In 1696, an English merchant called Henry Winstanley decided to build a lighthouse on the Eddystone rocks, a treacherous reef twelve miles southwest of Plymouth. While he had no engineering training, he did have plenty of self-confidence. He was so sure of his creation that he declared he wished to be inside it “during the greatest storm there ever was.”
In November 1703, the Great Storm struck. The lighthouse was gone by morning… and so was Henry Winstanley.
Built structures can look perfectly fine until the forces acting on them reach a point they can’t handle. When that happens, there’s no warning. It just fails.
This same principle can apply when you terrace a sloping garden using retaining walls.
A £30,000 Lesson
A couple came to us recently with a problem. Last year, they’d invested £30,000 in terracing their sloped garden with retaining walls, steps and new levels. Their builder was confident, and no designer or structural engineer was involved because neither the clients nor the builder thought to ask for one.
After heavy rain, the walls collapsed.
They were rebuilt, but even after the repairs the clients felt uncomfortable using the terrace. They asked us to advise on planting, and we suggested they get a structural engineer to assess it before the space was used as normal.
The safety body CROSS, which collects structural failure reports across the UK, notes that retaining walls are often built without any formal structural design – simply because many people don’t realise one is needed or would help.
In most cases, water is the underlying cause. And the good news is that failures like these are entirely preventable.
The Hidden Force Behind Most Retaining Wall Failures
Soil is heavy, but waterlogged soil is heavier. When wet soil presses against a wall that wasn’t designed to handle that load, you have a problem – one that can build slowly and invisibly over months or years.
Poor drainage is the most common cause of retaining wall failure. When water can’t escape from behind a wall, it builds hydrostatic pressure, pushing outward with considerable force. A wall can look solid for a long time while conditions behind it are quietly worsening.
The other common cause of retaining wall failure is simpler: the wall was never sized or built properly for the job – it had the wrong height or thickness, inadequate foundations, or the wrong materials were used for the ground conditions.
On a terraced site, each level needs to manage water, so it doesn’t simply push the problem onto the wall below. This is where a structural engineer earns their fee. Before anything’s built, they calculate the loads based on your specific site conditions.
A structural engineer will specify drainage from the start – weep holes, granular backfill, sometimes a French drain behind the lower tier. They’ll also determine wall thickness, construction methods and foundation design based on the actual loads and ground conditions on your site.
Getting a structural engineer involved early doesn’t complicate a project. It makes the build more straightforward because the contractor knows exactly what to do.
A retaining wall with no drainage, showing the build up of hydrostatic pressure at the base
A retaining wall with granular backfill and weep holes to reduce hydrostatic pressure build up
What the Rules Say (and What They Leave Out)
One of the most common questions we hear is: do I need planning permission for a retaining wall? The answer is patchier than most people expect.
Planning permission is needed for any retaining wall over two metres in height, or one metre if near a roadway. Building regulations approval can apply where a wall supports a building’s foundations. Build within 3.7 metres of a public road and you’ll need local authority approval under the Highways Act 1980.
However, planning permission may still be required if your local planning authority decides to treat multiple lower walls as a single large retaining system, or if hard-surfaced terraces are raised more than 300mm above the original ground level – which can be classified as a built structure under planning law, depending on the authority.
If excavations extend within three metres of a neighbour’s foundations, the Party Wall Act 1996 may apply – meaning you’ll need to give your neighbours written notice before work begins.
Here’s the important bit. A retaining wall well within your garden, away from any highway and not supporting a building – which covers most residential terracing projects – can fall entirely outside formal building regulation requirements. The law doesn’t automatically require you consult a structural engineer.
We’re garden designers, not planning consultants. Our contracts make clear that planning applications are the client’s responsibility. What we do is flag when a project warrants a pre-application enquiry with the local planning authority. Pre-application enquiries are usually free, takes days rather than weeks, and gives you a written position to build from.
Planning approval and structural safety are two separate questions, and satisfying one doesn’t answer the other. As the owner of a structure on your land, you are legally responsible if it collapses and causes damage or injury. Home insurance may not cover you if the wall was poorly built or neglected.
For terracing with walls approaching a metre or more, multiple tiers, or heavy clay subsoil – familiar territory across the East Midlands – a structural engineer’s input is worth building in from the start, rather than something to consider only if problems arise later.
If in any doubt about the rules and regulations where you live, check the UK Planning Portal.
What We Do
When a garden involves significant level changes or retaining structures, we’ll tell you your project needs a structural engineer.
Yes, it adds cost. A few hundred pounds for a modest wall, into four figures for a complex multi-tier scheme. Against the total budget of a serious garden project, it’s a small line item – and one that gives you confidence the job has been done properly.
We work with landscapers who understand and welcome structural engineer input. It tells them exactly what to build and gives everyone clear lines of responsibility if questions arise later. A good contractor sees it as a help, not a hurdle.
The couple with the collapsed walls had none of that in place. Their builder was probably capable in most circumstances. Installing retaining walls on a sloped site, though, is a structural engineering problem dressed up as a landscaping job.
If you have an existing terraced garden and something doesn’t feel quite right, a structural engineer is your first call. If you’re planning a terraced garden on a slope, talk to us first. We can help you work out when an engineer is needed and make sure it’s part of the project from the beginning.
We can help you transform your garden into a space you and your family can enjoy. If you’d like to discuss your project, get in touch below.