How Deep Should House Foundations Be in the UK?

The short answer: usually 1 metre, sometimes much more

For a typical house or extension on decent ground, strip foundations in the UK are commonly dug to around 1 metre deep and 600mm wide, with a minimum concrete thickness of 150mm to 225mm. Building Regulations do not set a fixed depth; instead they require foundations to reach ground that can safely carry the load, below the level affected by frost and seasonal movement.

As an absolute minimum, foundations should be at least 450mm deep to get below frost action, but that only applies on stable, non shrinkable ground, which is rare around here. On clay soils, which cover much of Northamptonshire, 900mm is normally the starting point and depths of 1.5m to 2.5m or more are not unusual where trees are involved.

What actually decides the depth

The building control officer or an approved inspector makes the final call when they inspect your open trenches before any concrete is poured. They are looking for firm, undisturbed natural ground of consistent bearing capacity. If the bottom of the trench is soft, made ground or full of roots, you keep digging.

Several factors push depths beyond the standard 1 metre. The big one locally is shrinkable clay, which swells in wet winters and shrinks in dry summers, moving anything founded too shallow within it.

• Soil type: shrinkable clays need deeper foundations than sand, gravel or rock
• Trees and hedges: roots dry out clay, so NHBC tables can require depths of 2.5m or beyond depending on species and distance
• Drains and services: foundations near a drain usually need to go down to at least the invert level of the pipe
• Sloping sites: trenches are stepped so each section sits at the correct depth
• Made ground or old backfill: foundations must pass through it to natural ground

Northamptonshire ground: what we usually find

Around Northampton and the surrounding villages, we dig through a real mix: boulder clay, Oxford and Lias clays, and pockets of ironstone and limestone brash. The clays here are classed as medium to high shrinkability, so 1m is a sensible budgeting assumption for an extension, but a mature oak, willow or poplar within influencing distance can push that to 2m or 3m.

Where trench depths get beyond about 2.5m, plain trench fill becomes slow, risky and expensive, and it is often cheaper to switch to an engineered solution such as piled foundations with ground beams. A soil investigation, typically a few hundred pounds for trial pits or boreholes, is money well spent on tricky plots because it removes the guesswork before you commit to a design.

What this means for your budget

Depth drives cost more than almost anything else in groundworks, because you pay for the digging, the muck away and every extra cubic metre of concrete. As a rough guide, trench fill concrete costs in the region of £110 to £150 per cubic metre supplied, so a trench that ends up twice as deep as hoped can add thousands to an extension.

Our honest advice: get trial holes dug or at least a proper look at neighbouring foundations before you fix your budget, allow a contingency of 10 to 15 percent for ground conditions, and never let anyone pour concrete before building control has seen the trenches. Redoing rejected foundations costs far more than doing them right once.