Foundation Types Explained

What's under your extension?

Foundations are the part of your extension you'll never see again — and they're also the most important. Get them wrong and everything above suffers: cracked walls, sticking doors, failed glazing. Get them right and your extension stands for a century.

But not every extension needs the same foundation. The right type depends on the soil, the load, and what's already there. Here's a plain-English guide to the main types.

Strip foundations

The most common type for residential extensions. A trench is dug (typically 1 metre deep, though this varies with soil), filled with concrete, and the brickwork is built up from there.

Best for: stable, well-drained soils like clay (at consistent depth), chalk or sand/gravel.

Cost: moderate — the standard against which others are compared.

Deep strip / trench fill

A variation of strip foundations where the trench is dug deeper — sometimes 2–3 metres — to reach stable ground below unstable top layers. This is common in areas with clay soil that shrinks and swells with moisture changes (common in much of South East England).

Trench fill is exactly what it sounds like: the trench is filled almost to the top with concrete, rather than just a shallow pour with brickwork built up.

Raft foundations

A raft (or raft slab) is a single, reinforced concrete slab that covers the entire footprint of the extension. Instead of transferring load down through walls to trenches, it spreads the load across the whole area.

Best for: soft or variable ground, or where load-bearing walls are positioned in ways that make strip foundations impractical.

Cost: typically higher than strip, due to the volume of concrete and reinforcement.

Piled foundations

When the ground near the surface is too weak to support the extension, piles are driven or bored deep into the ground — sometimes 5–10 metres — until they reach a firm stratum. Concrete ground beams then span between the piles and support the walls above.

Best for: very poor ground, made-up ground (fill), or near trees where clay desiccation could cause movement.

Cost: the most expensive option, but sometimes the only viable one.

How is the right type chosen?

The decision is based on:

1. Soil type — identified through a site investigation or trial pits
2. Proximity to trees — clay soils near trees often need deeper foundations
3. Existing foundations — your new foundation must respect what's already there
4. Load — a two-storey extension exerts more force than a single-storey
5. Building control — the inspector must approve the foundation design

What this means for your project

You can't choose your foundation — the ground chooses it for you. What matters is having a builder and structural engineer who assess the conditions properly and design accordingly.

During our consultation, we review your ground conditions (from site visits, trial pits or existing knowledge of the area) and ensure the foundation design is appropriate and fully compliant with building regulations.

Get in touch using the form on our contact page to discuss your project.