Foundations and Footings for Your Extension

Foundations are where the money disappears into the ground. Literally. You'll spend thousands of pounds on a hole full of concrete that nobody will ever see again, and if it's wrong, you'll spend thousands more putting it right. The rest of your extension sits on this. Every wall, every roof timber, every kitchen unit. Get the foundations wrong and nothing above them is stable.

The good news: for a typical single-storey rear extension, foundations are straightforward. Your structural engineer designs them, your groundworker digs and pours them, and building control inspects them. Your job is to understand what's happening, know what to check, and have enough contingency in the budget for the surprises that are genuinely common at this stage.

Foundation Types: What Your Structural Engineer Will Specify

You don't choose your foundation type. Your structural engineer does, based on ground conditions, soil type, loading from the structure above, and proximity to trees. But you need to understand what they've specified, because it directly affects cost and programme.

Two types cover the vast majority of domestic extensions in the UK.

Trench Fill

Trench fill is the modern default. A trench is excavated (typically 600mm, 1m or more deep), then filled almost entirely with concrete to within 150-200mm of ground level. It uses a lot of concrete. A 20m² extension needs roughly 11-13m³, which is two or three ready-mix truck loads. But it eliminates below-ground blockwork entirely, which means less labour, faster progress, and you're out of the ground in a day.

Trench fill costs more in materials and less in labour. The net effect is that it's typically 10-15% cheaper overall than strip foundations for the same extension. That's why most structural engineers specify it unless there's a reason not to.

Strip Foundations

The traditional method. A narrower concrete strip (minimum 150mm thick for single storey) is poured at the bottom of the trench, then concrete blockwork is built up from that strip to ground level. Strip foundations use far less concrete (1.6-2.0m³ for a 20m² extension versus 11-13m³ for trench fill), but the below-ground blockwork takes days of skilled labour that trench fill avoids.

Strip foundations still make sense in specific situations: where concrete prices are very high, where access prevents a ready-mix truck, or where the structural engineer needs more control over the foundation geometry.

Click to enlarge

Click to enlarge

Less Common Types

Pad foundations support point loads rather than continuous walls. If your structural engineer specifies a steel post or column (common with open-plan layouts and large bifold openings), it will sit on an isolated concrete pad. These are designed individually and aren't a substitute for perimeter foundations.

Piled foundations become necessary when good bearing ground is very deep, typically beyond 2.5-3m. This happens on heavy clay sites near mature trees, where NHBC guidance pushes the required depth beyond what trench fill can practically achieve. Mini-piling costs £5,000 – £15,000 on top of your normal foundation budget. The structural engineer specifies when piles are needed, and you don't get a choice about it.

Raft foundations spread the load across the entire footprint as a single thick concrete slab. They're less common for extensions but sometimes proposed when ground conditions vary across the site. If your SE specifies a raft, ask them to explain why the simpler options won't work. There should be a clear reason.

How Deep Do Foundations Go?

The minimum depth for foundations in the UK is 1m to firm ground, per Approved Document A. But "minimum" is doing a lot of work in that sentence. Actual depth depends on four things, and only one of them is under your control.

Soil type. Clay soils shrink and swell with moisture changes, which means foundations need to go deeper to reach stable ground. NHBC standards classify clay by plasticity: low plasticity needs a minimum 750mm depth, medium needs 900mm, high plasticity needs 1000mm or more. Sandy or gravel soils are more stable and often allow the standard 1m depth.

Trees. This is where foundation costs escalate fast. Trees extract moisture from clay soils, causing ground movement that can damage shallow foundations. The NHBC publishes tables linking tree species, distance from the building, and soil type to required foundation depths. An oak tree 10 metres from your extension on high-plasticity clay can push your foundation depth to 2m or beyond. Willow, poplar, and elm have similar influence zones.

Frost protection. Foundations must be deep enough that frost can't reach the bearing ground and cause heave. The minimum for frost protection alone is 450mm, but in practice this is never the governing factor because other requirements always push the depth well beyond it.

What building control finds on the day. Your structural engineer designs based on assumptions about the ground. Building control inspects the actual ground. If the soil at the bottom of your trench doesn't match the structural engineer's assumptions (softer than expected, different composition, unexpected tree roots), the inspector will require you to dig deeper. Unexpected depth is why a contingency on foundation costs is non-negotiable.

1m to 2.5m+

The range of foundation depths for UK extensions. A straightforward 1m trench on good ground is the best case. Clay soil near mature trees can push the depth to 2.5m or more, which may require switching from trench fill to piled foundations.