Foundations are the part of your new build that nobody ever sees. They go in the ground, get buried, and that is that. Which is exactly why they matter so much.

Get them right and they will support your home without complaint for a hundred years. Get them wrong and you will be dealing with cracked walls, sticking doors, and expensive remedial work long after you have moved in. In the worst cases, you will be looking at underpinning costs that dwarf what a proper foundation would have cost in the first place.

The foundation type you need depends on your site, your soil, what is growing nearby, and what you are building. There is no universal answer. This guide explains the main foundation types used in UK new builds, when each one is appropriate, and what London’s ground conditions mean for your project specifically. If you have not yet commissioned a ground investigation, our guide to surveys needed before a new build explains why that should come before any foundation design.

Why the Right Foundation Type Matters

A foundation does two things. It transfers the load of the building safely into the ground, and it prevents movement that would cause the structure above to crack or distort.

The challenge is that ground conditions vary enormously, even across a single plot. Soil can be strong or weak, stable or shrinkable, dry or waterlogged. What lies two metres down may be completely different from what is at the surface. And the presence of trees, underground services, water, or made ground from previous development all change the picture.

This is why foundation design is a structural engineering decision, not a contractor guess. Your structural engineer will use the results of your ground investigation to specify the right foundation type and depth for your specific site. If a contractor is telling you what foundation you need before a ground investigation has been done, that is a problem.

Never finalise a foundation design without a ground investigation report in hand. The cost of the investigation is small. The cost of redesigning foundations after work has started, or worse, after problems emerge, is not.

The Main Foundation Types Used in UK New Builds

Strip Foundations

Strip foundations are the most traditional and widely used foundation type for domestic buildings in the UK. A continuous trench is excavated under each load-bearing wall, concrete is poured to form a wide strip, and the walls are then built up from that concrete base.

There are two versions. Traditional strip foundations use a relatively shallow concrete strip with blockwork built up below ground to damp proof course level. Trench fill foundations are deeper and filled almost entirely with concrete, reducing the amount of blockwork needed below ground. Trench fill has become the more common choice for most new builds because it is faster to construct and less labour intensive, even though it uses more concrete.

Type	Typical Depth	Best Used When	Approximate Cost
Traditional strip	450mm to 1m	Good bearing soil, no significant trees nearby, simple layouts	£95 to £115 per linear metre
Trench fill	750mm to 2.5m+	Moderately shrinkable clay, deeper bearing required, faster programme preferred	£130 to £160 per linear metre

Strip foundations are well-suited to sites with firm, stable soil where bearing capacity is adequate and ground conditions are predictable. On London clay, depths are often pushed well beyond the standard minimum because of the soil’s shrinkable nature.

Raft Foundations

A raft foundation is a reinforced concrete slab that covers the entire footprint of the building rather than just the lines under the walls. Instead of transferring loads through individual strips, the raft spreads the total load across the whole ground area.

Rafts are used when soil bearing capacity is poor or variable, when differential settlement is a concern, or when the ground conditions make individual strip foundations impractical. They are also used on sites with highly shrinkable clay where the risk of uneven movement between different parts of the strip would be too high.

The raft needs to be stiff enough to resist the ground movement beneath it without cracking. This means thicker concrete and more reinforcement steel than a strip foundation, which pushes the cost up. Typically a raft foundation for a residential new build costs between £120 and £220 per square metre, compared to the linear metre pricing of strip foundations.

A raft is not just a thick floor slab. It is a structural element designed by an engineer to resist specific loads and movements. Do not let anyone tell you a standard ground-bearing slab will do the same job as a designed raft. It will not.

Pile Foundations

Pile foundations transfer the load of the building down through weak or unsuitable near-surface soil to a stronger layer of ground at depth. Piles are long structural columns, either driven or bored into the ground, that carry the building load through whatever poor material is at the surface to reach something capable of supporting it.

There are several types of pile used in residential construction:

• Bored cast in situ piles: A hole is drilled into the ground and filled with reinforced concrete. Suitable for most London sites, quieter than driven piles, and can be designed to very precise specifications.
• Driven piles: Pre-formed concrete or steel piles are hammered into the ground. Fast but noisy and vibration-intensive. Less commonly used in urban residential settings because of the impact on neighbouring properties.
• Mini piles or micro piles: Small diameter piles installed with compact equipment. Useful where access is restricted, headroom is limited, or the site is too constrained for full-size plant. Common on tight urban London plots.
• Screw piles: Steel piles with a helical flight that are screwed into the ground. Quick to install and with no spoil to remove. Suitable for lightly loaded structures and increasingly used for smaller residential projects.

Piles are connected at the top by a pile cap or a ground beam, which then supports the walls or frame above. Pile foundations are more expensive than strip or raft solutions, typically starting at £12,000 and rising substantially on complex sites, but on sites where the near-surface ground is genuinely poor they are often the only viable option.

Pad Foundations

Pad foundations are isolated blocks of concrete that support individual point loads, such as columns in a framed structure. They are less commonly used in traditional brick and block residential new builds, but they appear regularly in timber frame and steel frame homes where the structural load is carried through a series of columns rather than continuous load-bearing walls.

A pad foundation is essentially a thickened concrete base designed to spread the load from a single column or post into the ground below. They are relatively simple and economical for what they do, but they only make sense where the structural arrangement calls for point loads rather than continuous wall loads.

What London’s Ground Conditions Mean for Your Foundation

London is not a simple place to build. The ground across the city varies significantly, and several factors make foundation design here more complex than in many other parts of the country.

London Clay

Much of London, particularly south and west London, sits on London Clay. This is a highly shrinkable soil that expands when wet and contracts when dry. The seasonal movement this creates is one of the most common causes of foundation problems in the UK. The British Geological Survey’s interactive map lets you check the geology under your specific site, and it is worth doing before you commit to anything.

