If you’re planning to build a new home, add an extension, or even install a garden room in the UK, one of the most important things to consider is what’s beneath your feet — the soil.
And if that soil happens to be clay, you’ll need to take a bit of extra care when it comes to your foundations.
Clay soils are common across much of the UK — particularly in areas like London, Surrey, Kent, and the Midlands — and while they can support buildings perfectly well, they do have some unique challenges.
Let’s take a closer look at what makes clay different, why it matters, and how a structural engineer ensures your home stays stable for decades.
🪨 What Is Clay Soil?
Clay soil is made up of very fine mineral particles that hold water easily.
When it’s wet, it becomes sticky and expands.
When it’s dry, it shrinks and cracks.
This natural movement — known as shrink-swell behaviour — is what requires careful consideration in the foundation design.
⚠️ Why Clay Soils Can Cause Problems for Foundations
The biggest challenge with clay soils is that they change volume depending on how wet or dry they are.
Here’s what typically happens:
- 💧 In wet weather, clay absorbs water and expands.
- ☀️ In dry weather, it loses moisture and shrinks.
This constant expansion and contraction can cause ground movement, leading to:
- Cracks in walls and floors
- Uneven settlement
- Movement in extensions or conservatories
- Structural damage over time
In some areas, this movement can be made worse by trees and shrubs, which draw moisture out of the soil through their roots.
🧰 Key Foundation Considerations for Clay Soils
If your property sits on clay, it doesn’t mean you can’t build — it just means you need the right foundation design.
Here’s what homeowners and builders should keep in mind:
1. Site Investigation and Soil Testing
Before any building starts, it’s recommended to carry out a site investigation. For a small fee, typically, £1,000 – £2,000, the soil profile of the site can be determined leading to more certainty in choosing the right foundation type for your project.
This usually involves digging trial pits or boreholes and taking soil samples to determine:
- The type of clay (London Clay, Gault Clay, Weald Clay, etc.)
- The depth of stable soil (foundations should not be supported by made ground – the surface later of soil that is made up of rubble & organic material that is highly variable with low strength)
- Moisture content and shrink-swell potential
A geotechnical engineer or structural engineer will use this information to decide how deep and wide your foundations should be.
2. Foundation Depth
On clay soils, foundations generally need to be deeper than usual — often at least 1 metre below the ground level, and sometimes up to 2.5 metres — depending on the ground conditions and presence of trees. Where calculations suggests the foundation need to be deeper than 2.5metres, quite often it becomes more economical to use piles to save the excavation & material costs. Note that most trenches deeper than 1.2m would require some sort of lateral support to prevent the soil from caving in and creating a health & safety hazard.
The general idea is to place the foundation below the active zone of clay, where moisture changes are less extreme and therefore reduce the risk of foundation movement in the future.
3. Trees and Vegetation
Large trees (like oaks, willows, or poplars) can draw significant moisture from the ground, causing clay to shrink and creating a permanent moisture deficit in the soil.
If you’re building near trees, your foundation depth may need to be increased than the usual 1 metre.
If trees are removed just before construction, the opposite can happen — the clay rehydrates and expands, pushing foundations upward (called heave).
A structural engineer will take all this into account in the design and specify the right foundation type and any other measures to counteract the effects from heave.
4. Foundation Type
Different foundation types suit different ground conditions.
For clay soils, the most common options include:
- Deep Strip Foundations: Concrete foundations placed deep enough to reach stable clay.
- Trench Fill Foundations: A simpler version filled almost entirely with concrete.
- Raft Foundations: A reinforced concrete “slab” that spreads the load evenly across a larger area — ideal for weaker or variable soils.
- Piled Foundations: Used where clay is particularly unstable, with CFA or helical (screw) piles reaching solid ground below.
A structural engineer would decide which system works best for your site and your contractor can then price it off.
5. Drainage and Water Control
Good drainage is essential.
Avoid water collecting near the building, as excess moisture can cause clay to swell.
Gutters, soakaways, and proper landscaping help prevent long-term movement as well as ensuring the below ground drainage pipes are all in working order and not disjointed or cracked. For a small fee, CCTV surveys can also be carried out to determine the condition of the pipes.
👷♀️ How a Structural Engineer Can Help
A structural engineer plays a key role in ensuring your building on clay is safe and long-lasting. Here’s how they help:
- Assess the Site: They review the publicly available soil reports, review available tree reports identifying the tree species that needs to be considered.
- Design the Foundation: They calculate the required depth, width, and type of foundation to minize the risk of movement.
- Provide Drawings & Calculations: These are submitted to Building Control for approval.
- Monitor Construction: Some engineers offer site inspections to ensure foundations are built correctly.
Their expertise prevents costly future problems like cracking, settlement, or uneven floors.
🧩 Building Control and Clay Soils
In the UK, Building Control will review your foundation design to make sure it meets the Building Regulations, particularly for stability and resistance to ground movement. Sometimes the Building Control Officer can request the foundations to be dug deeper using their local knowledge, so the contractor should always seek confirmation/agreement from Building Control before going ahead and casting the foundations.
Your structural engineer’s design and calculations form part of this approval process — ensuring your project is both safe and compliant.
🏁 In Summary
Building on clay soils in the UK isn’t a problem — as long as it’s done right.
The key is understanding how the soil behaves and designing foundations that can cope with those changes.
Before you start digging, speak to a structural engineer.
They’ll help you design a foundation that keeps your home stable, secure, and standing strong for years to come — no matter how much the weather (or the clay) changes. 🌦️
HM Structures 
Leave a comment