Why foundations are the part you cannot see and cannot skip

Why ground conditions decide the design

Before foundation depth can be confirmed, the ground needs to be understood. Trial holes are dug to see what is actually beneath the topsoil: firm natural ground, soft or made-up ground, tree roots, old drains, or even old foundations from a previous building. Building control will require the foundation depth to reach ground capable of bearing the load safely, which sometimes means digging considerably deeper than first assumed.

Clay soils that shrink and swell with moisture, proximity to mature trees that draw moisture from the ground, and sloping sites all add complexity that a simple standard-depth strip foundation may not safely handle. This is why a quote given before proper ground investigation is genuinely provisional rather than a builder being evasive.

Common foundation types on domestic extensions

Strip foundations, a continuous trench of concrete beneath the new walls, are the most common approach on straightforward ground where a reasonable bearing depth is reached without excessive digging. They are relatively economical where ground conditions cooperate.

Trench fill foundations, where the trench is filled almost to ground level with concrete rather than leaving space for blockwork below ground, are often used where deeper digging is needed, reducing the amount of time an open trench needs to stay open and the amount of bricklaying required below ground.

Raft foundations spread the load across a wider concrete slab and are sometimes used on poorer ground or where deep piling would be disproportionately expensive. Piled foundations, driven or bored deep into the ground to reach a firm bearing layer, are used where ground conditions are genuinely poor or where a structure needs to bridge over made-up ground or old workings.

• Strip foundations for straightforward ground at reasonable depth
• Trench fill for deeper digs where less open trench time is wanted
• Raft foundations spreading load across poorer ground
• Piled foundations reaching a firm bearing layer on difficult sites

What makes foundations more expensive than expected

The single biggest driver of foundation cost overruns is depth. Every extra course of blockwork or extra cubic metre of concrete needed because ground conditions were worse than assumed adds real cost that is difficult to predict with total accuracy before digging starts. This is why sensible quotes flag foundation depth as a provisional item pending trial holes, rather than fixing an exact price before the ground is seen.

Nearby trees are a common complication in established Devon gardens. Building control often requires greater foundation depth near trees due to the risk of clay shrinkage as roots draw moisture, and removing a tree can sometimes make things worse in the short term as the ground rehydrates and swells. Old drains, services, or the remains of previous outbuildings found during digging can also add unplanned cost.

Access for concrete delivery matters too. A site that a concrete lorry can reach directly is far cheaper to serve than one requiring a pump, wheelbarrow relay, or a smaller specialist vehicle to get concrete into a rear garden.

• Deeper-than-expected ground adds real cost per extra course dug
• Nearby trees can require significantly greater foundation depth
• Old drains or buried structures found on site add unplanned work
• Poor access for concrete delivery increases cost of the pour itself

Building control's role in getting foundations signed off

Foundation depth and design are inspected on site by building control before concrete is poured, and again once poured, as part of the formal approval process for the extension. This inspection exists precisely because foundations cannot be checked once covered over, so it is one of the few points in a build where an independent check happens at the exact moment it matters.

A competent builder plans the dig to be ready for inspection at the right point and does not pour concrete before sign-off has been given, since doing so risks having to dig it out again if the depth or design is found unsuitable.

What this means for your budget and timeline

Because foundation work is genuinely one of the harder parts of a job to price with total precision in advance, it is worth building some contingency into your budget specifically for groundworks, and asking your builder how they would handle a foundation that needs to go deeper than expected once digging starts.

A frank conversation about this risk before work begins, rather than a surprise once trial holes are dug, makes for a far smoother project. It is also worth asking how foundation depth changes would be priced and confirmed under your contract, so there is no ambiguity if the ground turns out to need more than expected.

If you are planning an extension in Torbay or South Devon and want a realistic view of what your ground is likely to need before committing to a budget, get in touch through our free quote page and we will talk you through the groundworks side properly.

Common questions