Can a Wall on Strip Foundations Be Used to Retain Ground?

An extension wall built on strip foundations is designed to support the structure above and resist wind pressure — not to retain earth. If the ground level on one side of the wall is higher than on the other, the wall is simultaneously acting as both an external structural wall and a retaining structure.

Whether this is acceptable depends on the retained height, the wall construction, and the soil type.

What Changes When a Wall Retains Ground

Wind pressure is a relatively small lateral force that acts over the full wall height. Earth pressure is typically higher — particularly in clay — and acts at the base of the wall, generating an overturning effect that standard external walls are not designed to resist.

Strip foundations for external walls are designed for vertical load from the structure above, with a modest horizontal resistance. They are not automatically adequate as retaining wall foundations.

Small Retained Heights: BRE GBG 27

For small retained heights — up to approximately 1.725 m — the BRE Good Building Guide 27 provides design guidance that allows certain wall types to retain ground without bespoke engineering. The wall types covered are:

– Straight solid masonry wall
– Staggered plan masonry wall
– Piered masonry wall
– Reinforced hollow block wall

Each type has a specific maximum retained height for a given soil category. For walls within these limits, the standard masonry external wall construction may be adequate — but the wall must match one of these configurations, and the foundation depths must meet the guidance requirements (which are typically deeper than a standard strip foundation).

When Bespoke Engineering Is Needed

A bespoke retaining wall design is required when:

– Retained height exceeds GBG 27 limits
– The surface above has vehicle loads or a structural surcharge
– Groundwater may build up behind the wall
– The existing strip foundation was not designed for horizontal loads
– The wall construction does not match a GBG 27 wall type

Drainage

Regardless of the wall type, drainage behind a retaining wall is essential. Without a granular drainage layer and a drain at the base of the retained face, groundwater can build up and exert hydrostatic pressure that dramatically increases the force on the wall.

Paving or waterproofing on the retained surface can help, but a positive drainage layer and outlet is always the preferred approach.

What PorthouseDean Provides

PorthouseDean can confirm whether your proposed extension wall can retain the required height of ground using GBG 27, or design a bespoke retaining solution where it cannot. This can form part of the extension structural package or be provided as a standalone service.

Contact us to discuss the retaining requirements for your extension project.