How Deep Should Foundations Be? Types, Depth & Cost (2026 UK Guide)
The Quick Answer
For a typical UK house or extension, foundations are dug 900mm to 1.5 metres deep. Building Regulations set an absolute minimum of 450mm below finished ground level, which is enough to get below frost action, but almost nobody builds to that. On ordinary ground 750mm is the practical floor, and 900mm is where most jobs start.
On shrinkable clay the depth goes up. In clay, 900mm is the minimum, and near a mature tree it can be 1.5m, 2.5m or more. Occasionally it gets deep enough that piling works out cheaper than digging.
Foundation costs in 2026 run roughly £150 to £300 per linear metre for strip or trench fill on straightforward ground, which puts a typical single-storey extension at £3,000 to £8,000. Piled foundations start around £10,000.
What nobody can do is tell you your depth from a desk. It comes from the ground you actually have, and in Norfolk's clay (the same clay behind the county's subsidence problems) trees decide it as often as the soil does.
Why Foundation Depth Is Not One Number
A foundation does two things. It spreads the weight of the building onto ground that can carry it, and it does so at a depth where that ground stays still.
The second half is what drives the depth. Near the surface, ground is not stable: it freezes and thaws, dries out over summer and swells again through winter, loses moisture to tree roots, and compresses under load wherever topsoil has been left in place. Go deep enough and all of that stops. The ground sits at a more or less constant moisture content, the weather stops reaching it, and it holds the same shape all year.
So the useful question is not how deep foundations are in general, but how far down you have to go before this particular ground stops moving. Four things decide that:
- Frost. Water in the soil expands when it freezes and lifts whatever sits on it. Getting below roughly 450mm puts the foundation under normal UK frost penetration.
- Seasonal moisture movement. Clay shrinks when it dries and swells when it wets. That zone runs to around 900mm deep in the UK, and considerably deeper where trees are pulling water out of it.
- Load-bearing capacity. Topsoil and made ground cannot carry a building. The foundation has to reach undisturbed subsoil that will take the load above.
- What is already down there. Drains, old foundations, soft spots, a high water table, tree roots and buried services all change the dig.
Minimum Foundation Depth by Ground Type
Guide depths for domestic work. Treat them as starting points rather than a specification, because the trench gets inspected and the depth confirmed on site.
| Ground condition | Typical foundation depth |
|---|---|
| Gravel, sand, rock (non-shrinkable) | 450mm absolute minimum; 750mm in practice |
| Firm chalk or non-shrinkable subsoil | 750mm - 900mm |
| Shrinkable clay, no trees nearby | 900mm minimum |
| Shrinkable clay, trees within influence | 1.2m - 3m+, set by tree species and distance |
| Made ground, fill, soft spots | Dig through to competent ground, or raft/pile |
| High water table or peat | Specific design; often raft or piled |
The 450mm figure is the one that gets quoted online, and it causes more trouble than any other number here. It is a frost minimum for good ground rather than a general rule. On clay it is nowhere near enough, and a foundation at that depth will move with the seasons.
Types of Foundation
Five types cover almost all domestic groundworks.
Strip foundation. A continuous strip of concrete beneath the load-bearing walls, typically 600mm wide and 150-300mm thick, with blockwork built up off it to damp proof course level. This is the traditional approach and the cheapest in concrete terms. It suits shallower digs on decent ground.
Trench fill. The same trench, but filled with concrete to within a couple of courses of ground level instead of building blockwork up off a thin strip. It uses far more concrete and far less labour, and it closes the trench the same day, which matters when the sides are liable to collapse. On deep digs and on clay, trench fill is usually the sensible choice, and it is what most extensions in Norfolk end up on.
Raft foundation. A single reinforced concrete slab spanning the whole footprint, spreading the load over the entire area rather than concentrating it under the walls. It gets used on soft, variable or made ground where a strip would have to be impractically deep, and where the ground cannot be relied on to be consistent across the plot.
Pad foundation. Isolated square pads under point loads such as steel columns, posts, or a garden room frame. Common in extensions where a steel goalpost carries the wall that has been removed.
Piled foundation. Bored or driven piles taken down through unsuitable ground to a competent bearing layer, tied together with reinforced ground beams. Expensive, but it is the answer when good ground is a long way down, when tree influence would demand an unreasonably deep trench, or where an existing structure cannot be undermined.
