The foundations for a new house extension must be at least one metre deep. This figure increases depending on how much clay is in the soil and whether there are any trees nearby. This is because over time the roots of the tree will seep moisture out from the ground around it, including the clay soil in the new foundations which in turn will cause them to shrink. If the soil contains sand or chalk it could be susceptible to frost so the foundations must be deep enough so that frost cannot penetrate (usually at least as deep as 450mm). If there are any drains or sewers in the vicinity then these will also affect the depth of your extension foundations.