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Calculating the cost of getting your house out of the ground is probably the trickiest part of any estimate. Above ground it is possible to define almost exactly what materials and labour you require for the construction. Below ground is a potential minefield for the unwary. Nevertheless, by taking a step-by-step approach you can reduce the possibility of unexpected surprises!

General Ground Conditions

The best way to assess the site for the first time is to walk over it, carefully examining the various features.

Look for signs of unusual vegetation. This often indicates that the ground has been worked over. Are the owners trying to hide the site’s previous usage? Was there an existing property on the site? If so what was that property? Is it likely to have caused contamination in the ground or are there old basements lurking? These could cause significant problems when constructing new foundations. I would also recommend that you ask locals if they know anything about the history of the site- this is the moment nosey neighbours could be your best friends!

If you have visited on a wet day, how does the ground feel underfoot? Is it very muddy, possibly indicating a clayey substrate, or does the water appear to be draining away freely? If you visit the ground after a spell of dry weather are their fissures apparent in the surface indicating possible shrinkable clay?

Clay and Trees

Clay and trees can have a large impact on the foundations and drainage. Some clay soils can be prone to shrinkage, this means that during periods of dry weather the soil will shrink causing the foundations to move and cause significant structural damage. Conversely in wet weather the clay tends to swell, causing heave, thus creating more damage.

Trees can cause a similar problem. Trees tend to soak up more of the ground water and again increase the risk of shrinkage. Do not however imagine that simply chopping the tree down will solve the problem, as the ground will then swell further causing more damage to the foundations

The NHBC have a section in their standards, which provides a series of tables designed to assess the required foundation depths. This is certainly worth studying. The guide also provides a raft of advice for assessing the site. Whilst we do not advocate that an amateur should carry out the entire assessment themselves, it will certainly help you to filter out problem sites and avoid fruitless expense on detailed site investigations.

It is not however always obvious whether the ground contains shrinkable clay, so it will be worth having a word with the local building inspector to see whether shrinkable clay is a common trait of the land in your area. If so, it will certainly be worth arranging specialist soil tests to determine the characteristics of the ground. This combined with an engineers report to make recommendations on the ground conditions will cost around £1000-1500.

Armed with this advice, (which you should certainly obtain before legal completion of the site), you can then begin to assess the additional costs you are likely to encounter.

In low shrinkage soils which tend to be granular and more permeable, the foundations may only need to be a metre or so deep. On the other hand, if the site contains a highly shrinkable clay, which appears more plastic, with nearby trees, foundations could need to be up to 3.5 metres deep.

The concrete foundation including the additional excavation could cost an additional £200 per cubic metre. This could cost an additional £10-20000 on a medium sized house depending on the length of walls and depth of foundations.

In addition, the building inspectors and your insurer are likely to insist on systems to prevent damage caused by lateral pressures due to heave and possibly the provision of a suspended slab, again to reduce problems from heaving or shrinkage. This could easily add a similar amount of expenditure. The message is therefore clear… if the site features clay and there are trees, or even hedges you may have some unexpected costs. Take advice early.

Made Ground

What should you do if you encounter ground that has been worked over? This is often characterised by poor quality vegetation or an apparently level site in a decidedly sloping area.

My general advice is that unless the site is in a particularly desirable area, and is selling at a bargain price reflecting the problems, it is probably best to look elsewhere- as even the most seasoned builder may get stuck in the mud!

When looking at made ground you will certainly need to employ some expert help to assess the site. What is the ground filled with? How deep is the fill over the entire site? What is the fill made of…. is it contaminated? What foundations will you need to find a secure footing and so on? Given the number of variables, it is impossible to give general guidance on costs, as there are simply too many variables.

However, if you do not want to spend too much on a preliminary analysis, a morning spent with an experienced ground-worker and a JCB will certainly be edifying. Trial holes in several places should be no less than 3 metres deep… provided you hit good ground before then. If the landowner resists allowing you to carry out tests, leave the site to someone else.

Sloping Sites

Whenever you choose a site, it is worth spending some time assessing the amount of slope on the site. Over the years I have often been surprised how steep an apparently flat or gently sloping site turns out to be. This can of course give problems with drainage, meaning you may have to install a pump, adding another £6-8000 to your build cost.

Furthermore if you have to create a split level house, this can add significant costs in the form of retaining walls and waterproofing systems, all of which need to be constructed by companies experienced in this type of work if they are to be successful

Services and Cables

You need to be aware that there is always the possibility that there may be services hidden underground. Normally the existence of these would be revealed during the purchase process as easements, which will normally have been agreed between the service provider and the existing owner of the land.

Overhead cables can also be expensive to move, sometimes making the project completely unviable. On a project I was involved with, the local electricity authority wanted £25,000 to move an innocuous looking cable and transformer, so you should make enquiries about the costs of moving services before purchase.

Mining Extraction and Mining

In areas where there has been mineral extraction or coal mining you will need to take specialist advice to avoid problems with foundations and to a lesser extent drainage and services.

Summary

It is not possible to cover all aspects of groundwork’s in a couple of pages. But as ever, the trick with this is to go in with your eyes open. Take advice from the local building inspector and ensure that the property that you will ultimately build will have an appropriate end value.

This article was written by Adrian Wild, founder of HBXL Building Software and the HBXL Estimating Service.