Rear Extension Foundations on London Clay

Daniel Nikolla

Oct 20

1 min read

Updated: 7 days ago

Much of North London sits on clay, which shrinks and swells with changes in moisture. Foundations for a rear extension need to cope with that.

This guide covers the main issues that influence foundation design for extensions on clay soils.

Why clay matters

Clay shrinks in dry weather and swells in wet weather. Foundations that are too shallow can move with the seasons, causing cracks in the new extension or the existing house.

Foundation depth

Depth depends on soil, nearby trees and the depth of neighbouring foundations. Deeper foundations, or specially designed solutions, are common on clay.

Trees and drains

Roots draw moisture from clay and can cause movement some distance away. Drains near new foundations may need to be protected or bridged, and trees need to be considered early.

Joining new to old

The new structure must work with the old one. Differences in movement between them are best handled by careful detailing at the junction, rather than by hoping cracks will not appear.

Site investigation

Trial holes show what ground conditions really are. A small investigation early often saves a large redesign later.

Talk to Graal

Planning work on your home or a small development? Graal Structural Engineering provides structural design and project management support across North London. Call 07737188754 or request a free quote through our contact form and we will tell you what is needed and what it is likely to involve.