Foundation depth is not a builder’s judgement call. On shrinkable clay it is calculated from soil testing and tree data, which is why structural engineers near me is a search most Kingston homeowners run before they run any other one. The difference between the right answer and a rule of thumb can be two metres of excavation.
That matters here because the borough combines two conditions that push depths down: shrinkable clay soils across much of the area, and a high concentration of mature trees, particularly through Coombe and Kingston Hill where landscaped grounds and woodland are part of the character.
Why Clay Moves and Sand Does Not
Shrinkable soils change volume as their moisture content changes. They contract in dry spells and swell when rain returns, and that seasonal cycle transfers movement into anything founded too shallowly.
The NHBC definition is precise. A shrinkable soil is one containing more than 35 percent fine particles, meaning particles below 60 microns, with a modified Plasticity Index of 10 percent or greater.
Plasticity Index is the measure of volume change potential, established by Atterberg limits testing on samples from trial pits. It is graded as low, medium, high or very high, with a rough working guide of below 20 for low, 20 to 40 for medium and above 40 for high. Most London Clay falls into the high to very high range.
Sandy and gravelly soils sit outside the category entirely. They barely move with moisture, which is why foundations in them commonly sit at 600 to 900mm and stay there.
The Baseline Depths Before Trees Enter the Picture
On flat ground with no tree influence, formation depth, meaning the underside of the concrete, follows the soil classification.
- Low plasticity clay: minimum 0.75 metres
- Medium plasticity clay: minimum 0.9 metres
- High plasticity clay: minimum 1.0 metre
Those figures assume straightforward greenfield type conditions. Made ground, varying strata or a sloping site all change the answer, and a minimum embedment of 150 to 300mm into the underlying subsoil is typically specified on top.
So the frequently heard “900mm will be fine” is a defensible starting point on medium clay with nothing else going on. It is a guess on a site with a mature oak in the garden.
What Trees Do to the Number
Roots extract moisture from clay, producing localised shrinkage well beyond normal seasonal drying. The effect is concentrated, persistent and species specific.
NHBC Standards Chapter 4.2 is the reference the industry works to, and its tables cross reference four variables: soil plasticity, tree species, the tree’s mature height, and its distance from the building. High water demand species, principally oak, elm, willow and poplar, drive depths deepest.
The range this produces is wide. Engineered depths run from the 0.9 metre baseline up to around 3.0 metres for the most onerous combinations. A high shrinkage clay with an oak at half to one times its mature height sits at roughly 2.5 to 3.0 metres.
There is a practical threshold in there. A trench beyond about 2.5 metres becomes difficult to construct, and piled foundations with ground beams usually become the more economic solution at that point.
Soil investigation should be commissioned before depth is signed off on any site with mature trees within roughly 1.25 times the tree’s mature height. That means trial pits and laboratory Plasticity Index testing rather than a visual assessment.
Heave Is the Risk Nobody Expects
Subsidence gets the attention. Heave causes at least as much trouble and works in the opposite direction.
If a tree that has been drying the clay for decades is removed, the soil rehydrates and swells upward. That upward pressure can lift a foundation, and it does not care that the foundation was correctly designed for the conditions that existed when it was poured.
Two consequences follow for anyone extending in a well treed part of Kingston.
Removing a tree before or during construction needs specialist advice rather than a chainsaw. And where heave is a risk, depth alone is not the answer. Compressible boards or void formers are specified on the ground side of the trench, and NHBC looks for heave precautions where foundations are deeper than 1.5 metres and within the zone of tree influence.
Those voids have to stay clear. Bridging them with hardcore, mortar droppings or offcuts of insulation transfers the pressure straight into the structure and defeats the point of installing them.
Protected Trees Add a Legal Layer
In Kingston, and particularly around Coombe, Tree Preservation Orders are common. A protected tree brings two separate obligations.
You cannot cut down, top, lop, uproot or wilfully damage it without the council’s written consent. And the root protection area calculated to BS 5837:2012 effectively excludes excavation from a zone around the trunk, which can rule out the position the extension wanted before foundation depth is even discussed.
On the Coombe Hill project Extension Architecture took from 200 to 600 square metres, an order applied to a single young deciduous tree north east of the existing garage, and that constraint influenced where new structures could sit.
Drains Complicate Excavation Too
A detail that catches people during digging rather than during design.
Where a drain runs near a foundation, it needs protecting from the load above. Drains within about a metre of a foundation typically need encasing in concrete, and the foundation may have to be deepened to pass beneath them.
That is an easy variation to absorb if the drainage position is known in advance. It is an expensive one to discover with an excavator already on site.
What This Means for Budgeting
Foundations are the single largest cost variable on a domestic extension, and the spread reflects exactly the factors above.
Standard strip foundations on stable ground for a 25 square metre extension commonly run £3,000 to £6,000. Engineered solutions, meaning deeper trench fill, a raft, or mini piles, run £8,000 to £15,000 and beyond.
That is a five figure swing decided by soil classification, tree proximity and species. Which is why a ground assessment before design carries more weight than most homeowners expect: it does not merely inform the budget, it can change what is buildable on the plot.
The Order of Work That Avoids Surprises
Commission trial pits and Plasticity Index testing early, particularly where mature trees are present. Survey the trees, including any overhanging from neighbouring gardens, and establish which are protected.
Then let the structural engineers near me search produce someone who designs from that data rather than from a standard detail, with calculations going to Building Control under Part A. Foundation type, depth, width and reinforcement all follow from ground bearing pressure and tree influence rather than from preference.
A kingston architect working locally will usually have the trees flagged before the first sketch, because on this ground the design responds to what is underneath it rather than the other way round.
