The short answer
There is no single legal foundation depth in the UK. As a starting point, footings on firm ground are rarely shallower than about 0.75–1.0m, partly to get below the zone affected by frost and seasonal moisture. On shrinkable clay the figure rises sharply, and where trees are nearby, NHBC guidance can require depths of 1.5m to 3.0m or more depending on the species, distance and clay shrinkability. The governing rule is Part A of the Building Regulations, which requires foundations to reach firm, undisturbed ground capable of carrying the load without harmful movement. The real depth is never final until the trench is open: your Building Control surveyor inspects it and can require you to dig deeper if the bearing soil is poorer than assumed.
"How deep?" is the most common foundation question and the one with the least satisfying answer, because the depth is dictated by the soil rather than a table. Here is what actually sets it.
Typical depths
- Firm ground, no trees~0.75–1.0m
- Frost / seasonal zoneGenerally below ~0.75m
- Shrinkable clayOften 1.0–1.5m+
- Clay near trees1.5–3.0m or more
- Final depthConfirmed in open trench
What sets the depth
- Reaching firm ground: the trench must pass through topsoil and any soft, made-up or disturbed material to reach undisturbed soil that can carry the load.
- Frost and seasonal movement: shallow footings can be heaved or softened by frost and wet/dry cycles, so a minimum cover of firm soil above the bearing level is needed.
- Clay shrinkage: firm clays shrink and swell with moisture. Foundations must sit below the zone where this movement is significant.
- Trees and hedges: roots draw moisture from clay, deepening the shrinkage zone. The required depth depends on species, mature height and distance from the building.
| Situation | Indicative depth | Driver |
|---|---|---|
| Firm gravel/sand, no trees | ~0.75–1.0m | Below frost/seasonal zone |
| Shrinkable clay, no trees | ~1.0–1.5m | Below clay movement zone |
| Clay with nearby trees | 1.5–3.0m+ | Root-driven moisture change |
| Made-up / soft ground | Until firm stratum | Reach load-bearing soil |
Indicative ranges only; the figure is set by site conditions and confirmed by Building Control. Sources: NHBC Standards Chapter 4.2; Planning Portal Part A.
Why trees change everything on clay
On shrinkable clay, the controlling factor is usually trees. NHBC's foundation depth tables relate the required depth to the tree species water demand (high, moderate or low), its mature height, and the distance from the foundation. A high-water-demand tree such as an oak, poplar or willow close to the extension can push the trench past 2.5–3.0m, at which point trench fill becomes awkward and piled foundations are often more economic. Removing a mature tree shortly before building can also cause heave as the clay recovers moisture, which itself needs designing for.
How depth is confirmed and signed off
Foundation depth is checked under your Building Regulations application. The surveyor inspects the open trench before concrete is poured, looking for firm undisturbed soil at the design level, with no roots, soft pockets or standing water. Only when they accept it should you order the pour. If the ground is poorer than assumed, expect to dig deeper, step the foundation on a slope, or switch to a different foundation type — all of which are normal outcomes of an honest inspection rather than a fault.
Stepped foundations, frost and other depth factors
Depth is not always uniform along a foundation. On a sloping site, the trench is usually stepped down the slope in stages so each length sits at a consistent depth into firm ground while keeping a sensible top level for building — the steps follow rules in Part A on how far each one can rise or run. Where an extension meets the existing house, the new foundation generally needs to match or relate to the existing foundation depth, which sometimes means a short length of underpinning so the two sit compatibly.
- Frost heave: shallow footings in frost-susceptible soils can be lifted as ground water freezes and expands; adequate cover of firm soil above the bearing level protects against this.
- Made-up ground and fill: old gardens, demolition sites and infilled ponds can hide soft material that must be dug through to reach undisturbed soil — this can push depth well beyond a textbook figure.
- High water table: a trench that fills with water needs pumping to inspect and pour, and a very high water table may make deep trench fill impractical, favouring a raft or piles instead.
- Sulfates and aggressive ground: certain soils and groundwater attack ordinary concrete, so the mix may need to be specified accordingly at the same time depth is decided.
Each of these can change the practical depth on the day, which is why an experienced groundworker and an early ground investigation matter as much as the number on the drawing.
Finding out the depth before you dig the whole trench
Because the real depth is only certain in the open trench, the way to avoid a nasty surprise is to investigate the ground before the full dig and the budget are fixed. A modest amount of ground investigation turns an unknown into a planned figure and is far cheaper than discovering deep clay or made ground halfway through.
- Trial holes: a digger excavates one or two holes to expose the soil profile, the water table and any soft or made-up material, so the likely bearing depth is known in advance rather than guessed from a drawing.
- Soil sampling on clay: where shrinkable clay and trees are involved, samples establish the clay's plasticity (its shrinkability), which feeds directly into the NHBC depth tables that set how deep to go for a given tree and distance.
- Desk check: historic maps and the council can reveal old ponds, quarries, mine workings or infilled ground that hide soft material and force deeper foundations.
This early information also lets the right foundation type be chosen alongside the depth. If the tables point to depths beyond roughly 2.5m near a thirsty tree, trench fill becomes awkward and piling with ground beams is often more economic — a decision best made at design stage, not in a half-dug trench. Where any doubt exists, a structural engineer uses the investigation to specify the depth and type, so Building Control can approve the design and confirm it at the open-trench inspection without the programme stalling. The number on the drawing remains provisional until the surveyor sees the ground, but good investigation makes that number a reliable plan rather than a hopeful estimate.
Frequently asked questions
Is there a minimum legal foundation depth?
No fixed national minimum exists. Part A requires foundations to reach firm, undisturbed ground that carries the load without harmful movement. In practice that rarely means shallower than about 0.75–1.0m, and far deeper on clay near trees.
How deep should foundations be near a tree?
It depends on the species, its mature height and distance from the building, plus the clay's shrinkability. NHBC tables commonly give 1.5m to 3.0m or more for high-water-demand trees close to the foundation.
Can foundations be too shallow?
Yes. Shallow footings risk frost heave, seasonal clay movement and inadequate bearing, leading to cracking and subsidence. This is why Building Control inspects the open trench before any concrete is poured.
Sources & further reading
- NHBC — Standards Chapter 4.2 (building near trees)
- Planning Portal — Approved Document A (structure)
- LABC — foundations and Building Control inspections
Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.