Extension Foundations Explained: Types, Depth and Cost

Foundations are the least glamorous part of an extension and one of the most important, because everything above them depends on getting them right. The extension foundations types you can choose from are not really a free choice: the ground, the trees nearby and the load of what you are building largely decide which one you need, and Building Control has to agree before any concrete is poured. This guide explains the four main foundation types, how deep they typically go, what moves the cost, and what an inspector will want to see. Treat the figures as a guide and rely on your structural engineer and Building Control for the specifics of your site.

The four main foundation types

Most domestic extensions use one of four systems.

Strip foundations. A continuous strip of concrete poured in the bottom of a trench, with the walls built up off it in blockwork or brick below ground. This is the traditional method and still used where ground is good and the trench does not need to be very deep. It uses less concrete than trench fill but takes longer, because the below-ground walls have to be built by hand.

Trench fill foundations. The trench is dug and then filled with concrete almost to the top, so the wall starts only a course or two below ground level. Since the late 1990s this has become the most commonly specified foundation for single and two-storey extensions, because it is faster, needs less bricklaying below ground, and performs well on moderately shrinkable clay soils. It uses more concrete than a strip footing, but saves labour and time.

Raft foundations. A single reinforced concrete slab spread across the whole footprint of the extension, distributing the load over a wide area. Rafts suit weaker or variable ground, or shallow situations where deep trenches are impractical. They use more steel and concrete, so they typically cost more than trench fill.

Pile foundations. Deep columns (piles) driven or bored into the ground to reach firm strata, tied together with concrete ground beams. Piles are the answer for difficult ground: made-up or filled land, very soft soil, or sites close to large trees where a normal trench would have to go impractically deep. They need specialist equipment and design, and are the most expensive option, but sometimes the only sensible one.

How deep do extension foundations go?

There is no single number, because depth is set by the ground, not by preference. The Building Regulations require foundations to reach ground that can carry the load and to sit below the level affected by frost and seasonal movement. As a rough guide, the minimum is around 450mm to get below frost, but in practice most extensions need to go to about 1 metre or more to reach firm, stable strata.

The big variable is clay soil and trees. Shrinkable clay swells and shrinks with moisture, and tree roots dry it out, so on clay near trees foundations often have to go much deeper, sometimes 2 metres or more, to reach ground that will not move. The NHBC publishes tables relating foundation depth to soil type and the species and distance of nearby trees, and your engineer will use these. This is exactly the kind of thing that turns up as an unwelcome surprise once the trench is open, so read our guide to hidden extension costs before you set your budget.

What Building Control checks

Foundations fall under Part A (Structure) of the Building Regulations, and they are inspected before you build on them. In practice, your builder digs the trench and then calls Building Control (or your approved inspector) to inspect it before any concrete goes in. They will check the depth, the width, the ground at the base of the trench, and how close it is to drains, trees and boundaries. If the ground at the bottom is not good enough, they can require you to dig deeper or change the design on the spot. Do not let anyone pour concrete before this inspection, because covering up an unapproved trench is a serious and expensive mistake to unpick. Our guide to Building Control sign-off explains how the inspection stages fit together.

What drives the cost

Foundations are one of the first big spends of an extension, and the total depends on:

• Foundation type. Trench fill is the usual baseline; a raft or piled solution costs more because of the extra concrete, steel or specialist rigs.
• Depth. Every extra course of depth on clay or near trees adds concrete, spoil to cart away, and labour. A deep foundation can cost several times a shallow one.
• Access and spoil. A tight urban plot where a mini-digger cannot reach, or where spoil has to be barrowed out and skipped, pushes prices up.
• Ground surprises. Old drains, previous foundations, soft spots or a high water table all add cost once the digging starts.

As a broad 2026 guide, foundations for a typical single-storey extension of around 20m² often fall somewhere in the low thousands, with trench fill commonly cheaper than a reinforced raft, but the numbers vary widely with depth and ground. Always get itemised quotes, and ask specifically what happens to the price if the trench has to go deeper than assumed. For the whole project budget, see our single-storey extension cost guide.

The bottom line

You rarely get to pick your foundation on looks or price alone. The ground, the trees and the load decide it, an engineer designs it, and Building Control signs off the open trench before anything is poured. The government-backed Planning Portal guidance on foundations is a sound reference for the rules. Get the ground investigated early, keep a contingency for going deeper, and treat the foundation as the part of the build least worth cutting corners on.

Frequently asked questions

What are the main types of extension foundations? The four main types are strip foundations (a concrete strip with walls built up off it), trench fill (a trench filled with concrete nearly to the top, the most common choice today), raft foundations (a reinforced slab over the whole footprint), and pile foundations (deep columns for difficult ground). The ground conditions usually decide which you need.

How deep should extension foundations be? The minimum is around 450mm to clear frost, but most extensions need about 1 metre or more to reach firm ground. On shrinkable clay near trees, foundations may have to go 2 metres or deeper. The exact depth is set by your engineer and confirmed by Building Control on site.

Which foundation type is most common for extensions? Trench fill has been the most commonly specified foundation for single and two-storey domestic extensions since the late 1990s, because it is quick to build, needs less below-ground bricklaying, and copes well with moderately shrinkable clay. Strip, raft and pile foundations are used where conditions call for them.

Do foundations need Building Control approval? Yes. Foundations are covered by Part A (Structure) of the Building Regulations and must be inspected before concrete is poured. Building Control checks the depth, width and ground at the base of the open trench and can require a deeper or redesigned foundation if the ground is inadequate.

Why do foundations near trees have to be deeper? Trees dry out shrinkable clay soil, which then swells and shrinks with moisture and can move a shallow foundation. To reach ground that stays stable, foundations near large trees on clay often have to go much deeper, guided by NHBC tables based on the tree species, its distance and the soil type.