Foundations Explained

Foundations need to be dug according to a predetermined plan, and to have been accurately surveyed and set out. It is surprising just how often this is not done, and occasionally the ramifications can be very serious indeed, as completed houses turn out to be in the wrong place and subsequently have to be demolished. Your plans should identify all the load-bearing walls and the width of the trenches to be excavated. The depth of excavation is harder to predetermine and this is routinely decided by the building inspector on site. This is where things can get a little bit tricky, because if you have a difficult site, the foundation trenches may have to go down two metres, sometimes even more, below ground, which is expensive and potentially dangerous.

What your building inspector or warranty provider is looking for is principally a good bearing on solid ground. However, you can never be certain just what lies beneath the ground until it's opened up. This has led to professionals becoming more and more cautious about foundations and specifying loads more concrete or, increasingly, engineered or piled foundations.

Typical costs

It's a difficult task to present foundation costs in a simple manner because each site is different and the ways foundations are built can vary a lot. However, on simple sites with straightforward issues, the foundations (which include excavation, concrete, blockwork and the ground floor) should cost around 100/m² of footprint. Thus a 100m² bungalow might expect simple foundation work to cost around £10,000. Note that this amount doesn't alter for two or three storey houses, one of the reasons that they are slightly cheaper to build on a cost/m² of internal floor area basis than bungalows.

On problem sites, this figure can increase to around 200/m² of footprint. Very occasionally, costs can rise above this. Note that these figures don't include anything for site excavations where, for instance, you might be tackling a sloping site.

Typical foundation work
1 Digging out the trench to a level agreed with the building inspector on site before.
2 Concrete is laid to a depth of at least 150mm.
3 Blocks are laid up to finished floor level.

Surveys: Official vs Unofficial

Today, many people recommend that you undertake a professional ground survey before you start work. Trial holes are dug around the site so that a view can be taken on the best means of placing the foundations. The professional you need for this work is a structural engineer and in hiring such a person, you are effectively placing the risk for the success of your foundations onto them, or more particularly, their insurance policy. Consequently, engineers tend to be ultra conservative in their assessments and recommend engineered solutions, such as rafts or, more likely, piling. The problem here is that such foundation systems are very much more expensive than the more traditional methods. Instead of costing around £60/m², the cost spirals to over £100/m², and sometimes rather more than this.

DIY Surveys

Whilst not suggesting in any way that you should take on the risk of designing your own foundations, it is worth making an assessment of your site and the likelihood of it encountering problems. Things that engineers are typically looking for are the presence of large trees (easy to spot!), boggy ground and clay soils, all of which are observable without requiring trial holes. Conversations with neighbours and local builders may well reveal a lot of background information that will help you form a picture of the chances of your site requiring specialist foundation work. A key person to seek advice from is the local building inspector.

Risk

Whatever you do, you can never entirely eliminate the risk of cost overruns below ground. You may find features like wells or mine shafts that lay in your way. It is, therefore, essential that you hold back some contingency funds for unforeseen eventualities. The good news is that whilst foundation costs can sometimes double, they rarely treble: there is an upper limit at which some solution can usually be found.

Basements

If your ground conditions appear difficult, you might do well to consider building a basement. If you are expecting to spend, say, £30,000 on getting out of the ground, then you are maybe halfway towards the cost of a basement and you may find that you are able to add considerably more value to the house than the additional cost of a basement build.

Foundation Methods

1 The traditional method involves pouring concrete to a depth of no less than 150mm and then building up to floor level with bricks and blocks.
2 Raft foundations are thick, reinforced floor slabs, strong enough to build the house walls off.
3 The 'trenchfill' method is the most common- it's quick and easy but uses more concrete than strip foundations.