Nobody can tell you the depth of your foundations before a hole is dug, and any quote that states one confidently is making an assumption you should read. Depth is decided by the ground, by what is growing near it and by what is already buried there, and it is the single biggest source of movement between a quoted price and a final bill.
What the building inspector is actually looking for
A foundation has to reach ground that can carry the load without moving seasonally, and it has to be deep enough not to be affected by frost or by the drying and swelling of the soil above. The inspector visits the open trench before concrete is poured, which is why the trench inspection is a fixed point in every extension programme and why covering it early is never acceptable.
Clay, trees and why the foundation depth for single storey extension work grows
Shrinkable clay changes volume as it wets and dries, and a tree drawing water out of it magnifies the effect. Where clay and trees meet, foundations go deeper, and near a large tree they may need to be reinforced or piled with a suspended floor over them. The species, the mature height and the distance are all part of the calculation, so identifying the trees on and near the plot is real design information rather than landscaping detail.
What else is found in the ground
Old foundations, backfilled ground from a previous building, a drain nobody recorded, a soakaway, or a public sewer. A drain crossing the footprint has to be diverted or built over with the water company's agreement, and a public sewer near the works needs a build over agreement in its own right. Each of these is a delay as well as a cost, which is why the drainage survey is worth doing before the quote rather than after.
How the depth of foundations for single storey extension work appears in the quote
Honest builders price foundations as a provisional sum with a stated assumed depth, and set out what happens to the price if the actual depth differs. That is not a hedge, it is the only way to price something nobody can see. What you should insist on is that the assumption is written down, that the rate for additional depth is agreed in advance, and that you are shown the trench before it is filled.
Questions people ask about single storey extension foundations
How deep should foundations be for a single storey extension?
It depends on the ground, nearby trees and what is already buried. It is determined on site at the trench inspection, which is why quotes carry an assumed depth.
What is a provisional sum for foundations?
An allowance for work that cannot be priced until the ground is open. Agree the assumed depth and the rate for going deeper before work starts.
Can I build over a drain?
Sometimes, with the water company's build over agreement and a suitable design. A private drain may be diverted instead, but neither should be discovered on the day.
Do trees always mean deeper foundations?
On shrinkable clay, usually. On gravel or chalk the effect is much smaller. The soil type decides whether the tree matters.