Why Are My Foundations Specified at 1 Metre Deep?

When a structural engineer specifies foundations at 1 metre depth on a residential project without a ground investigation, it is almost always a conservative starting point — not the final word.

Why 1 Metre?

The Building Regulations minimum founding depth for residential strip foundations is 450 mm on most soil types and 900 mm on clay. One metre sits comfortably above both thresholds and provides a reasonable margin for minor ground variability, shallow drains or undetected soft spots.

Where no ground investigation data is available — which is common on small domestic projects — specifying 1 metre is a practical way to capture most normal ground conditions without requiring expensive pre-construction surveys. It tells the builder to excavate to at least that depth, then pause for Building Control inspection.

1 Metre Is a Minimum, Not a Maximum

The 1 metre depth shown on drawings is the minimum below which the engineer believes the ground will be acceptable. If the Building Control Officer inspects the formation at 1 metre and finds soft ground, ash fill, roots or disturbed material, they will ask for deeper excavation until suitable bearing strata is reached.

Your builder recommending 2 metres is not necessarily wrong — on some sites (particularly in London clay) going deeper is the only way to reach stable ground below the seasonal moisture fluctuation zone. If the BCO agrees, deeper excavation is the right call.

What Triggers a More Specific Design?

Where trees are within 20 metres and the soil is clay, a 1 metre assumption is rarely sufficient. In those circumstances a proper tree and soil assessment is needed and the foundation depth is calculated against the actual shrinkage potential of the clay and the influence zone of each tree species. The result may be 1.5 metres, 2 metres or more — and will be detailed on the structural drawings accordingly.

The BCO Has Final Say

Regardless of what the drawings say, it is the Building Control Officer who looks in the trench on the day and decides whether the bearing strata is acceptable. Your engineer’s drawings are the design intent; the BCO’s inspection is the site-specific confirmation.

If you need revised calculations based on deeper measured foundation depths found on site, contact PorthouseDean with the details and we can update the documentation.