Whenever we want something built, whether it’s a house, tower block or just a garden path, we need stable foundations on which to build. The types and sizes depend on the ground’s stability, the weight of the structure or extension, and other factors we’ll consider later. But, apart from lightweight structures and shallow foundations like a pathway, you must get a qualified structural engineer or architect to design them.
Generally speaking, typical trench-fill concrete foundations of a standard design, of 20m x 0.6m x 1m, cost on average £2,400-£3,000. However, the cost of laying foundations of any type and other Groundworks prices varies depending on your land’s size in square metres.
This guide deals with typical extension and house foundation types, concrete foundation costs, and the regulations that govern foundations and groundworks in general.
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Main Topics
Laying Foundations Cost*
The table below shows typical prices you can expect to pay for laying a foundation in the UK. You can use the data below as a cost calculator and benchmark against quotes you receive.
The prices shown assume a standard width of 0.6m and a standard depth of 1m. Also, they include excavation, removing waste soil and pouring ready mixed concrete.
| Foundation Type | Dimensions | Estimated Cost |
|---|---|---|
| Strip Fill | 5m x 0.6m x 1m | £500-£600 |
| 7m x 0.6m x 1m | £700-£840 | |
| 10m x 0.6m x 1m | £1,000-£1,200 | |
| 15m x 0.6m x 1m | £1,500-£1,800 | |
| 20m x 0.6m x 1m | £2,000-£2,400 | |
| 25m x 0.6m x 1m | £2,500-£3,000 | |
| Trench Fill | 5m x 0.6m x 1m | £600-£750 |
| 7m x 0.6m x 1m | £840-£1,100 | |
| 10m x 0.6m x 1m | £1,200-£1,500 | |
| 15m x 0.6m x 1m | £1,800-£2,300 | |
| 20m x 0.6m x 1m | £2,400-£3,000 | |
| 25m x 0.6m x 1m | £3,000-£3,800 | |
| Pad Foundations | Needs building design because of many significant variables. | |
| Raft Foundations | Needs building design because of many significant variables. | |
| Slab Foundations | Needs building design because of many significant variables. | |
*We compiled these estimated prices from various online sources for excavating trenches and pouring ready mixed concrete. All building plots have different ground conditions and slopes, so hire a qualified structural engineer or architect for accurate quotes. Alternatively, use the form on this page to get up to four quotes.
Strip Foundation Costs
Strip foundations support a line of loads, such as load-bearing walls or load-bearing columns placed close together.
To construct, strip foundations include an excavated trench of at least 1m depth (unless instructed by the building inspector) and a solid concrete base. Then, contractors build from the concrete using blockwork or engineering brick until they reach approximately ground level. From then on, the wall changes to bricks, stones, or continues with block, whatever your preference.
Typically, the cost of foundations per metre varies from £100-£120 for a standard width of 0.6m and depth of 1m. However, in practice, the absolute minimum concrete thickness is 150mm, depending on the foundation’s purpose.
Trench Foundation Costs
Trench foundations are excavations filled with concrete to just below ground level. These provide a firm base and are simple to construct. Moreover, they’re less labour intensive, needing no block laying below ground level. The reduction in labour costs compensates for the higher cost of concrete.
Typically, a builder or groundworker charges £120-£150 per metre for a foundation of at least 1m deep and 0.6m wide.
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Raft Foundations
A raft foundation is a steel-reinforced concrete slab stretching over more than the entire area so that load-bearing walls at various locations can transfer their loads over a large area. Thus, the foundation soil has much-reduced stress. Furthermore, because the raft foundation is an unbroken area, there won’t be differences in the settlement in the underlying soil as with strip or pad foundations.
Because the concrete thickness and amount of reinforcing depend on the specific site conditions, geology, and project specifications, it’s challenging to specify a typical project. However, the soil removal costs are generally lower than strip and pad foundations. Also, the concrete takes less time to pour and less time to cure. Therefore, the overall cost is usually less than for a strip foundation. Generally, the average price of the concrete grade for this application is about £110/m3.
