Retaining walls and Eurocode 7 in a planning submission

Whether a retaining wall needs planning permission depends on its height, and on a sloping site the height is the one thing nobody agrees on. Retaining wall planning permission height limits come from Part 2 of Schedule 2 to the General Permitted Development Order 2015: one metre where the wall is adjacent to a highway used by vehicular traffic, two metres anywhere else. Measuring that one metre on a slope is where the argument starts, and it is worth settling with the council before the ground is cut.

When a retaining wall needs permission

Class A of Part 2 permits the erection, construction, maintenance, improvement or alteration of a gate, fence, wall or other means of enclosure, within the height limits. A garden retaining wall that encloses land falls inside that description.

Three situations take you outside the class.

Height above the limits, which is the common one on a site with a fall across it.

A wall that forms part of a wider engineering operation, such as regrading a sloping plot to form terraces and a level garden. The cut and fill is development in its own right, and describing the result as a boundary wall does not change that.

A wall adjacent to a listed building, or within the curtilage of one, where the class is withdrawn and listed building consent may also be needed.

Hard surfacing is separate again, dealt with under Part 1 Class F for a dwelling, and a retaining structure supporting a new driveway is usually assessed with the driveway rather than on its own.

If the position is unclear, a Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that proposed works are lawful. On a terraced garden scheme that certificate costs far less than rebuilding a wall.

Height thresholds and the sloping site problem

The Order measures height above ground level. On a retaining wall there are two ground levels, and the Order does not spell out which one applies.

Practice generally takes the height from the lower ground level, on the basis that the wall is one metre or two metres above the ground adjoining it, and that is the reading most authorities apply to a retaining structure. But it is a matter of interpretation rather than a settled rule, and the answer can differ between authorities. The honest position is that nobody should design a 1.8 metre retaining wall on the assumption that the higher side counts.

Two practical points follow.

The boundary position matters as much as the height. A wall set one metre inside your boundary, with a planted slope between it and the neighbour, raises fewer questions than the same wall built on the line.

Where the wall is adjacent to a highway used by vehicular traffic, the tighter one metre limit applies, and a footway of such a highway counts as part of it. That catches most front garden retaining walls on a sloping street.

Design standards and the geotechnical basis

Design follows the Eurocode 7 framework in BS EN 1997, with the UK National Annex, and BS 8002 remains the practical code of practice for earth retaining structures. The design is a limit state exercise, with partial factors applied to the actions and to the soil strengths.

The ultimate limit state checks are sliding along the base, overturning about the toe, bearing failure of the founding stratum, and overall stability of the slope the wall sits in. The serviceability check is deflection, which on a garden wall rarely governs and on a wall supporting a building sometimes does.

All of it depends on the soil properties used. For a modest garden wall, trial pits with hand shear vane readings and a visual description may be enough. For anything retaining more than about 1.5 metres, or on a site with a history of movement, a Geotechnical Assessment with laboratory testing is the sensible basis, because the difference between assumed and actual shear strength is the difference between a wall and a rebuild.

Where the wall supports or is supported by an existing building, a Structural Survey of that building comes first, and the same questions arise as when reusing existing foundations on a redevelopment site.

Surcharge from vehicles and structures

A wall designed for retained soil alone will fail if something stands on the soil.

The zone of influence is the one people miss. A wall at the bottom of a garden with a new garden room built close behind it is carrying that building's loads through the soil, and the designer needs to know the structure is coming. Tell them at the start rather than after the wall is built.

Drainage behind the wall

Water is the most common cause of retaining wall failure, and it is also the least costly thing to get right.

The standard arrangement is a free-draining granular backfill or a drainage composite against the rear face, a perforated land drain at the base wrapped in geotextile, and a positive outfall to a point with capacity. Weep holes through the wall are a fallback, not a drainage system, and they block.

Three things to decide and record on the drawing.

1. Where the drain discharges. A land drain that ends in made ground has not solved anything, and discharge to a highway drain or an ordinary watercourse needs separate consent.
2. Whether the design assumes drained or undrained conditions. If drainage is assumed, the drainage has to be maintainable, which means a rodding point.
3. How surface water from the terrace above is intercepted, so it does not simply run over the top.

Where the wall also forms part of a habitable space, water resistance moves from drainage into waterproofing design under BS 8102, which is a different exercise with its own grades of protection.

Neighbouring land and the right of support

Land has a common law right of support from neighbouring land in its natural state. Excavating at the toe of a slope and removing that support gives your neighbour a civil claim, regardless of what the planning permission says.

Two statutory routes also engage. Excavating within three metres of a neighbouring structure to a depth below its foundations triggers a section 6 notice under the Party Wall etc. Act 1996, with one month's notice and drawings. Building a new wall on the line of junction triggers a section 1 notice.

The sequence of excavation and temporary support is a designer duty, and the propping arrangement, batter angles and dig sequence belong in a temporary works design, as set out in temporary works design and designer duties. On a cut into a slope above a neighbour's garden, that design is the whole job.

Drawings and calculations to submit

For a planning application involving a retaining structure, the pack that avoids a request for further information looks like this.

1. A topographical survey with existing levels, including levels on the neighbouring side of each boundary.
2. Proposed levels, with the cut and fill volumes stated.
3. Cross sections through the steepest part of the site, at a scale that shows the wall and the ground on both sides.
4. The wall type and its construction, whether mass gravity, reinforced concrete cantilever, crib, gabion or reinforced soil.
5. Drainage behind the wall, with the outfall identified.
6. Structural calculations by a chartered engineer where the wall exceeds about 1.5 metres or supports a structure.
7. A tree report where excavation falls within a root protection area.

A freestanding garden retaining wall is not normally building work under the Building Regulations, although a wall forming part of a building, or supporting one, is. Confirm that with your building control body rather than assuming, because the answer depends on what the wall does.

This is general guidance, not legal advice. Check the position with your local planning authority and a chartered structural or geotechnical engineer before relying on it, and remember that a Lawful Development Certificate is the only formal confirmation that works are lawful. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.