Before a single brick of an extension goes up, the real work happens in the ground. Foundations are the part nobody sees once the job is done, and they are also the part building control cares about most. If you are planning an extension, it pays to understand what is going in beneath it and why the depth is rarely a round number you can guess in advance.
Why foundations matter more than people think
An extension adds weight, and that weight has to be carried down to ground that will not move. The problem is that most ground does move. Clay swells when it is wet and shrinks when it dries out, and that seasonal push and pull is enough to crack a wall that sits too shallow. The whole point of a foundation is to reach past the zone where the soil is still shifting and land on something stable. Get that right and the extension behaves as if it was always part of the house. Get it wrong and you see it later in the plaster.
The two types you will usually hear about
For most domestic extensions it comes down to two options.
Strip foundations are a band of concrete poured at the bottom of the trench, with blockwork built up off it to ground level. They use less concrete, which can suit firmer, shallower ground.
Trench fill foundations are exactly what they sound like: the trench is dug to the required depth and filled with concrete almost to the top. There is more concrete in the ground, but far less bricklaying below ground level, so on deeper digs it is often quicker and more practical overall.
Which one is right is not a matter of preference. It follows the ground conditions, the depth needed, and what the structural drawings specify. On a lot of extensions we pour trench fill simply because the required depth makes it the sensible call.
How deep is deep enough?
There is no single answer, and anyone who quotes you one without seeing the site is guessing. That said, here is the honest shape of it.
The absolute minimum under building regulations (Approved Document A) is 450mm, which is really about getting below the reach of frost. In practice, extensions are almost always dug deeper than that. On clay, a metre is common, and building control will often want the new foundation to match or exceed the depth of the existing house so the two do not move independently.
Then there are trees. This is the big one people forget. A thirsty species like oak, willow or poplar draws huge amounts of moisture out of clay in summer, and that drying reaches surprisingly far. The NHBC foundation depth tables (Chapter 4.2) cross-reference the tree type, its mature height, how close it is, and how "plastic" the clay is, and they can push a required depth well past two metres. A removed tree can be just as much of a problem, because the ground slowly re-wets and swells over years. None of this is guesswork on site; it is worked out from the drawings and the soil before we dig.
What building control will expect
Every extension foundation needs building regulations approval, and the inspection points are not optional. The key one is simple: the trench has to be inspected before any concrete goes in. That is the moment the building control officer confirms the depth, the ground you have reached, and that it matches the approved drawings. Pour first and you may be digging it back out. A good groundworker plans around that inspection rather than treating it as a hurdle.
Drains, levels and the bits that catch people out
Foundations rarely sit in isolation. If a drain or sewer runs near the line, the foundation may need to go deeper or be bridged. Old foundations from a previous structure, made ground, soft spots, a sloping site - all of it changes the dig. This is why a firm extension price only really lands once someone has looked at the ground properly, and why the cheapest quote on paper is often the one that has assumed the easiest possible conditions.
Getting it right from the start
The foundation is the one part of an extension you cannot easily fix afterwards, so it is worth doing properly and to the drawings. This is our core work - see our foundations and footings service for what we cover. If you are planning an extension in Newmarket or the surrounding villages and want a straight answer on what your ground is likely to need, get in touch for a no-obligation quote. We will look at the site, the drawings and the conditions, and price the groundwork honestly rather than hoping the ground is kinder than it looks.