Foundations are the most important part of building a new house extension, conservatory or any other structure. They ensure that your building remains stable for years to come, reducing the risk of subsidence or other structural issues.

So how much will it cost to lay concrete foundations at your property? We’ll go through the costs of different types of foundations, no matter what you’re building.

How much do foundations cost in the UK?

How much concrete foundations cost depends on the type of foundations you want and what you need them for. The average cost of concrete foundations for a permanent structure (not a shed or greenhouse) is £6,250.

Type of concrete foundation for a medium-sized house extension	Average cost
Strip foundations	£6,000
Strip foundations per linear metre	£250
Trench foundations	£6,500
Trench foundations per linear metre	£271

These prices are just for the materials and labour associated with laying your foundations. It doesn’t factor in the other associated costs for your new home, extension or conservatory, like applying for planning permission, getting a building regulations certificate or engaging a surveyor or structural engineer.

What factors affect the cost of concrete foundations?

• Type of structure: The type of building you’re putting on top of the foundations will affect the cost. A two storey extension will require deeper foundations than a conservatory, so you’ll pay more in labour and material costs.

• Type of foundations: Strip foundations are generally cheaper than trench foundations, though they often take longer to lay. Bear this in mind when considering the foundation type.

• Project area: The larger the area, the more you’ll pay. However, in very big construction areas, you may pay less per linear metre than smaller areas.

• Soil type: Different types of soil will require different depths of foundations to keep your build stable. The deeper the foundations need to be, the more you’ll pay.

• Access: How easy it is for your builders to access the area will affect the cost. If they have to hire specialist equipment to fit through narrow gaps – or even through terraced houses! - it’ll cost more.

• Where you live: Your location will affect the cost of your concrete foundation. If you live in London or the south east, where the cost of living is higher, you’ll pay more than the average listed above, while in areas of Scotland or the north of England you may pay less than the average.

At HaMuch, we use our tradespeople’s hourly and daily rates to calculate project costs across the UK. Take a look at the table further down the page to find an estimate for concrete foundations in your area.

What is the difference between a trench and a strip foundation?

Trench and strip foundations are similar, as they both involve digging trenches, but they have slightly different reinforcements. Both types of foundation involve digging trenches down to a depth of around 1 metre. Strip foundations are filled with concrete and steel reinforcements, whereas trench foundations are filled with layers of sand, clay or rubble before finishing with concrete.

What is the difference between a concrete slab and a foundation?

While many people talk about laying concrete foundations for sheds, hot tubs or greenhouses, they actually mean laying a concrete slab. There’s a difference between concrete slabs and a foundation – mainly that concrete slabs are poured at the same thickness all the way to the edges of the area.

Concrete slabs are only suitable for placing light, temporary structures on. They usually sit on the surface of your garden or land, so can only take the weight of things like sheds and hot tubs. For new homes or extensions, the concrete foundations need to go deep into the ground to be able to spread the weight across a bigger area.

What is the minimum depth for a concrete foundation?

The minimum depth for a concrete foundation depends on the type of foundation, what building is being constructed on the foundations and the soil type. However, all foundations must comply with the British Standard 8004:2015 Code of Practice for foundations.

Generally, strip or trench foundations should be at least 600mm wide and 1 metre deep. In strip foundations, the concrete layer should be at least 150mm deep within the foundation hole.

However, just working to the minimum depth does not mean that your foundations are safe and stable. Things like nearby trees or drains can mean that your concrete foundation needs to be deeper.

Can I pour my own foundation?

It’s not recommended to pour your own foundation if it’s for a permanent structure like an extension or conservatory. The foundations literally stop your build from sinking, so why risk causing problems further down the line by doing a DIY job? Unless it’s your trade, it’s not a good idea to pour your own foundation.

If you do decide to dig your own foundations, you’ll need to follow excavation plans drawn up by an architect or structural engineer. You need to make sure you dig to the depth specified on these plans to ensure you meet building regulations.

Can I pour my own concrete slab?

Pouring your own concrete slab is much more straightforward than digging your own foundations. If you want to build a base for your shed, hot tub or decking, you can do this yourself if you’re used to doing DIY.

You can create a frame out of wood to mark the area, then dig down to the correct depth (usually around 150mm). Add gravel up to 100mm, then pour and screed the concrete on top. Allow the concrete to cure, then remove the frame. You’re done!

How do you put a new foundation under an existing house?

Putting a new foundation under an existing house is called underpinning. This is usually done when a house has suffered from subsidence, usually caused by a water leak that has washed away the existing foundations or tree root ingress that has broken them down.

Underpinning can cost significantly more than laying new concrete foundations – around £17,000.

If you’re ready to get quotes for concrete foundations, post your job on HaMuch. We’ll send it to builders in your area who are available to help.

Estimates for common extension builder jobs;

Rates for other trades