Foundations are the part of a build that nobody admires once the walls go up, yet they are often the hardest cost to pin down before the digger arrives. Two extensions of the same size can end up with very different bills, depending on the soil, the depth, and how easy the site is to reach.
The average concrete foundation cost in the UK is around £7,000 for a new build or large extension, and a typical single-storey extension usually falls between £4,500 and £6,500. Strip foundations start at about £90 per metre, while raft or piled foundations on poor ground cost considerably more.
Average Concrete Foundation Cost in the UK
Total cost depends mostly on the size of the building and the type of ground beneath it. These typical ranges cover most domestic projects:
- Porch or Small Garage: Around £2,500 to £4,000
- Single-Storey Rear Extension: Around £4,500 to £6,500
- Two-Storey Extension: Around £6,000 to £9,000
- New Detached House: Around £10,000 to £20,000 or more
These figures cover digging, concrete, and labour for standard foundations on good ground. Every foundation cost UK estimate should still be checked against a site survey, and soil or access can change the total quickly.
Concrete Foundation Cost by Type
The type of foundation has the biggest single effect on concrete foundation cost. Your engineer or building control officer will usually confirm which one your ground needs.
Strip Foundations
Strip foundations are the standard choice for most UK homes and extensions. The builder digs a trench under each wall and pours a layer of concrete about 225mm deep at the bottom. Standard 600mm strips cost about £90 to £150 per linear metre, and wider strips for softer ground run from about £140 to £220 per metre.
Trench Fill Foundations
Trench fill foundations use the same trench but fill it almost to the top with concrete. This method uses more concrete than a strip, but bricklayers can start sooner, and the crew spends less time working below ground. Digging and filling a 7m by 5m footprint to a depth of 1m usually costs about £5,600 to £6,000.
Raft Foundations
Raft foundations spread the weight of the building across one reinforced concrete slab. Builders usually choose a raft in these situations:
- Soft or uneven ground
- Made ground or old fill
- Sites where deep trenches are not practical
Rafts cost about £100 to £190 per square metre, depending on slab thickness and reinforcement. Most domestic rafts total £6,000 to £15,000 or more.
Pad Foundations
Pad foundations are separate blocks of concrete placed under single load points such as columns, posts, or steel frames. Each pad usually costs about £300 to £800, depending on its size and depth. Pads suit garden rooms, carports, and framed buildings better than solid masonry walls.
Piled Foundations
Piled foundations carry the weight of a building down to firmer ground using concrete or steel piles. Engineers specify them when the soil near the surface is weak, very wet, or affected by large trees. Expect about £400 to £900 per pile, or roughly £8,000 to £15,000 for a typical extension.
Where the Money Goes in a Foundation Budget
Every foundation quote combines several separate costs. Seeing each one helps you compare quotes properly:
- Excavation: Machine digging usually costs about £85 to £125 per cubic metre, including the operator. Hand digging on tight sites costs more and takes longer.
- Muck-Away: Excavated soil has to go somewhere, and a grab lorry typically charges £250 to £450 per load. Reusing spoil for landscaping on site can cut this cost.
- Concrete Supply: Ready-mixed foundation concrete usually costs about £100 to £160 per cubic metre delivered. Common foundation mixes include FND2 or C25, depending on the ground and the design.
- Groundwork Labour: Groundworkers typically charge about £120 to £250 per day each. Most domestic foundations need a small team for several days.
- Reinforcement and Formwork: Rafts, ground beams, and wide strips often need steel mesh or bars, plus timber shuttering on sloping sites. Steel prices have climbed, with concrete reinforcing bars up 12.5% in the year to August 2026, while ready-mixed concrete prices dipped slightly.
- Engineer and Ground Investigation: A structural engineer’s design typically costs £500 to £1,500. Trial pits cost about £300 to £600 each, and boreholes run from about £600 to £2,000.
- Building Control: Fees for an extension usually range from about £500 to £1,200. The inspector checks the open trenches before any concrete goes in.
Concrete and excavation usually make up the largest share of the bill. Asking for each item to be priced separately makes quotes far easier to compare.
Extension Foundation Cost and Other Common Projects
Project size and layout decide much of the final figure. These examples show what different domestic builds typically need.
Single-Storey Rear Extension
The typical extension foundation cost for a single-storey rear extension with about 24 metres of wall runs from £4,500 to £6,000 for strip foundations and £5,000 to £6,500 for trench fill. Clay ground, nearby trees, or a sloping garden can push those figures higher. Rear gardens with no side access may also need a concrete pump, and pump hire adds to the total.
