Retaining wall design
Design of a new retaining wall for a garden, basement, driveway or boundary.
About this service
A retaining wall holds back a difference in ground level, and the soil behind it is always pushing. That push grows with height, with water trapped in the ground and with anything sitting on top, such as a parked car, a shed or the corner of a neighbour's house. Even a garden wall over about a metre high deserves proper design rather than a builder's rule of thumb.
In London the soil itself adds to the challenge. London Clay drains poorly, so water builds up behind walls that have no drainage path, and the clay swells and shrinks with the seasons. Terraced gardens in Haringey, Lewisham and Croydon often step down steep plots, while lowered front gardens for driveways and new basement lightwells create cuts right next to pavements and houses. Each situation needs a wall chosen for its own loads and access.
Our design covers the full wall: the type, the base or footing, the reinforcement or masonry specification, and the drainage behind it. We also look at how the wall will be built, because digging out for the base can briefly leave the ground unsupported. Every design goes through our three-step quality control process, with final sign-off by our Head of Engineering, before it reaches your builder. Fees start from £600 + VAT.
When you need it
- You're changing ground levels in a garden or driveway
- An old retaining wall needs rebuilding
- A basement or lightwell needs a retaining wall
What's included
- Assessment of soil, water and surcharge loads
- Wall type selection and design
- Drainage details
- Calculations and drawings
How it works
- Site informationWe gather levels, the retained height, what sits above the wall and what is known about the soil. Photographs and a simple survey are usually enough for a garden wall; larger walls may need ground data first.
- Choose the wall typeWe compare practical options such as reinforced concrete cantilever, mass concrete, reinforced blockwork or gabions, weighing height, space for a base, access for machinery and how the finished face should look.
- Calculate and detailWe check sliding, overturning, bearing pressure and the strength of the wall stem, then detail reinforcement, movement joints, drainage stone, weep holes and any pipe to carry water away.
- Build sequenceFor walls near buildings, boundaries or the highway, we set out how the excavation should be dug in short lengths or supported so the ground and neighbouring structures stay stable during construction.
- Issue and supportYou receive calculations and drawings ready for pricing and, where needed, for Building Control. We answer contractor queries and review any changes they propose before work proceeds on site.
What you receive
Why drainage and surcharge decide most retaining wall designs
Most retaining walls that fail in London gardens do not fail because they were too thin. They fail because water had nowhere to go. A saturated clay backfill can roughly double the pressure on a wall compared with drained ground, so the drainage layer, weep holes and outlet are as important as the concrete. We make sure that water discharges somewhere sensible. If you are rebuilding a wall that has already leaned, a retaining wall condition assessment first can explain what went wrong.
Surcharge is the other big factor. A car on a driveway, a house foundation close behind the wall or a stack of building materials all add load. Walls near a neighbour's foundations may also bring in the Party Wall etc. Act 1996, as our party wall guide explains. Deeper cuts for lightwells are covered in our basement conversion guide.
Projects that used this service
Frequently asked questions
Do I need a structural engineer for a garden retaining wall?
For low walls under about a metre with nothing close behind them, a well-built standard detail may be fine. Above that height, or where the wall supports a driveway, a building, a boundary or sloping ground, an engineer's design is strongly advised. The loads rise quickly with height and water, and a failed wall can damage property and injure people.
What is the best type of retaining wall for clay soil?
There is no single best type, but on clay a reinforced concrete cantilever wall with good drainage behind it is a common, robust choice. Gravity walls in mass concrete or gabions suit lower heights where there is space. Whatever the type, a drainage layer and weep holes are essential, because water trapped in clay greatly increases the pressure on the wall.
Does a retaining wall need Building Control approval?
It depends on what the wall does. A wall that supports or forms part of a building, such as a basement or lightwell wall, will generally fall under the Building Regulations. A free-standing garden wall often does not. Your local council's Building Control team can confirm for your site.
Can I rebuild a leaning retaining wall on the same line?
Often yes, but the new wall should be designed for the real loads rather than copied from the old one. The lean usually tells you something, such as missing drainage, a shallow base or extra load behind it. Rebuilding usually involves temporary support of the ground while the old wall comes down, especially near a boundary or footpath.