None of this is a matter of preference. The ground survey and the site conditions dictate which one you get.
Building Regulations and Foundations
Foundations are covered by Part A of the Building Regulations, and for most domestic work the design either follows the traditional strip foundation rules in Approved Document A or comes from a structural engineer's calculation.
In practice that means four things:
- Building Control must inspect the trench before you pour. This is a mandatory inspection stage. The inspector looks at the depth, the bearing ground and the condition of the trench, and can require you to go deeper. Pour before that inspection and you may be told to break it out.
- Concrete specification matters. Ordinary foundations typically use a GEN1 mix. Where the ground contains sulphates, which is common in some clays and in made ground, you need a sulphate-resisting designated mix such as FND2, because ordinary concrete degrades in sulphate-bearing ground over time. A soil chemical test tells you which one you are dealing with.
- Foundations near drains have rules of their own. Under Part H, where a drain runs close to or beneath a foundation, the trench is generally filled with concrete to protect the pipe and stop the foundation loading it. Finding an unrecorded drain in the trench is one of the more common mid-dig surprises.
- Foundations must not undermine neighbouring structures. Digging deeper than the neighbour's foundation within 3 metres of their wall brings the Party Wall etc. Act 1996 into play, and that requires notice before work starts.
The trench inspection is worth taking seriously rather than treating as paperwork. It is the point where somebody independent confirms the ground is what everyone assumed it was, and it is regularly the point where the depth changes.
Foundations in Clay Soil: Why Norfolk Is Different
Much of Norfolk sits on clay-rich ground, and clay is the reason foundation depth is a live question here rather than a box to tick.
Clay is a shrinkable soil. It takes on water and swells, then dries out and shrinks. Over a wet winter and a dry summer that cycle moves the ground vertically, and a foundation sitting inside the moving zone rides up and down with it. A few years of that produces cracking, which is the mechanism behind most of the county's clay-related subsidence.
The fix is depth: get the foundation below the zone that moves. That is why 900mm is the minimum in clay, and why any competent groundworker treats 900mm as the start of the conversation rather than the end of it.
Two clay-specific issues catch people out.
Heave. Clay does not only shrink. It swells too. If mature trees are removed from a clay site, the ground they had been drying out slowly rehydrates and rises, sometimes for years afterwards. That upward pressure can lift a foundation. Where the risk exists, the design usually includes a compressible layer against the trench sides and beneath ground beams, so the clay has somewhere to expand into.
Sulphates. Some clays and many made-ground sites carry sulphates that attack ordinary concrete. Where the ground testing picks it up, the mix has to be specified accordingly.
None of this is exotic. It is standard practice on clay, provided whoever is pricing the job knows they are on clay and has priced for it. A quote based on a 750mm dig for a site that needs 1.5m is not a cheaper version of the same job. It is a different job, and the difference turns up later.
Foundations Near Trees
Trees on clay are the biggest single driver of foundation depth in the UK, and the reason two identical extensions on the same street can need very different digs.
A mature tree draws a large volume of water out of the soil every day through the growing season. On clay, that dries the ground out well beyond the canopy and well below normal seasonal depth. Build a shallow foundation into that dried zone and you have two problems: the ground moves as the tree's demand varies from year to year, and if the tree ever comes down, the ground rehydrates and heaves.
The depth needed depends on how shrinkable the clay is (its plasticity, which soil testing measures), the species of tree (oak, poplar, willow and elm have high water demand; birch and rowan are much lower), and how far the tree stands from the foundation relative to its mature height. The NHBC Standards set out tables combining these into a required depth, and Building Control and warranty providers will expect the design to follow them.
In practice, a high water demand tree close to a high-plasticity clay site can push a foundation to 2.5 metres or more. At that depth trench fill gets expensive and awkward to dig safely, and piling with ground beams often works out both cheaper and safer.
Two things are worth knowing before you plan around a tree. Removing it is not an automatic fix, because on clay the removal creates a heave risk that then has to be designed for. And the tree may have a Tree Preservation Order or sit in a conservation area, in which case you cannot touch it without consent, whatever it is doing to your foundation design.
How Much Do Foundations Cost?
