Kitchen Extension Foundations - what size and how deep.

Most foundations for kitchen extensions are exactly the same for single and other two storey extensions subject to local sub soil ground conditions. The usual and minimum foundation depths for most areas having good to medium quality ground bearing sub soils is 1M deep. However, this can change dramatically depending upon specific site conditions.

Most kitchen extension foundations are usually specified as trench fill foundations as these are relatively easy, quick and cost effective to install. A typical size may be 600mm wide x 800mm deep with the invert of the trench 1M deep allowing 200mm of earth or pavings to be formed over the top of the foundations. They also have advantages of safety over a strip foundation as it reduces the need for men working inside the trenches.

When sub-soil ground conditions are of poor quality bearing pressures or are shrinkable clay then the kitchen extension foundation design will need to be adjusted to a more site specific engineered approach rather than a simplistic generic accepted design.

Alternative design solutions for a kitchen extension foundation could be simply making foundations deeper until a better bearing sub soil is located. Highly shrinkable soils will require the use of heave protection boards as well as extra depth to the foundations. This is usually not economical if the depth of the foundations needs to go beyond 2.5M deep.

For really dubious and poor quality sub-soils such as land fill or non-virgin ground, piling is usually the solution for a kitchen extensions foundations. There is a variety of piling techniques and solutions but all demand access for some form of mechanical plant to install the kitchen extension piles.

A pile cap is then usually formed over the top of the pile and a series of ground beams span across or between the piles to form a perimeter support and the combination of the piles and ring beams act as the main foundations for the kitchen extension.