Foundation and Ground Works

If you’re looking to have an extension built, solid foundations are more than likely to be required to prepare for laying the brickwork.

The groundwork depends on the layout of your plot and the extension but usually entails clearing the area around the building and pit digging ready for the foundations. As the name suggests, groundworks applies to everything that occurs below ground level and involves preparing the ground for solid foundations to be laid. Groundworks usually costs between 10-15% of the cost of the final build. However, if difficulties arise then it can be considerably more. It is imperative to get the foundations and ground works right if your extension is to be a success.

Preparing the ground

First the ground has to be cleared of plants, shrubs and turf. This is normally done by unskilled labourers rather than builders. For single story extensions labourers will clear and dig the area needed for the extension with an extra metre around it. They dig down to a sufficient enough level for the floor and supporting layer to be put down.

Laying foundations

Foundations are essential in construction. Essentially they act as a means of distributing the weight of a structure over a sufficient surface, so as to prevent subsoil from spreading which could cause unequal settlement. The foundations (sometimes referred to as footings) required will be dependent on loading as well as the conditions of the ground. Soil conditions can have an effect on foundation dimensions. An engineer usually arranges for a couple of trial holes to be dug before hand, as a means of determining the condition of the ground. Changes to the design of the foundations may be required, for example due to tree roots, water courses and soil type.

Methods used in foundations

Foundation material will differ depending on the property and the extension itself. There are two methods used today, both of which are covered in the Building Regulations part A1/2:

• Deep strip – the less expensive method, this standard method involves using as little concrete as possible. Strip foundations tend to feature usually a steel reinforcing bar. Used for the more simple extensions.

• Trench fill – trenches are dug and then filled with concrete. This method tends to be the most expensive because it uses a greater amount of concrete compared to strip. However it’s fast and reduces the potential risk for flooding between the digging stage and the pouring of concrete. A trench fill foundation usually applies in cases of large-scale extensions and is one of the most commonly used methods in new builds.

Builder Guide has a range of different builders who are waiting to quote you on your foundation requirements and can offer detailed information on what type of foundation you’ll need and the cost of the work involved.