Walls that hold,
fences that stay up.
A retaining wall is a structure carrying tonnes of wet soil. Drainage behind it is not a detail — it is most of the design.
London gardens slope. Terracing them is what makes a steep or split-level plot usable, and it is the element with the least margin for error in domestic landscaping, because a retaining wall is holding back a mass of saturated soil that gets heavier every time it rains.
The load is not mainly the soil — it is the water in it. Soil behind a wall that cannot drain becomes hydrostatic pressure, and that pressure is what pushes walls over. This is why a properly built retaining structure has a drainage layer behind it, a positive route for that water to escape through weep holes or a land drain, and a geotextile keeping the fines out of the drainage layer so it does not silt up and stop working. A rendered blockwork wall with nothing behind it is a wall that will lean.
Founding matters equally. The wall is built off a foundation sized for the height it retains and the ground it sits on, taken below the zone where London clay shrinks and swells seasonally. Where a wall is close to a mature tree or holds back more than about a metre, it stops being a landscaping detail and becomes something that warrants structural design rather than judgement.
Fencing is the lighter cousin of the same problem: posts carry wind load, and wind load is what takes fences down. The failure point is nearly always where the post meets the ground.
Materials
- Brick
- Matched to the house where the wall is seen from it. The right choice on a period property and the most permanent of the options.
- Rendered block
- A clean contemporary face at lower cost than brick, and it takes a colour. The block behind it does the structural work.
- Gabion
- Stone-filled cages that drain freely by design, which suits a wet retaining situation and gives a strong texture.
- Fencing
- Close-board for privacy, slatted for light and air. Posts concreted below movement, and set out to the boundary rather than eyeballed.
How it is built
- Establish what it retainsHeight, ground type and water are what size the structure. Above roughly a metre it is designed, not assumed.
- FoundationExcavated to firm ground and cast to suit the load — the part most failed garden walls never had.
- Build with drainage behindA free-draining backfill and a land drain at the base, because water pressure is what pushes walls over, not soil weight.
- Weep holes or outfallA route out for the water that collects behind. Omitting it undoes everything above.
- Coping and finishA capping that throws water clear of the face, so the wall does not stain and saturate.
What decides the specification
The things that change the answer on a London job — and that nobody can judge from a brochure.
Drainage behind the wall
A free-draining backfill, a geotextile separating it from the retained soil, and a positive outlet — weep holes or a perforated land drain running to somewhere real. Without an outlet the drainage layer just fills up, and the wall carries full water pressure. This is the single most common reason garden retaining walls fail.
Height, and when it needs engineering
Low terracing is landscaping. Once a wall retains roughly a metre or more, is surcharged by a driveway, patio or building above it, or is supporting a neighbour's ground, it is a structure and the design should be checked by an engineer rather than assumed. Retaining a neighbour's land also brings the Party Wall etc. Act into play.
Gabions, and where they earn their place
A gabion is a free-draining gravity structure: water passes straight through, so hydrostatic pressure is never generated. That makes them well suited to wet ground and to sloping plots, and they can be planted into. They need a level, compacted founding and correctly tensioned baskets, or they belly outward.
Boundary heights
A fence or wall on a boundary is normally permitted development up to two metres, and up to one metre where it adjoins a highway used by vehicles. Conservation areas, listed buildings and some estate covenants restrict it further, and a planning condition on the original development can override permitted development entirely.
Whose boundary is it
Ownership decides who may build, and title plans are frequently ambiguous about it. Working on or replacing a boundary structure is agreed with the neighbour before it starts — a fence in the wrong position by 150mm is a legal problem, not a landscaping one.
Fence posts and wind
Fences fail at the post base. Concrete-set timber posts rot at the ground line where wet and air meet; concrete posts or post supports move the failure point away from the timber. Closeboard is more wind-resistant than panel fencing, which acts as a sail.
What it costs
Every garden is priced from its own survey, because the cost is decided by things that are invisible until the ground is opened: how far the spoil has to be carried, what the existing sub-base is made of, whether levels have to be built up or cut away, and how the site drains. We quote after measuring, not before.
Book a free site surveyRetaining Walls & Fencing, answered.
How high can a garden wall or fence be without planning permission?
Generally up to two metres anywhere on the boundary, reduced to one metre where the boundary adjoins a road used by vehicles or the footpath beside it. Those limits are tighter in conservation areas and for listed buildings, and a condition attached to a previous planning permission can remove permitted development rights altogether.
Does a retaining wall need drainage?
Yes, and it is the most important part of the design. Soil behind a wall holds water, and that water generates pressure the wall was probably never designed to carry. A free-draining backfill separated by geotextile, with weep holes or a land drain taking the water somewhere, is what keeps the wall vertical.
When does a retaining wall need a structural engineer?
When it retains roughly a metre or more, when there is a load above it such as a driveway, patio or building, when it supports a neighbour's ground, or when it is close to a mature tree on shrinkable clay. Below that, sound construction and proper drainage are usually sufficient, but the height alone is not the whole test.
Are gabion walls a good idea in a London garden?
They work particularly well on wet or sloping ground, because water passes through them and no hydrostatic pressure builds up behind. They also take planting and read as a softer, more textural element than render or brick. They do need a properly compacted level founding and correctly tensioned baskets, or they bulge.
Who owns the fence between two gardens?
It is decided by the title deeds, and it is often genuinely unclear — the common belief about the left-hand or right-hand boundary is not a legal rule. Where the plan is ambiguous, the practical answer is to agree it with the neighbour in writing before any work starts.
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