There is no single depth that suits every extension, but there are sensible rules of thumb and a clear process for getting it right. This guide explains typical foundation depths in the UK, what changes them, and what to expect on Cornish ground before anyone puts a digger in.
For a typical single or two storey extension on decent ground, strip foundations are commonly dug to about 1 metre deep and around 600mm wide, with a minimum of 150mm to 225mm of concrete in the bottom. Building Regulations do not fix a single depth. Approved Document A simply requires foundations to reach ground that can carry the load, below topsoil, soft fill and anything affected by frost or seasonal movement. In practice, 450mm is the absolute minimum to get below frost action, and most building control officers will want to see considerably more than that.
The honest answer is that the final depth is confirmed in the trench, not on paper. Your building control inspector looks at the exposed ground during the dig and either approves the formation or asks you to go deeper until you reach firm, consistent bearing material. A good groundworks contractor prices for a realistic depth, then adjusts on site with the inspector rather than guessing beforehand.
Soil type is the biggest factor. Soft clay, made ground, old backfill and peaty or waterlogged soil all push depths down because they cannot reliably carry the weight of a wall. Shrinkable clays, which swell and shrink with moisture, often need trenches of 1 metre or more even without other complications.
Trees and drains are the two things that most often catch homeowners out. Roots dry the ground beneath them, so foundations near trees may need to go well below root influence, sometimes 1.5 metres to 2.5 metres or more depending on the species and its distance from the trench. Similarly, if your foundation runs close to a drain, the trench usually has to be taken down at least to the invert level of that pipe so the new load does not crush or disturb it.
Cornwall is generally kinder than much of the south east because we have very little of the highly shrinkable clay that forces deep trenches around London. Around St Austell you often hit weathered granite, shillet or firm gravelly subsoil at a sensible depth, which can mean foundations toward the shallower end of the normal range once the inspector is happy.
That said, this area has its own quirks. Parts of mid Cornwall sit over historic mining and china clay workings, and some plots include old fill or disturbed ground that looks firm but is not. Where there is any doubt, a couple of trial holes dug before pricing, or a soil investigation on trickier sites, is far cheaper than discovering a problem halfway through the build. On genuinely poor or filled ground, the answer may be trench fill, a raft or piles rather than simply digging ever deeper.
Traditional strip foundations use a layer of concrete in the bottom of the trench with blockwork built up to damp proof course. Trench fill, where the trench is filled with concrete to near ground level, costs more in concrete but saves labour and time, and it is often the practical choice for deeper digs or wet trenches that need concreting quickly. For most extensions the difference in overall cost is smaller than people expect, and site conditions usually make the decision for you.
Either way, expect the sequence to run: set out, dig, building control inspection of the open trench, concrete pour, then another inspection before work continues. Weather matters too. In a wet Cornish winter, open trenches can deteriorate fast, so a contractor who digs and pours promptly protects both your programme and your budget.
Because depth is confirmed on site, treat foundation costs as a range rather than a fixed figure. Ask for a quote based on a stated depth, for example 1 metre, with a clear rate for any additional depth the inspector requires. That way there are no surprises and no incentive for anyone to cut corners.
If you are planning an extension in the St Austell area, we are happy to look at the site, talk through likely ground conditions and dig trial holes if needed, so your quote reflects reality rather than optimism.
Not always. On straightforward sites, standard strip or trench fill foundations agreed with building control are fine. An engineer becomes worthwhile where there are trees, suspected fill, mining history or a sloping site, because a designed solution can prevent expensive over digging.
Yes. The inspector approves the ground they see in the open trench, and if it is soft or inconsistent they will ask for more depth. This is why a good quote states an assumed depth plus a rate per additional metre, so any extra is transparent.
On a typical extension, digging, inspection and concreting are often done within two to five working days, ground and weather permitting. Difficult access, hitting rock or groundwater, or waiting on inspections can extend that, so build a little slack into your programme.
Got a job in mind across Cornwall or Devon? A quick call or message with Robert is usually all it takes to get a quote moving.