On London Clay, strip foundations typically need to go deeper than they would in other parts of the country. The standard minimum of 450mm that applies in stable conditions is rarely sufficient. Most domestic foundations on London Clay are designed to at least 900mm to 1m, and where trees are present that depth can increase dramatically.

Trees on Clay: the Biggest Variable

If your site has trees nearby, and in London that is more often than not, their interaction with clay soil is one of the most significant factors in your foundation design. Tree roots extract moisture from clay soil, causing localised shrinkage that can extend well beyond the visible canopy. The NHBC Standards Chapter 4.2 provides detailed guidance on foundation depths near trees based on species, mature height, and soil shrinkability.

The practical implications are significant. A mature oak 15 metres from your building on London Clay could require foundation depths of 2.5 to 3 metres under NHBC guidance. That is a very different piece of groundwork to a standard trench fill at 900mm, and the cost difference is substantial.

Trees that have been removed are also a consideration. When a tree is removed, the clay soil it was desiccating begins to rehydrate and swell. This is called heave, and it pushes upward rather than downward. Foundations on a site where trees have recently been removed need to account for heave risk, which may mean a different foundation type or the inclusion of compressible material beneath the slab.

Never remove a tree from a clay site without getting structural engineering advice first. The rehydration of clay after tree removal can cause more damage than the tree itself.

Brownfield and Made Ground

Large areas of London were developed industrially before the current wave of residential development. Brownfield sites often have made ground: fill material of unknown composition placed over the original ground at some point in the past. Made ground can be anything from demolition rubble to industrial waste, and its bearing capacity is unpredictable.

On made ground, strip foundations into the fill material are almost never appropriate. The solution is usually to pile through the made ground to reach a bearing stratum below it, or to use a raft designed to bridge over variable compressibility in the fill. Either way, you cannot design the foundation until you know how deep the made ground goes and what lies beneath it, which is exactly what a Phase 2 ground investigation is for.

High Water Table

Near the Thames and its tributaries, the water table can be close to the surface. This affects foundation design in two ways. First, excavation becomes harder and more expensive because groundwater needs to be managed during the dig. Second, any below-ground structure needs to be designed to resist hydrostatic pressure. Basements in particular require waterproofing systems that are significantly more complex and expensive on high water table sites.

How the Right Foundation Type Gets Chosen

Your structural engineer leads this process. Here is how it works in practice.

• The ground investigation report is reviewed. Soil type, bearing capacity, shrinkability, groundwater level, and contamination are all assessed.
• Any trees within the zone of influence are identified and species are checked against NHBC Chapter 4.2 tables to determine foundation depth requirements.
• The structural loads from the building design are calculated. A heavier structure or one with unusual loading patterns may push toward a more robust foundation solution.
• The engineer selects the foundation type that is safe, appropriate for the ground conditions, and buildable on the specific site.
• Foundation drawings and specifications are produced for building control approval and for the contractor to price and build from.

This process is not optional and it is not something that can be shortcut. Building control will inspect your foundations before the concrete is poured and before any work covers them up. If the foundations do not meet the approved specification, you will be asked to rectify them at your own cost.

Never pour concrete before building control has inspected the foundation trenches. Once the concrete is in, nobody can verify the depth, the soil conditions at the base, or the trench preparation. If your contractor wants to pour before the inspection, stop them.

Foundation Costs: What to Budget

Foundation costs vary significantly based on type, depth, soil conditions, and site access. Here is a realistic guide for a London new build in 2026.

Foundation Type	Typical Cost Range	Key Variable
Strip (traditional)	£10,000 to £18,000 for a 3 bed house	Trench length and depth
Trench fill	£13,000 to £22,000 for a 3 bed house	Concrete volume and depth
Raft	£120 to £220 per m² of footprint	Slab thickness and reinforcement spec
Mini piles plus ground beam	£15,000 to £40,000+	Number of piles and depth to bearing
Full piled solution (brownfield or poor ground)	£25,000 to £60,000+	Ground conditions, pile diameter, depth

These ranges are starting points. On a constrained London site with deep clay, nearby trees, and restricted access for plant, foundation costs can easily exceed these figures. The ground investigation report is what gives you the information to price foundations accurately before you commit. Pricing foundations without it is guesswork.

If you want to understand where foundations sit within the overall cost of a new build, our guide to new build costs in London covers the full breakdown.

Common Foundation Mistakes and How to Avoid Them

Assuming standard depths will do

The 450mm minimum depth in Building Regulations Approved Document A is an absolute floor that applies in ideal conditions. Virtually no London site qualifies as ideal. Clay soil, nearby trees, made ground, and high water tables all push depths up. Your structural engineer sets the depth, not Building Regulations.

Skipping or skimping on the ground investigation

The ground investigation is what tells your engineer what they are designing for. Designing foundations without it is like specifying a roof structure without knowing what loads will sit on it. The investigation costs a fraction of what a foundation redesign or remediation would cost. Do it properly and do it early.

Not accounting for trees

Trees are one of the most common causes of avoidable foundation problems in London. Any tree within a distance of its mature height from your building on clay soil needs to be assessed against NHBC Chapter 4.2. This includes trees on neighbouring land that you have no control over. If they are there now, they need to be in the foundation design.

Removing trees without engineering advice

Removing a tree to make way for a new build seems straightforward. On clay it can create a heave problem that is worse than the shrinkage risk the tree presented. Always get structural engineering input before removing any significant tree on a clay site.

Not allowing for groundworks contingency

Foundations are where the unexpected happens most often. You dig down and find something that was not in the ground investigation. Made ground deeper than expected. An old wall. A drain not on any record. A pocket of weak soil. Build a contingency of at least 15% into your groundworks budget specifically because of this risk.