Garage or Garden Room Base
Garages and garden rooms often use a reinforced slab or a shallow raft instead of deep trenches, and most single garage bases fall within the range shown earlier. Lightweight garden rooms on pads can cost far less, often around £1,200 to £2,000. Heavier masonry garages still need proper footings, and building control will confirm what is required.
New-Build House
New houses usually need trench fill or engineered foundations under every external and internal load-bearing wall. Detached homes with around 40 metres of trench fill can need about 24 cubic metres of concrete before any allowance for over-digging. Ground investigation is especially worthwhile here, and the foundation design depends on its results.
How Much Concrete a Foundation Needs and What It Costs
Working out the concrete volume gives you a clear view of the biggest material cost. Multiply the length, width, and depth of the trench, all in metres:
- Measure the total length of all walls that need foundations.
- Multiply by the trench width, usually 0.6m for domestic strips.
- Multiply by the concrete depth, such as 0.225m for a strip or about 1m for trench fill.
- Add 5 to 10% to cover uneven trench sides and over-digging.
For a rear extension with 24 metres of wall, a standard strip needs about 3.2 cubic metres of concrete, or roughly £320 to £520 at typical ready-mix prices. The same extension on trench fill needs about 14.4 cubic metres, and that comes to around £1,440 to £2,300 before the extra allowance. Our free online concrete calculator does the same sum for any trench size.
If you’re confused, share your foundation drawings or trench sizes with our experts at Pro-Mix Concrete, and we will price the concrete as a separate line so you can see how much of your quote is material and how much is labour.
Factors That Push Concrete Foundation Costs Higher
Some sites need deeper, wider, or more complex foundations than others. These conditions add the most to a typical budget:
- Clay soil that swells and shrinks with the weather
- Trees within a few metres of the new walls
- Foundations that need to go deeper than 1 metre
- Narrow or rear-garden access for machines and lorries
- High groundwater that floods trenches
- Old drains or services found during the dig
- London and South East labour rates, often 15 to 40% higher
Clay is one of the most common reasons for deeper foundations, and much of London is built on it. The British Geological Survey projects that more than 26% of London properties could be affected by clay shrink-swell by 2070. That risk explains why many London builds carry a higher foundation budget from the start.
How to Keep Your Foundation Costs Down
Careful planning before the dig saves more money than any discount on the day. These steps help keep the budget under control.
Investigate the Ground First
Trial pits or a soil report cost a few hundred pounds but can prevent thousands in redesign later. Knowing the soil type early lets your engineer choose the most economical foundation instead of designing for the worst case. Surprises found halfway through a dig are the most expensive kind.
Order the Right Amount of Concrete
Accurate volumes stop you paying for concrete you do not use or for a second, part-filled delivery. Part loads often carry a surcharge, and a shortfall can leave a trench half-poured while you wait for more. Measuring carefully and adding a sensible margin avoids both problems.
Plan Access and Spoil Before the Dig
Clear access for diggers and delivery lorries keeps labour hours down. Where lorries cannot reach the trenches, a concrete pump is usually cheaper than barrowing concrete by hand for a whole day. Reusing excavated soil on site for landscaping also reduces the number of muck-away loads.
Is a concrete slab cheaper than strip foundations?
For small outbuildings, a slab on its own often costs less, but it cannot replace footings under load-bearing walls. Larger buildings on poor ground may need a raft instead, and rafts usually cost more than standard strips.
How long do concrete foundations take to cure before building?
Most builders start blockwork on strip or trench fill foundations within a few days, once the concrete has hardened enough to walk on. Concrete keeps gaining strength for about 28 days, and full structural loads are best added after that.
Bottom Line
The concrete foundation cost for most UK projects ranges from about £4,500 to £6,500 for a single-storey extension to £10,000 or more for a new house. Strip foundations remain the cheapest option on good ground, while rafts and piles add cost where the soil is weak or affected by trees. Getting a ground survey, pricing each item separately, and ordering the right volume of concrete keeps the final bill close to the original estimate.
Open trenches do not wait well, and rain, collapsing sides, or a late delivery can quickly add days and cost to a foundation job. At Pro-Mix Concrete, we supply foundation concrete across London and the surrounding areas, delivered ready-mixed or mixed fresh on site, with line and boom pumps available when access is tight. Each load is batched to the grade your engineer specifies, ready to pour as soon as building control signs off the dig.
Call the Pro-Mix team on 0800 772 3808 with your trench sizes and pour date. We will confirm the volume, the mix, and a delivery slot that lines up with your inspection.
- Dennis Broderick is the founder and owner of Pro-Mix Concrete Company, a trusted name in ready-mix concrete solutions across the UK. With over 20 years of hands-on experience in the construction and concrete industry, Dennis brings unmatched expertise, practical insights, and a commitment to quality on every project - from residential driveways to large-scale commercial developments.
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