Foundation type is decided by ground conditions and what is nearby, not by the size of the extension going on top. Two identical single-storey extensions on the same street can end up with very different foundation designs once trees, drains, made-up ground or a sloping plot are taken into account. Because foundations are entirely invisible once the job is finished, they are also the part of the budget most likely to be underestimated by anyone pricing from a drawing alone. This guide covers the main foundation types, when each is used, and the factors that push a straightforward dig into something more involved.
Strip foundations
A strip foundation is a continuous trench of concrete under the load-bearing walls, wide enough to spread the building's weight into ground of reasonable, consistent bearing capacity. It is the traditional approach for straightforward single-storey extensions on firm, undisturbed ground, and remains the most common foundation type for domestic work where site conditions are unremarkable.
Trench fill foundations
Trench fill is a deeper, narrower trench filled almost to ground level with concrete rather than built up with blockwork. It has become the default choice for most modern domestic extensions because it reduces the amount of time bricklayers spend working below ground and copes better with deeper excavations, which are increasingly common due to tree influence and services.
Raft foundations
A raft foundation is a reinforced concrete slab spanning the whole footprint, spreading load over a wide area rather than concentrating it in trenches. Rafts are used where ground conditions are poor, where deep trench excavation is impractical, or where existing foundations nearby cannot be undermined by a deep dig. They generally need a specific structural design rather than a standard building control detail.
Piled foundations
Where ground conditions are genuinely unsuitable for shallow foundations — soft clay, made-up ground, or proximity to trees with a history of subsidence — piles transfer load down to firmer strata well below the surface, tied together with a reinforced ground beam. Piling is significantly more expensive than trench-based methods but is sometimes the only reliable option, and is common on infill or previously developed plots.
Comparing the options
The table below sets out typical depths, when each type is used, and a broad indicative cost comparison. Ground investigation and, in some cases, an engineer's design are needed before a final choice is confirmed.
| Foundation type | Typical depth | When it's used | Indicative cost range |
|---|---|---|---|
| Strip | 600mm–1m | Firm, consistent ground, standard single-storey extension | £90–£150 per linear metre |
| Trench fill | 1m–2.5m+ | Standard for most modern extensions, deeper digs, tree influence | £110–£190 per linear metre |
| Raft | Slab, minimal depth below ground | Poor ground, restricted excavation depth, nearby structures | £150–£250+ per m² of footprint |
| Piled | 3m–10m+ depending on strata | Very poor ground, significant tree influence, made-up ground | £250–£500+ per m² of footprint |
Figures are broad indicative UK ranges and vary considerably by region, ground conditions and access for plant. A site investigation and, where needed, an engineer's design are required before a firm price can be given.
Trees, NHBC influence distances and building over drains
Trees are one of the most common reasons a foundation ends up deeper or more expensive than expected. NHBC guidance sets influence distances for different species based on their mature height and water demand, and where an extension falls within that distance, foundations typically need to go deeper or be redesigned to avoid future ground movement as roots take up moisture. Building over or near an existing drain run raises its own requirements — Building Regulations (Part H, alongside structural provisions) generally require the drain to be protected with a lintel and bridging over it, or diverted, and building control will want to see this detailed before work starts.
- High water-demand species (e.g. oak, willow, poplar) have the largest influence distances
- Removed trees can also affect ground moisture recovery and need separate consideration
- Drains under or near foundations usually need a designed lintel or diversion, agreed with building control
Why ground investigation matters
A trial pit or borehole, sometimes combined with a desktop study of geological records, tells the designer what is actually in the ground rather than what is assumed. On infill plots or where the history of the site is unclear — old outbuildings, filled ponds, previous industrial use — skipping this step is a common cause of foundation problems discovered mid-dig, which is the most disruptive and expensive time to discover them.
Groundworks in wet weather and over winter
Groundworks are one of the stages most exposed to weather, and a wet spell can turn a straightforward dig into a genuinely difficult one — trench sides can slump in saturated clay, and concrete should not be poured into standing water or onto waterlogged ground without extra preparation. Winter digs also bring a frost risk to freshly poured concrete, which needs protecting if temperatures are forecast to drop before it has cured sufficiently. None of this makes winter groundworks impossible, but it is sensible to build some contingency into the programme and to expect a competent groundworker to pause rather than push on with a pour in genuinely unsuitable conditions, since a foundation poured badly is far more costly to put right later than a few days' delay.
What building control checks at foundation stage
Foundations are one of the notifiable stages building control physically inspects on site, typically once the trench or excavation is open and again once reinforcement (where used) is in place, before concrete is poured. The inspector is checking the depth reached matches what was specified for the ground conditions found, that there is no evidence of soft spots, tree roots or made-up ground that would need the design revisiting, and that any drains or services running through the area have been protected as detailed. Because concrete cannot easily be un-poured, this inspection genuinely needs to happen before the pour, not as a formality afterwards, and a good contractor will build the inspector's visit into the programme rather than treating it as an interruption.