Guide prices for 2026 domestic work:
| Job | Typical cost |
|---|---|
| Strip or trench fill foundation (per linear metre, standard ground) | £150 - £300 |
| Deep trench fill on clay or near trees (per linear metre) | £250 - £500 |
| Single-storey extension foundations (typical, complete) | £3,000 - £8,000 |
| Two-storey extension foundations | £5,000 - £12,000 |
| Raft foundation (typical domestic footprint) | £6,000 - £15,000 |
| Piled foundations with ground beams | £10,000 - £25,000+ |
| Ready-mix concrete, delivered (per m³) | £110 - £160 |
| Soil investigation / trial hole and testing | £400 - £1,500 |
| Underpinning existing foundations (per linear metre) | £1,000 - £2,000 |
In rough order of impact, the price moves on depth (every extra 300mm means more excavation, more concrete and more spoil), access (whether a machine and a concrete wagon can reach the trench, or whether everything has to be barrowed and pumped), spoil removal (a deep clay dig produces a lot of material and all of it has to be carted away), concrete specification (sulphate-resisting mixes cost more), and whether the design needs an engineer's calculation.
Two costs people forget to budget for. The soil investigation runs to a few hundred pounds and is most of what stands between you and a mid-dig surprise. And a contingency for depth is worth carrying, because Building Control can require you to go deeper on the day, and that cost lands on you either way.
Signs of a Foundation Problem in an Existing Building
Most people never think about their foundations until something above them starts cracking. What to look for:
- New diagonal cracks, wider at one end, running through brickwork and showing up inside and out in the same place
- Cracks wider than 3mm, particularly around door and window openings
- Doors and windows sticking where they did not before
- Cracking where an extension meets the original house, usually a sign that the two are on different foundation depths and moving independently
- Sloping or springy floors
Not every crack is a foundation problem, and most are seasonal or cosmetic. The pattern worth taking seriously is diagonal, still growing, and visible on both faces of the wall. Our guide to the signs of subsidence in Norfolk covers how to tell the difference and what to do next.
Where the ground is the cause, the repair is usually underpinning: extending the existing foundation down to stable ground in sequenced sections. It is disruptive and expensive — our underpinning cost guide sets out the real numbers — which is the strongest argument there is for getting the depth right the first time.
Drainage and Foundations
Water is behind most foundation problems, so where your water goes matters about as much as how deep you dig.
A soakaway discharging too close to a building saturates the ground beneath it, which is why the rules require at least 5 metres between a soakaway and any structure. Leaking drains do the same thing more slowly, washing fines out of the ground and softening it, and a downpipe emptying at the base of a wall manages it faster than either.
If you are building an extension, the surface water off its roof has to be dealt with properly, and on clay that means a designed soakaway rather than an assumption. Our soakaway cost and installation guide covers how one gets sized. Where existing drains cross the footprint, they need locating before the dig rather than during it, and our drainage services in Norfolk cover surveying and diverting them as part of the same job.
FAQ: Foundations
How deep should foundations be?
For a typical UK house or extension, foundations are 900mm to 1.5 metres deep. The absolute minimum under Building Regulations is 450mm below finished ground level to get below frost action, but 750mm is the practical minimum on good ground and 900mm is the minimum on shrinkable clay. Near mature trees on clay, the required depth can be 1.5 to 3 metres or more. The final depth is confirmed by Building Control inspecting the open trench.
How deep should foundations be in clay soil?
At least 900mm, and often considerably deeper. Clay shrinks and swells with seasonal moisture change, and a foundation inside that moving zone will move with it. Where trees are within influencing distance, the depth is set by the clay's plasticity, the tree species and its distance relative to mature height, following the NHBC Standards tables, which can require 2.5 metres or more.
What is the minimum depth for foundations in the UK?
450mm below finished ground level is the absolute minimum, set to get the foundation below normal frost penetration. In practice this is only appropriate on non-shrinkable ground such as gravel or rock, and most domestic foundations are dug to at least 750mm, with 900mm the minimum on clay.
What are the different types of foundation?
Five types cover most domestic work: strip foundations (a concrete strip under load-bearing walls with blockwork built off it), trench fill (the trench filled with concrete almost to ground level), raft foundations (a single reinforced slab spanning the whole footprint), pad foundations (isolated pads under point loads such as steel columns), and piled foundations (piles taken down to competent ground and tied with ground beams). The choice is dictated by ground conditions, the depth required and site access.
