The short answer
A strip foundation in the UK typically costs £80–£180 per linear metre dug and poured, with a standard domestic trench-fill foundation often around £100–£150 per metre. The rate covers excavating the trench, removing the spoil, and filling with concrete to the required depth. The biggest variable is depth: a 1m foundation on firm ground is at the lower end, while a 2m-plus foundation in shrinkable clay near trees can double or triple the per-metre figure because it uses far more concrete and generates far more spoil. Strip foundations must be designed and built to Building Regulations Part A and inspected by Building Control in the open trench before concreting.
Strip and trench-fill foundations are the most common domestic footing, priced per linear metre of trench. But the rate hinges on depth, which is set by the ground rather than chosen. The sections below explain the rate, the difference between strip and trench-fill, and what drives the cost.
At a glance
- Typical trench-fill~£100–£150/m
- Full range~£80–£180/m
- Biggest variableTrench depth
- Inspected byBuilding Control (open trench)
- Standard min depth~1m on firm ground
Typical strip foundation costs
Strip foundations are priced per linear metre of trench because that is how they are set out — lines of foundation under the walls. The per-metre rate is dominated by depth, since a deeper trench means more excavation, more spoil to remove and, for trench-fill, far more concrete. The table shows indicative rates at common depths; the deeper the foundation, the higher the per-metre figure, often steeply so once you pass 1.5m.
| Foundation depth | Indicative cost per metre | Notes |
|---|---|---|
| ~1m, firm ground | £80–£120 | Standard trench-fill |
| ~1.5m | £110–£160 | Deeper dig + concrete |
| ~2m, clay/trees | £150–£220 | More spoil + concrete |
| Traditional strip (masonry) | £90–£140 | Less concrete, more labour |
| Reinforced strip | £140–£200+ | Steel for poor ground |
Indicative figures for guidance only. Depth and ground conditions drive the rate.
Strip versus trench-fill
There are two common forms, and the per-metre cost differs. A traditional strip foundation has a relatively shallow band of concrete at the bottom of the trench, with the foundation brought up to ground level in masonry (blockwork). It uses less concrete but more labour and time laying the substructure brickwork in a confined trench. A trench-fill foundation pours concrete almost to the top of the trench, with only a few courses of masonry above. It uses far more concrete but is much faster, avoiding the slow, awkward work of bricklaying deep in a narrow trench.
Trench-fill has become the default for most domestic work, especially where foundations are deep, because the labour saving outweighs the extra concrete and it is quicker and safer than working at the bottom of a deep trench. Traditional strip can be more economical where the foundation is shallow and concrete prices are high relative to labour. The choice is partly cost and partly practicality — a 2m foundation is almost always trench-filled because no one wants to lay blockwork two metres down. When comparing quotes, it is worth knowing which method is being priced, as the balance of concrete versus labour differs and so does the per-metre rate.
Converting per-metre into a total
Because strip foundations are priced per linear metre, turning that into a project total is a matter of measuring the run of foundation under the walls. For an extension or a house, add up the perimeter and any internal load-bearing walls that sit on a foundation — that total length, multiplied by the per-metre rate for the required depth, gives the foundation cost. A modest rear extension might have 15–25 linear metres of foundation; a full house has considerably more once internal walls are included.
Two things make the total more than a simple length-times-rate sum. First, corners and junctions need slightly more material and labour than a straight run, and complex outlines with many returns cost more per metre than a simple rectangle. Second, and more importantly, the depth that sets the per-metre rate is itself uncertain until the trench is dug, so the total carries the same depth risk as the rate. A foundation priced at 1m that turns out to need 1.8m does not just cost more per metre — every metre of the run is affected, so the increase applies across the whole length. This is why a strip foundation total should be treated as firm only once the depth is confirmed by a soil survey or trial hole, and why a contingency against deeper-than-expected foundations is sensible. Measuring the run accurately and confirming the likely depth before pricing turns the per-metre rate into a reliable total rather than an optimistic estimate.
What drives the cost up
Depth is the dominant factor, and it is set by the ground, not the builder. The standard minimum is around 1m on firm soil, but shrinkable clay — particularly near trees, whose roots dry and shrink the soil seasonally — can demand 2m or deeper under NHBC and Building Control rules. Doubling the depth more than doubles the cost, because both the concrete volume (for trench-fill) and the spoil to dispose of increase. Trees within influencing distance are the classic reason a foundation goes far deeper than expected, and catch many homeowners out.
Ground conditions also bring other costs: made-up or soft ground may need a reinforced strip or rule out a strip foundation altogether in favour of a raft or piles; a high water table needs pumping and complicates the pour. Spoil disposal rises with depth and any contamination, and access affects how cheaply concrete can be placed — pumped or direct from the truck is far cheaper than barrowing. Trench width set by the wall and load adds concrete too. Because Building Control inspects the open trench and can require a deeper or reinforced foundation on the day, the safest way to price strip foundations is against a structural engineer's design informed by a soil survey, so the per-metre figure reflects the depth the ground actually needs rather than an optimistic assumption that gets corrected, expensively, on site.
Frequently asked questions
How deep does a strip foundation need to be?
A common minimum is around 1m on firm ground, but shrinkable clay or nearby trees can require 2m or more. Building Control inspects the open trench and confirms the required depth before concrete is poured, so the final depth is set by the ground.
What is the difference between strip and trench-fill?
A traditional strip foundation has a shallow concrete band with masonry built up to ground level; trench-fill pours concrete almost to the top of the trench with little masonry above. Trench-fill uses more concrete but is faster, especially for deep foundations.
Why do trees affect foundation cost?
Tree roots dry out and shrink clay soil seasonally, causing ground movement. To avoid the foundation moving, Building Control and NHBC rules require it to be taken deeper near trees — often well beyond the standard 1m — which significantly increases the per-metre cost.
Sources & further reading
- Checkatrade — foundations cost guide
- NHBC — foundation depths near trees
- Planning Portal — Building Regulations (Part A)
Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.