In short: most UK extensions sit on trench-fill foundations, typically 900–1,200mm deep (shallower on rock, much deeper in clay or near trees). They are designed by a structural engineer under Building Regulations Part A and checked by building control in the open trench before any concrete is poured. Groundwork for a typical 20m² extension runs roughly £4,700–£9,500, more in the South East.
Foundations are the part of an extension nobody sees and nobody should cut corners on — they carry the whole build. Here is what type most extensions use, how deep they go and why, what building control checks, and what the groundwork costs in 2026.
Strip or trench fill?
Two foundation types cover most domestic extensions:
- Trench fill — a trench (commonly around 600mm wide) dug to firm ground and filled almost to the top with concrete. It uses more concrete but very little below-ground bricklaying, so it is quick and is the modern default for most single and two-storey extensions.
- Traditional strip — a thinner concrete base with blockwork built up off it to ground level. It uses less concrete but more labour below ground.
Which one suits your extension depends on the ground, the depth needed and access for a concrete pour — a structural engineer specifies it.
How deep do they go?
Depth is set by the ground, not by a fixed number. As a rough guide:
| Ground | Typical foundation depth |
|---|---|
| Rock | from around 450mm |
| Gravel or sand | around 750–900mm |
| Clay | around 900–1,200mm |
| Clay near trees | 1,500–3,000mm (deeper the closer the trees) |
Clay shrinks and swells with moisture, and tree roots make that worse, which is why foundations near trees go so much deeper. The real depth is confirmed on site: building control inspects the open trench before concrete goes in, because the ground can only be judged once it is dug.
Getting out of the ground: the first courses
Once the concrete is in, the wall is built up to ground level in blockwork (or engineering brick), with the cavity kept clean. A damp-proof course (DPC) goes in at the right height and ties to the existing house, and the cavity is closed and weeped correctly. Getting these first courses level and true is what everything above depends on — it is also where tying neatly into the existing wall matters, which we cover in joining an extension to an existing wall.
What the groundwork costs
Foundations are one of the bigger single items in an extension budget:
| Item (typical 20m² extension) | Rough cost |
|---|---|
| Trench-fill concrete (per linear metre of trench) | £170–£280 |
| Full groundwork & foundations (20m²) | £4,700–£9,500 |
| Typical mid-range (outside South East) | £6,000–£7,500 |
Deeper digs, soft or contaminated ground, high water tables, trees and tight access for a concrete pump all push the figure up — the South East can add 25–55%. It is worth having the ground and design settled before work starts, because surprises in the trench are the most common reason groundwork overruns.
Who designs and signs it off
Extension foundations are not a guess on the day. A structural engineer designs them for the load and the ground under Building Regulations Part A, and building control inspects the open trench before concrete is poured. This applies even when the extension itself is permitted development and needs no planning application — building control approval is separate and always required.
Where we fit in
We build the extension off the foundations — the blockwork, brickwork and getting cleanly out of the ground and tied into the house. On many jobs the groundworks and the bricklaying are priced separately; you or your main contractor supply the materials and we lay them. See what a bricklayer charges for an extension and our single-storey extension cost guide for how the wider budget fits together.
Figures are 2026 UK market guides drawn from published cost data and our own experience; treat them as ballpark. Foundation depth and design must be confirmed for your site by a structural engineer and building control.
Planning an extension?
If you are planning an extension in Hampshire, Berkshire or Surrey and want the brickwork done right from the ground up, see our extension work or send us the details and we will come and look.
Frequently asked questions
How deep do extension foundations need to be?
It depends on the ground. As a rough guide, foundations are around 900 to 1,200mm deep in ordinary conditions, but they can be shallower on rock and much deeper in clay or near trees, where 1.5 to 3 metres is not unusual. The depth is confirmed by building control on site once the first trench is dug, because the ground can only be judged accurately when it is open.
What type of foundation does an extension need?
Most modern UK extensions use trench-fill foundations: a trench, commonly around 600mm wide, dug to a firm bearing and filled almost to the top with concrete. Traditional strip foundations, with a thinner concrete base and blockwork built up off it, are also used. A structural engineer specifies which suits your ground and design.
How much do extension foundations cost?
For a typical single-storey extension of around 20 square metres, the groundwork and foundations commonly cost about £4,700 to £9,500, with a mid-range often £6,000 to £7,500 outside the South East. Trench-fill concrete alone works out around £170 to £280 per linear metre of trench. Deeper digs, poor ground, trees and difficult access all push it up.
Do you need a structural engineer for extension foundations?
Yes, in almost all cases. Under the Building Regulations (Part A, Structure), an extension needs foundations designed for the load and the ground, and that design normally comes from a structural engineer. Building control then inspects the open trench before any concrete is poured.
Do extension foundations need building control approval?
Yes. Foundations are one of the key stages building control inspects. An inspector checks the open trench for depth, width and firm bearing before the concrete goes in, and signs off the work as it proceeds. This is separate from planning permission and is required even when the extension is permitted development.
What is a trench-fill foundation?
A trench-fill foundation is a trench dug down to firm ground, usually around 600mm wide, then filled almost to ground level with concrete. It uses more concrete than a traditional strip foundation but far less bricklaying below ground, which makes it quicker and, on many sites, the practical choice for an extension.