What is the difference between strip and trench fill foundations?
A strip foundation is a relatively thin concrete strip at the base of the trench, with blockwork built up from it to damp proof course level. Trench fill pours concrete almost to the top of the trench instead. Trench fill uses much more concrete but far less labour and closes the trench the same day, which makes it the usual choice for deep digs and clay sites where the trench sides are unstable.
How much do foundations cost?
Strip or trench fill foundations run roughly £150 to £300 per linear metre on standard ground in 2026, rising to £250 to £500 per linear metre for deep digs on clay or near trees. A typical single-storey extension comes in at £3,000 to £8,000 and a two-storey extension at £5,000 to £12,000. Raft foundations are £6,000 to £15,000 and piled foundations start around £10,000. Depth, access and spoil removal are the biggest cost drivers.
Do foundations need Building Regulations approval?
Yes. Foundations fall under Part A of the Building Regulations, and Building Control must inspect the open trench before any concrete is poured. The inspector checks the depth and the bearing ground and can require you to dig deeper. Pouring before that inspection risks being told to break the foundation out.
How close to a tree can you build foundations?
There is no single distance, because it depends on the tree species, its mature height and the shrinkability of the soil. High water demand species such as oak, poplar, willow and elm influence the ground much further than low demand species such as birch or rowan, and the effect is far greater on high-plasticity clay. The NHBC Standards tables combine these factors into a required foundation depth, and Building Control will expect the design to follow them. Removing the tree is not an automatic solution, because on clay the ground then rehydrates and heaves.
What concrete is used for foundations?
A GEN1 designated mix covers ordinary domestic foundations. Where soil testing shows sulphates in the ground, which is common in some clays and in made ground, a sulphate-resisting designated mix such as FND2 is required, because ordinary concrete deteriorates in sulphate-bearing conditions. Ready-mix delivered runs about £110 to £160 per cubic metre in 2026.
Can you build an extension on the existing house foundations?
No. An extension needs its own foundations, designed for its own loads and taken to a depth appropriate to the ground. Tying new shallow foundations into an older, deeper structure is a common cause of cracking where the extension meets the house, because the two then move independently. Where the new foundation goes deeper than the neighbour's within 3 metres of their wall, the Party Wall etc. Act 1996 also applies.
What happens if foundations are too shallow?
The foundation sits inside the zone where the ground moves seasonally, so the building rises and falls with it. Over time that produces diagonal cracking, sticking doors and windows, and in clay ground the pattern of movement associated with subsidence. The remedy is underpinning, which means extending the existing foundation down to stable ground in sequenced sections, and it typically costs £1,000 to £2,000 per linear metre. That is far more than digging correctly in the first place.
The Bottom Line
Foundation depth is not something you can look up. It is the depth at which your particular ground stops moving, and it comes out of soil type, frost, moisture, load and whatever else happens to be buried on site. For most UK domestic work that lands between 900mm and 1.5 metres, with 450mm as a frost minimum that rarely applies in practice.
In Norfolk, clay makes the question sharper. 900mm is the floor rather than the target, and mature trees on clay can push the requirement to 2.5 metres or beyond, at which point piling starts to make more sense than digging. Get it wrong and nothing happens straight away. It shows up as cracking three winters later, and the fix is underpinning at several times what the extra depth would have cost.
So: do the soil investigation, price the job for the ground you actually have, and expect Building Control to have the final say once the trench is open. Budget for them to want more. Of everything in a build, the foundations are the part you cannot go back to once the walls are up, which is why they deserve more thought than they usually get.
Planning an extension or new build in Norfolk? KiKK brings groundworks and clay ground-condition expertise to foundations across Norwich and Norfolk: soil assessment, excavation, and concrete, dug to the depth the ground actually needs. Contact KiKK →
This article provides general guidance and is not a substitute for site-specific structural design. Foundations should be designed in accordance with Part A of the Building Regulations and, where relevant, the NHBC Standards, on the basis of a soil investigation of the actual site. KiKK Groundworks provides groundworks, foundations, drainage, driveways, and civil engineering across Norwich and Norfolk. NRSWA accredited, fully insured, owner-led.