Pad Foundations
Pad (or pile) foundations support concentrated loads from a single point, such as structural columns. Generally, pad foundations reach deeper than other types and have vertical steel reinforcing. The average cost for concrete suitable for pad foundations is £110/m3.
Additional Costs
To choose the correct concrete grade, type of foundation and depth of concrete, you need data about the surrounding environment. This information comes from specialist contractors such as surveyors, geologists, soil technologists, tree surgeons, and many more.
The following table shows some outside specialists and their fees that an architect will use to determine the correct foundation specifications. As before, these vary with the specific site conditions. Therefore, use these as a benchmark for your research.
| Additional Cost | Average Cost |
|---|---|
| Structural Engineer | £800 |
| Flood Risk Assessment | £600 |
| Tree Survey | £250 |
| Soil Survey | £1,200 |
In addition to the above expenditure, there are also extras related to an extension’s foundations cost when restoring the area. Typically, these might include:
- Garden turf needed to patch damaged lawns costs around £15/m2 for materials and labour.
Alternatively, if you’re laying the foundations for a new garage, you might want to resurface the driveway:
- Block paving driveways for a typical UK drive cost about £3,500-£4,000.
- Tarmac for a 50m2 driveway ranges from £2,000-£3,500.
- To cover a 40m2 medium-sized driveway, Gravel costs around £1,500-£3,500.
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Laying Groundworks Cost Factors
The groundworks, foundations, and prices necessary to provide a stable base on which to build, depend on several factors.
Type of foundation
The type of ground on which you’re building, and the height of the water table dictate the foundation most suitable for the structure. Of the two most common types, strip foundations are usually cheaper in materials than trench-fill, but the latter takes less time and are less labour intensive.
Size of foundation
The overall size of any project determines the amount spent on materials and labour. Small trenches and foundations take much less time than extensive groundworks and, therefore, will cost less.
If you want to build in areas where soil suffers from shrinkage and periodic drying out, you will probably have to dig deeper trenches and pour thicker foundations to reduce the risk of subsidence. The ground near established trees and areas where the water table fluctuates significantly suffers from shrinkage.
Type of ground
The plot’s soil type significantly affects the amount of excavation and the size of the foundations. For example, sandy soil needs different foundation dimensions than clay, chalk or bedrock.
The property’s location
The property’s location affects the quotation. Areas of higher cost of living, such as London and the Southeast, have labour charges up to 20% higher than elsewhere in the country.
The foundation’s purpose
The foundation’s purpose significantly affects the trench’s dimensions and concrete depth. For example, a house’s foundations will be smaller and shallower than a multistorey office block. But, they will be too extensive for a garage or patio. Also, you might need a combination of foundation types in the same structure, such as strip foundations for loadbearing walls, while structural columns need pad foundations.
Site access
Poor access to the site significantly increases the cost. For example, if the project is in a back garden where the only access is a narrow side gate, you will have trouble using mechanical excavators and delivering materials. Instead, you must dig the trenches by hand and wheelbarrow the waste soil to the skip in the front driveway. Fortunately, ready mixed concrete suppliers have pipe extensions and conveyors to transfer concrete from the road. And, if the foundations are a long way from the street or uphill, you can hire concrete pumps at £300-£400/day or boom pumps at £600-£800/day. By the way, the approximate price of ready mixed concrete is £80-£150/m3, delivered locally. But, many concrete companies charge a delivery excess dependent on distance from the depot.
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Foundation Project Timelines
The durations for each of the following project steps vary. So, change them depending on your situation. For simplicity, let’s assume we’re constructing foundations for an extension.
Excavation
Generally, excavation duration varies from three or four days to three weeks depending on the factors mentioned earlier. For a typical 3m single-storey rear extension, you need to dig strip foundations about 450mm wide and a minimum of 1m deep to hold at least 200mm concrete.
