How Deep Do Footings Need to Be for an Extension?

The short answer: usually about 1 metre, but the ground decides

Building Regulations require footings to reach ground that can carry the load of your extension without moving. For a typical single or two storey extension on decent ground, building control will normally expect a minimum of around 1 metre. Shallower depths of 750mm or so are sometimes accepted on gravels or solid ground, but 1 metre is the sensible figure to plan around.

The final depth is not fixed on a drawing. It is confirmed on site by the building control inspector, who looks into the open trench before any concrete is poured. That is why experienced groundworkers dig to the expected depth, then hold off until the inspection is done, because the inspector can and does ask for more depth if the ground at the bottom of the trench is not good enough.

Why Northamptonshire ground often means deeper digs

Much of Northampton and the surrounding villages sits on clay, including bands of shrinkable boulder clay and Oxford clay. Shrinkable clay swells when wet and shrinks in dry summers, and that seasonal movement happens in the top layer of ground. Footings have to sit below the zone that moves, which is why clay sites routinely need deeper foundations than sandy or gravelly ones.

Trees and large shrubs make this worse. Their roots draw moisture out of clay, so the shrink and swell zone extends much deeper near vegetation. Depending on the tree species, its height and its distance from the extension, footing depths of 1.5m to 2.5m are common, and in bad cases the numbers stop making sense for a traditional dug footing. Removing a thirsty tree does not instantly fix this either, because the clay can rehydrate and heave upwards for years afterwards.

• Broadleaf trees like oak, willow and poplar have the biggest influence and the widest reach
• A tree that has been recently removed still counts when depths are assessed
• Old drains, ponds or backfilled ground near the dig can also force changes to the design

Trench fill or strip footings, and what happens past 2.5 metres

Most domestic extensions locally use trench fill, where the trench is dug to depth, typically 600mm wide, and filled with concrete to near ground level. It is fast, safe and avoids anyone working at the bottom of a deep trench laying blocks. Traditional strip footings, with a concrete strip and masonry built up off it, still have their place on shallower digs.

Once required depths head past about 2.5m, open trenches become expensive and genuinely dangerous, and the sides need supporting. At that point an engineer will usually recommend an alternative such as mini piles with a reinforced ground beam. It sounds drastic but on a difficult site it can work out comparable in cost to a very deep trench fill, with far less spoil to cart away.

What this means for your budget and timeline

Groundworks quotes for extensions are usually based on an assumed depth, commonly 1 metre, with an agreed rate for any extra depth the inspector requires. Ask for that rate up front so there are no surprises. Deeper digs mean more muck away, more concrete and sometimes a pump, so a footing that goes from 1m to 2m can add a four figure sum depending on the size of the extension.

If you have trees near the build, it is worth getting a soil investigation or at least an engineer's assessment before you commit to a design. A few hundred pounds spent early is much better than discovering the problem with an open trench, a booked concrete lorry and an inspector shaking their head.