How deep should foundations be for a single-storey extension?

Foundations may sit out of sight, but they are the most critical part of any extension. They carry the entire load of the structure above and must be designed to suit both the building and the ground it sits on.

There isn’t one fixed answer when it comes to foundation depth for a single-storey extension. Instead, the correct depth depends on several site-specific factors, including soil conditions, nearby trees, drainage runs and local building control requirements.

This guide explains what influences foundation depth and what you should expect before work begins.

Why ground conditions matter so much

The type of soil beneath your home plays a major role in determining how deep foundations need to go. Different soils react differently to weight, moisture and temperature changes.

• Clay soils expand and shrink as moisture levels change. Because movement can occur down to around 750–900mm, foundations in clay usually need to extend beyond this depth to remain stable.
• Sandy or chalky soils are more vulnerable to frost. Foundations in these conditions generally need to be set below the frost line, typically deeper than 450mm, to prevent movement during colder months.

Building control will usually assess soil conditions during excavation and confirm whether deeper foundations are required.

Is a soil investigation necessary?

A formal ground investigation isn’t always required for small residential extensions, but it can be helpful on complex or higher-risk sites. A soil survey provides detailed information about subsoil conditions across the plot.

In many cases, foundation depth is agreed on site using the experience of the designer and the judgement of the building control officer once excavation begins.

Typical footing dimensions for domestic extensions

For most single-storey residential extensions, strip foundations are commonly used.

• Foundation width is often around 600mm, allowing adequate bearing for cavity walls
• Depth is determined by soil conditions and must reach stable, undisturbed ground

As a general rule, foundations should extend at least 300mm into firm, virgin soil and sit below the local frost depth to prevent seasonal movement.

Practical guidance from building control specialists

When preparing foundations, several best-practice principles apply:

• Ground next to foundations should slope away from the structure to prevent water build-up
• Foundations must rest on undisturbed soil, not backfilled or recently disturbed ground
• Every site is unique, so foundation depths should always be confirmed before concrete is poured

These checks are usually carried out during site inspections as work progresses.

Budgeting for foundation work

Foundation costs vary depending on depth, soil conditions, access and construction method.

• Strip foundations typically range from £110–£130 per m²
• Trench-fill foundations generally cost around £130–£165 per m²

Deeper excavations, poor ground conditions or restricted access can increase costs significantly, so allowances should always be made early in the budget.

Other factors that influence foundation depth

Nearby buildings

When building close to neighbouring structures, foundations should not undermine existing footings. Excavation is often carried out in short sections to avoid destabilising nearby walls.

Trees and root systems

Trees remove moisture from the soil, which can cause shrinkage — particularly in clay. The closer and larger the tree, the deeper foundations may need to be to prevent long-term movement.

Drainage and sewers

Loads from foundations spread outward at roughly a 45-degree angle. If drains fall within this zone, foundations may need to be deepened or redesigned to avoid damage.

Made ground and backfill

If the soil has previously been disturbed — for example, after drainage works — excavation must continue until firm, untouched ground is reached before foundations are poured.

Former landfill sites

Sites with historic landfill often require specialist solutions such as reinforced raft foundations, as stable ground may be several metres below the surface.

Getting foundations right from day one

Correctly designed and installed foundations are essential for the safety, durability and long-term performance of your extension. Because conditions vary so widely from site to site, professional advice and experienced contractors are vital.

Taking the time to assess ground conditions properly at the start can prevent structural issues, delays and unnecessary costs later in the build.

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