Ask ten builders how deep extension foundations need to be and you will hear "a metre" more often than not. The honest answer is that there is no fixed legal depth in the UK: Building Control decides on site based on your ground conditions. Here is what actually determines the depth, and what to expect in and around Northampton.
Building Regulations do not state a single required depth. Approved Document A simply says foundations must reach ground that can safely carry the load. In practice, for a typical single or two storey extension on decent ground, strip foundations around 1m deep and 600mm wide are the common starting point, with a minimum of 450mm to get below frost action.
The final say belongs to the Building Control inspector, who will look at the open trench before any concrete goes in. If they are not happy with what they see at 1m, they will ask you to go deeper. That is why sensible groundworkers quote foundation depth as a range rather than a promise.
Much of Northamptonshire sits on shrinkable clay, including bands of Oxford Clay and boulder clay. Clay swells when wet and shrinks in dry summers, and that seasonal movement can crack an extension built on shallow footings. On shrinkable clay, depths of 1m to 1.5m are routine even without any other complications.
Trees and large shrubs make this worse because their roots dry the clay out well below the surface. NHBC guidance links foundation depth to the tree species, its mature height and its distance from the trench. A thirsty species such as oak, willow or poplar within influencing distance can push a required depth to 2.5m or even 3m. Beyond about 2m of digging, trench fill or an engineered solution such as piles with ground beams often becomes more economical than a conventional strip footing.
Once trenches are dug, you notify Building Control and the inspector visits before concrete is poured. They check the depth, that the trench bottom is on consistent, load bearing ground, and that it is clean and dry. They will also look for anything that changes the design, such as old drains, soft spots, backfilled ground or roots crossing the trench.
Drains deserve a special mention. If your foundation runs near or over an existing drain, the footing usually needs to go at least as deep as the pipe invert, and pipes passing through the foundation need lintels or rocker joints to protect them. This catches out a lot of homeowners because it can add depth that had nothing to do with soil quality.
Depth is one of the biggest variables in groundworks pricing. Every extra 300mm of depth means more excavation time, more muck away lorries and more concrete, and trench fill concrete is a significant cost at 2m plus. As a rough guide, going from a 1m strip footing to a 2.5m trench fill on the same extension can double or more the foundation element of the job, though the exact figure depends on access, soil type and concrete prices at the time.
If you suspect difficult ground, a soil investigation before you commit to a design is money well spent. A couple of trial holes or boreholes typically costs a few hundred pounds and removes most of the guesswork, which protects you from mid dig surprises when the digger is already on site and being paid for by the day.
Yes. Almost every extension needs Building Regulations approval, and the foundations must be inspected and approved by Building Control before concrete is poured. This applies whether or not you needed planning permission.
You can start there, but do not pour concrete until the inspector has seen the trench. On Northamptonshire clay, especially near trees, it is common to be asked to go deeper, so budget with some contingency for extra depth.
Removed trees can matter as much as living ones, because clay that a tree has dried out will slowly swell as moisture returns, which can heave shallow foundations upwards. Tell your groundworker and Building Control about any trees removed in roughly the last 10 to 25 years so the depth or design can allow for it.
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