard UK 8-wheeler ready-mix truck can hold up to 8 cubic metres (m³) of concrete. Smaller 'minimix' trucks are available for restricted access sites, holding roughly 3 to 4 cubic metres.

When should I call Building Control for foundation inspection?

You must call your Building Control officer once the trenches have been dug and cleaned out, but *before* any concrete is poured. They need to verify the depth and the bearing capacity of the soil.

Foundation Calculator

Estimate concrete volume for trench foundations.

Trench Width (mm)

Trench Depth (mm)

Total Length (m)

Concrete Order

Groundwork Tip: Before the concrete truck arrives, ensure your trenches are 'pegged' to indicate the correct pour depth. Ready-mix companies usually allow 30-45 minutes for offloading before charging 'waiting time' fees.

Groundwork Mastery: The UK Foundation Guide

Structural Integrity

Foundations are the most critical part of any build. In the UK, trench-fill foundations are the standard method for residential extensions, where concrete is poured directly into a deep, narrow trench to within 150mm of the ground level.

Our **foundation concrete calculator** helps you accurately estimate the volume required, reducing the risk of being 'short' on the day of the pour—which can lead to weak 'cold joints' in the foundation.

Standard Trench Specs

Standard Width600mm

Standard Depth1000mm

Waste Allowance10% Recommended

Soil Types & Heave

UK soil varies dramatically. In **Clay soil**, foundations must often be deeper to reach stable ground that isn't affected by seasonal moisture changes. If you are building near trees, the roots can shrink the clay, leading to subsidence.

In these cases, you may be required to install **'clayboard'** (a compressible material) to the side of the trench to protect the foundation from lateral soil pressure.

Ready-Mix Logistics

When ordering, specify the 'consistence' (slump). For trench-fill, a **S3 slump** is standard—it is fluid enough to flow into the corners of the trench without excessive tamping, but stiff enough to support its own weight.

Reinforcement

While standard mass-concrete foundations don't always require steel, Building Control may insist on **steel reinforcement bars (rebar)** or mesh if the ground is unstable or if the foundation 'steps' up a hill.

Pro Tip: The Clean Out

Before pouring, ensure the bottom of the trench is completely free of loose soil, standing water, and organic material (like roots). Any 'soft spots' left in the trench can lead to the foundation settling unevenly later on, causing cracks in your new walls.

Foundation & Soil FAQ

Project Disclaimer

Foundations must be designed by a qualified structural engineer and approved by Local Authority Building Control. Never pour concrete into an un-inspected trench. Our calculator is for material estimation only and does not replace professional structural advice.