You must also take into account existing drains and service conduits. In general, you should divert these away from the foundations. Alternatively, dig foundations deep enough to go underneath the services.
Remember that the building inspector will inspect the foundations for depth, width, soil stability, and water table. If the excavation continuously fills with groundwater, you must install a pump and carry out any other remedial instructions.
Pour concrete
Concrete starts to cure within two hours of mixing unless the ready mixed load has a curing inhibitor added at the mixing stage. The actual setting time depends on the humidity, concrete type, and ambient temperature. Furthermore, curing time can reduce to 30 minutes in hot weather. Therefore, you must unload and level the concrete as quickly as possible.
Concrete curing time
Prevent the concrete from curing too quickly. The surface becomes weak and dusty if it dries out too fast. Therefore, keep the surface covered with plastic sheeting for temperatures above 20⁰C and an insulation blanket for temperatures below 4⁰C.
Even though concrete starts to set rapidly, it’s not strong enough to build on for 48-72 hours. And it doesn’t reach its full strength for around 28 days. Even if you think the concrete has cured enough, don’t lay any blocks or bricks yet, as the building inspector might disagree.
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How are Foundations Laid?
It’s best to hire a professional to lay the foundations, as they must pass inspection. However, for your information, these are the general steps.
1 Datum
Hire a surveyor to find a level datum point, usually the damp proof course level marked on the plans. The builder takes all heights from this and will measure the excavation depth and top of concrete from this point.
2 Mark out perimeters
Use string lines to mark out the walls of the extension. Peg them in place with wood or steel pegs.
3 Mark trenches
Depending on the width of the trench, mark up the sides of the excavation. Ensure all lines are square to the existing building. Some professional digger drivers like the trench sides to be marked, while others use the centre lines. Ask which they prefer.
4 Dig trenches
Always dig the trenches referring to the site datum level. Remember, the building inspector has the last say on how deep to dig depending on the soil’s condition. So, be prepared to go deeper if instructed. Once the building inspector is happy, they will sign off the trenches.
5. Concrete depth pegs and pour
Hammer pegs into the excavation base as a guide for concrete thickness. If the plans specify steel reinforcing mesh, now is the time to install this. Then, pour the concrete to the required thickness. Finally, vibrate the concrete to remove all voids and use a rake to level off the surface. Take time to get the level spot-on, as it will save time later.
Finally, allow the concrete to cure until hard enough to commence work, then ask the building inspector to visit and pass this stage.
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Building Regulation & Planning Permission
It’s essential to have the building inspector approve your excavations and concrete foundations before constructing the structure. Otherwise, you run the risk of structural collapse if there is anything wrong, and you’ll be fined and ordered to replace your foundations.
The following rules and regulations cover the building of foundations.
Party Wall
Your project mustn’t interfere with your neighbour’s property. Therefore, if your foundation is within 3m of your neighbour’s wall, you must conform to the Party Wall etc. Act 1996. Use a surveyor for this. To help, the UK government publishes assistance for this legislation.
Planning Permission
As such, you don’t need Planning Permission for foundations. However, you might need permission for the structure above. Use the services of an architect to submit a planning application for the intended construction. If in doubt, speak to your local authority’s planning department before beginning.
Building Regulations
As always, the Building Regulations ensure your finished building is safe, strong, and well made.
The foundations must comply with Approved Document A. This regulation deals with the foundations and ensures load-bearing and party walls are sound. In addition, Regulation 7 ensures the materials are of adequate quality, and the professionals build the foundations as per current building practice.
Next Steps
Anything you build on your property must have adequate foundations, from a simple garden shed or path to a ten-bedroom detached mansion. It’s essential to get quotations from qualified and experienced groundworkers to compare the foundation’s cost before agreeing to a specific contractor.
Complete the form on this page, and we’ll ensure you receive up to four quotes from reputable contractors so that you can choose in the comfort of your home.
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