What types of retaining wall are used in domestic construction?

Common types in domestic and garden construction include: gravity walls (mass concrete, stone, or block walls that rely on their own weight to resist overturning - suitable for walls up to about 1m); cantilever walls (reinforced concrete or block walls with an L-shaped base slab that uses the weight of soil on the base to resist overturning - used for walls 1-3m); timber sleeper walls (railway sleepers or large hardwood timbers on concrete pads with deadman anchors - suitable for garden terracing up to 1m); and gabion walls (wire baskets filled with stone - flexible, draining, and well-suited to walls in wetter ground).

Do retaining walls need planning permission?

Retaining walls in domestic gardens are generally Permitted Development (no planning permission required) if they do not exceed 1m in height at the front of the property or 2m elsewhere. Walls over these heights, or retaining walls that form part of a highway or that affect a neighbour, require planning permission. In conservation areas, even lower walls may require permission. Note that Building Regulations approval is separate from planning permission - any retaining wall that could affect the safety of people or a building may require Building Regulations approval regardless of planning.

What drainage is needed behind a retaining wall?

Drainage is critical for retaining walls. Water building up in the retained soil significantly increases the lateral pressure on the wall - a fully waterlogged soil can exert twice the pressure of a dry soil of the same type. All retaining walls should have: a free-draining granular backfill (clean 10-20mm gravel, not topsoil) against the back face; a perforated land drain at the base of the retained ground, connected to a suitable outfall; and weep holes through the wall face at regular intervals (typically 1m centres) to allow any water that builds up to escape. Omitting drainage is the most common cause of retaining wall failure.

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Retaining Wall

Q: What is a retaining wall?

A retaining wall is a structure designed to hold back a mass of soil or fill material on one side where the ground level changes significantly - for example, at the edge of a cut-and-fill driveway, a terraced garden, or an excavation. Unlike a simple boundary wall, a retaining wall must be structurally designed to resist the lateral (sideways) pressure of the retained soil plus any loads on the surface above (surcharge), without sliding, overturning, or experiencing structural failure in the wall itself.

/ rɪˈteɪnɪŋ wɔːl /

Also known as: earth-retaining wall, gravity wall, cantilever wall, sleeper wall

Definition

A retaining wall is a structure designed to hold back a mass of soil or fill where ground levels differ significantly on each side - for example at a terraced garden, a cut-and-fill driveway, or an excavation. Unlike a simple boundary wall, a retaining wall must be structurally designed to resist the lateral pressure of the retained earth, water in the soil, and any surcharge loads above, without sliding, overturning, or structural failure. All retaining walls require drainage behind them.

In practice

For garden walls retaining up to 1m of soil, mass concrete or dense concrete blockwork walls on a strip footing are common. The wall should be a minimum of 300mm thick for 1m retained height, founded at least 500mm below ground on the lower side. For retained heights of 1-2m, an L-shaped reinforced concrete cantilever wall or reinforced blockwork wall (with vertical rebar tied into the base slab) is needed - this requires structural design, particularly to size the base slab correctly to resist sliding and overturning.

Drainage is the most critical element of any retaining wall. Clean granular free-draining material (gravel) must be placed against the back face of the wall, with a perforated land drain at the base leading to a suitable outfall. Weep holes (50-75mm diameter, at 1m centres) through the wall face provide secondary drainage. The most common cause of retaining wall failure - bulging or collapse - is water building up behind the wall, which dramatically increases the lateral pressure on the structure.

Building Regulations

Planning (front garden)Walls over 1m by highway require planning permission

Planning (elsewhere)Walls over 2m require planning permission

Building RegulationsMay require approval if affecting structural safety of a building

Design standardBS 8002 - Earth retaining structures

Retaining walls over 1m high adjacent to a highway, or over 2m high elsewhere, require planning permission. Where a retaining wall is close to a building's foundations or could affect structural stability, Building Regulations approval (Part A - Structure) is required and a structural engineer's design is needed. The Party Wall etc. Act 1996 may also apply where a retaining wall is close to a boundary with a neighbour and excavation is required. Always check these requirements before starting work.

Full Building Regulations guidance

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