Domestic strip foundations in the UK are typically 1m deep, and 450 to 600mm for a garden wall. In the US, footings must reach below the local frost line, which ranges from 300mm in the south to 1.2m or more in northern states.
Those are starting points, not answers. The real depth depends on what the ground is made of, what’s growing near it, and what the building above weighs — and on most projects it’s confirmed by an inspector standing in the open trench rather than decided in advance.
What foundations are actually doing
Two jobs. They spread the building’s load over enough ground area that the soil can carry it without settling, and they reach down to a stratum that stays stable through the seasons.
The second job is the one that drives depth. Near the surface, soil moves. It freezes and heaves in winter, shrinks and swells with moisture, and gets disturbed by roots and burrowing. A foundation sitting in that zone rises and falls with it, and the building cracks. Going deeper reaches ground that stays put.
Depth by structure
| Structure | Typical depth | Typical width |
|---|---|---|
| Low garden wall (under 1m) | 300–450mm | 300–450mm |
| Garden wall (1–2m) | 450–600mm | 450–600mm |
| Garden room / large shed | 450–600mm | 450mm |
| Single-storey extension | 900–1,000mm | 600mm |
| Two-storey extension | 1,000mm+ | 600–750mm |
| Detached house | 1,000mm+ | 600–900mm |
| Near mature trees on clay | 1,500–3,000mm | To engineer’s design |
The width matters as much as the depth. A footing must be wide enough that the pressure it puts on the soil stays below what that soil can bear. The rule of thumb for domestic strip footings is roughly twice the wall thickness, but on soft ground it goes wider.
Soil type changes everything
| Soil | Bearing capacity | Behaviour |
|---|---|---|
| Rock | Very high | Stable, minimal depth needed |
| Gravel / sand | 100–300 kN/m² | Good bearing, drains well, stable |
| Firm clay | 100–150 kN/m² | Good bearing, but shrinks and swells |
| Soft clay | 50–75 kN/m² | Poor bearing, significant movement |
| Peat | Very low | Unsuitable — needs piles or full removal |
| Made ground | Unpredictable | Must be dug through to natural ground |
Clay is the problem soil across much of the UK and parts of the US. It holds water and expands, then dries and shrinks, and the movement is seasonal. A foundation at 600mm in shrinkable clay will rise and fall by several millimetres each year, which is enough to crack brickwork. A metre down, the moisture content is far more stable.
Made ground — a filled-in pond, an old quarry, demolition rubble, anything humans put there — is the one that catches people out. It looks solid, compacts under a boot, and then settles unevenly for decades. You dig through it until you reach undisturbed natural ground, however deep that turns out to be.
Frost depth
Water in soil expands roughly 9% when it freezes, lifting anything sitting above it. When it thaws, the ground settles back — but not evenly, and not necessarily back to where it started.
In the UK, 450mm is generally accepted as below the frost line, which is why garden walls at that depth are fine. The 1m used for buildings is driven by clay movement, not frost.
In the US the frost line varies enormously — around 300mm in Georgia, 1m in Chicago, 1.5m in northern Minnesota. Local building codes publish the figure and it isn’t negotiable. There’s a state-by-state breakdown in our guide to frost line depth and footings.
Trees
Trees on shrinkable clay are the single biggest cause of foundation depth increasing beyond the standard. A mature oak can draw hundreds of litres of water a day out of the ground in summer, drying and shrinking the clay well beyond its canopy.
NHBC guidance sets required depth based on tree species, mature height and distance from the foundation. A high water-demand species close to the building can push a foundation to 2.5m or beyond, at which point piles usually become cheaper than a deep trench.
Removed trees create the opposite problem. The clay that had been kept dry slowly rehydrates and swells — heave — which can lift a foundation over several years. Removing a tree before building can be worse than leaving it.
Stepping down a slope
On a sloping site the foundation follows the ground in steps rather than running at one level. Each step should be no more than the thickness of the foundation itself, and consecutive steps should overlap by at least twice the step height or 300mm, whichever is greater.
Stepping keeps the excavation depth reasonable on the high side while maintaining full depth on the low side. Trying to hold one level across a steep slope means either an enormous trench at the top or an inadequate one at the bottom.
Who actually decides
Not you, and not your builder. In the UK, building control approves the depth after inspecting the open trench. You dig to what you expect, then call for inspection before pouring anything. The inspector may well tell you to go deeper.
That’s why concrete for foundations should never be ordered until the trench has passed inspection. A load booked for the morning and a trench that needs another 300mm is an expensive combination.
In the US the process is similar — a footing inspection before the pour, checking depth, width, bearing and any reinforcement.
Drainage reduces heave
Frost heave needs three things together: freezing temperatures, frost-susceptible soil, and water. Remove any one and the problem largely disappears.
You cannot change the climate or easily change the soil, but you can control water. Free-draining granular backfill against a foundation, a perimeter drain at footing level, and surface falls that carry water away from the building all reduce heave substantially — often more cheaply than digging deeper.
Working out the concrete
Volume is trench length × width × depth, and the figure to use is the trench you actually dug rather than the one on the drawing. Over-dig of 50mm on each side of a 12m trench adds 1.2m³, which is a full extra part-load.
Use the footing calculator with your measured dimensions, and add 10% — trenches are never as neat as planned, and sides slump.
Common questions
How deep should foundations be for an extension?
Typically 1m for a single-storey extension on normal ground, more on clay or near trees. Building control confirms the depth at trench inspection, so don’t order concrete until that’s done.
How deep for a garden wall?
300 to 450mm for a low wall, 450 to 600mm for anything over a metre tall or on soft ground. The footing should be roughly twice the wall thickness wide.
What happens if foundations are too shallow?
Seasonal ground movement transfers into the structure. Symptoms are diagonal cracks in brickwork, doors and windows sticking, and cracks that open in summer and close in winter. Underpinning is the remedy, and it costs many times what the extra depth would have.
Can I dig foundations by hand?
For a garden wall, yes. For a 1m deep building foundation, a mini digger is faster, safer and cheaper than the days it would take by hand. Trenches over 1.2m deep also need shoring — an unsupported trench collapse is genuinely dangerous.
Do foundations need reinforcement?
Plain mass concrete is standard for domestic strip footings. Reinforcement comes in where ground conditions are poor, the footing spans soft spots, or an engineer specifies it after a soil investigation.