Small Retaining Walls: Design vs Guidance — Which Approach Do You Need?

For small retaining walls — the kind you’d build in a domestic garden to create a terrace, or at the edge of a driveway — the choice between bespoke structural design and following standard guidance can make a significant difference to cost and programme.

Not every small retaining wall needs a structural engineer. But some do, and using the wrong approach can result in a wall that fails or that Building Control refuses to accept.

What Standard Guidance Covers

The BRE Good Building Guide 27 (GBG 27) provides design charts for small retaining walls that can be built without bespoke engineering calculations — provided the wall conforms to specific types and dimensions.

The guide covers up to approximately 1.725 m retained height (the exact limit varies by wall type and soil category). For walls within these limits, the guide gives sufficient information for a competent builder to construct a safe wall without a structural engineer.

Wall Types Covered by GBG 27

1. Straight solid masonry wall — plain brick or block wall with no piers
2. Staggered plan wall — a wall that zigzags in plan, using the staggered form to increase lateral stiffness
3. Piered wall — a straight wall with buttress piers at defined intervals
4. Reinforced hollow block wall — concrete blocks with hollow cores filled with reinforced concrete

Each type has different capacity. Piered and reinforced hollow block walls can span greater heights than straight plain walls.

Soil Type Matters

GBG 27 divides soils into two categories:
– Category A: sands, gravels, and non-plastic silts — lower lateral pressure
– Category B: clays and plastic silts — higher lateral pressure (because clay swells with moisture and generates hydrostatic-type pressures)

A wall adequate for Category A soil may be undersized for Category B. If your site is in clay (common in most of London and South East England), use the Category B limits.

When Bespoke Design Is Required

The guidance does not cover these situations — a structural engineer must design the wall:

– Retained height exceeds GBG 27 limits for the applicable wall type and soil category
– The surface above the retained area has a surcharge (vehicle loads, a structure, a steeply sloping bank)
– Groundwater is present behind the wall
– The wall relies on the foundation to resist overturning, but the foundation depth is constrained
– The wall is adjacent to a building or structure that could be affected by movement or failure

PorthouseDean’s Role

PorthouseDean can design retaining walls that fall outside GBG 27’s scope. For walls within scope, we can confirm whether your proposed wall type meets the guidance or identify any issues that push it outside it.

Contact us to discuss whether your retaining wall needs a bespoke design or can follow standard guidance.