Foundations are one of the biggest “unknowns” in any building work. It takes excavation to reveal the condition of the ground is that you have at your site, hence why foundation depth (and price) can differ so much from project to project.
1) Soil Type and Ground Conditions
Different soils behave differently. With soil types, clay can expand and contract with moisture levels, while sandy or made-up ground may require further consideration. This is one of the reasons why two rear extensions on the same street can require different foundations. For advice from Exeter Builders, contact https://www.silverferndevelopments.co.uk/exeter-builders/
2) Nearby Trees and Vegetation
It is particularly important for clay, which can gather the moisture of the trees. If there are large trees near the building, Building Control may require deeper foundations to reduce movement. It is one of the major causes why foundation costs also go up.
3) The Load of the Structure
One-off extensions may require different foundations than a double-storey build. The foundations can be exposed to width or depth (or a little of both) by heavier loads, and often a choice of design.
4) Existing Drains and Services
Perhaps this may mean a different design if you are constructing near drains, manholes, or something similar. Sometimes foundations need to excavate deeper, or the drainage has to be provided with protection such as fabric, etc., and/or re-directed altogether, which costs more money.
The best way to negate surprises is to know there are going to be some ground-related “unknowns” in your budget and that you hired a builder who explains the process well. A great quote will also say what is in, along with possibly adding the possibility of everything that could change when they begin development.