Retaining walls and permitted development limits

A retaining wall in a garden can be permitted development, or it can be two separate pieces of development needing permission, and the difference usually turns on how much earth you move. The wall itself may fall within the permitted development right for means of enclosure in Part 2 Class A of the General Permitted Development Order 2015, with its 1 m and 2 m height limits. The excavation and regrading behind it is an engineering operation, and there is no householder permitted development right that covers a substantial change of garden levels.

When a retaining wall needs permission

Part 2 Class A of the GPDO permits the erection, construction, maintenance, improvement or alteration of a gate, fence, wall or other means of enclosure, within stated height limits. A boundary wall that happens to retain a modest level difference is generally treated as falling inside that right.

The doubt arises with a wall whose purpose is retention rather than enclosure. A structure built mid-garden to hold back a cut terrace is not enclosing anything, and several authorities take the view that Part 2 Class A does not apply to it at all. There is no neat statutory answer, and honestly the position varies between councils.

What is clearer is the earthworks. Under section 55(1) of the Town and Country Planning Act 1990, development includes engineering operations, and cutting into a slope or building up a platform is an engineering operation. Part 1 of the GPDO gives householders rights for extensions, outbuildings and hard surfaces. It gives no right for general excavation or filling.

So the useful question is not whether the wall is permitted development. It is whether the whole project, wall plus earthworks plus any retained level change visible from outside, amounts to development requiring an application.

Height limits and how they are measured

The limits in Part 2 Class A are straightforward to state and awkward to apply.

Measurement is where disputes begin, because the Order does not define the datum. Practice measures from the natural ground level immediately adjacent to the wall, and where there is a level difference either side, from the lower side. That produces the outcome most homeowners find surprising: a 900 mm retaining wall with a 1.2 m fence on top of it is commonly treated as a 2.1 m means of enclosure, which exceeds the limit.

Two further points. The natural ground level is the level before the works, so you cannot raise the ground first and then measure a short wall from the new level. And an Article 4 direction can remove Part 2 Class A entirely, which is common in conservation areas and in areas with a strong pattern of open front gardens. Check the council's Article 4 register before you assume anything.

Level changes as engineering operations

The scale of the earthworks is what moves a garden project from minor works into an application. A council will look at three things.

The volume moved, because a scheme that exports or imports lorry loads of soil is plainly an operation of some substance. The visual effect, particularly whether a new terrace or platform is visible from the highway or from neighbouring gardens and raises a new outlook or overlooking issue. And the effect on drainage and on neighbouring land, because changing where water goes is a planning and a civil matter at the same time.

An illustrative example. A sloping rear garden is cut back 1.5 m at the house end to form a level terrace, held by a 1.5 m reinforced masonry wall running across the garden and returning along both boundaries. The wall alone might arguably sit inside the 2 m limit. The 40 cubic metres of excavation, the new retained level across the whole width, and the raised terrace looking directly into the neighbour's kitchen are what make it an application.

Raising levels is scrutinised more heavily than cutting, because a raised garden creates overlooking and shifts surface water onto adjoining land. If your scheme raises levels near a boundary, expect to be asked for sections rather than plans.

Boundary position and highway frontage

The 1 m limit for frontages applies to a highway used by vehicular traffic, including the footway of such a highway. A public footpath that carries no vehicles falls under the 2 m limit instead, so establish the status of the route before designing.

Building at the line of junction with a neighbour brings the Party Wall etc. Act 1996 into play. Section 1 deals with new walls built at or astride the boundary, and section 6 requires notice where you excavate within three metres of a neighbouring structure to a depth below its foundations, or within six metres where a line drawn down at 45 degrees from the base of their foundation would cut your excavation. Those notices are separate from planning and they have their own timescales.

There is also a common law duty not to remove support from your neighbour's land. Cutting into a slope at the boundary without retaining it gives your neighbour a claim regardless of what the planning position is, and that is the claim that actually costs money.

Highway authority interests run alongside. A retaining wall supporting the highway, or built close enough to affect it, will need the highway authority's agreement and may need a licence.

Structural design and building control

A freestanding garden retaining wall generally falls outside Building Regulations control. That is not permission to build it badly.

Retaining structures are designed to Eurocode 7, BS EN 1997, which covers earth pressures, bearing capacity, sliding and overturning. Any wall over about a metre, and any wall retaining a slope above a neighbour or a building, should be designed by a structural or geotechnical engineer, with the soil parameters taken from an actual investigation rather than assumed. Where the wall supports a building or forms part of building work, building control is involved and a Structural Survey or engineer's calculations will be required.

On a sloping site with unknown ground, a Geotechnical Assessment pays for itself. Clay shrinkage, perched water and made ground all change the design, and the same considerations arise where the works go below floor level, as our note on cellar conversion or new basement excavation sets out.

Drainage behind a retaining structure

Water is what brings retaining walls down. Saturated backfill exerts pressures far above those from dry soil, and a wall designed without drainage is a wall designed for a load it will eventually see.

The standard arrangement has four parts:

1. Free-draining granular backfill immediately behind the wall, separated from the retained soil by a filter geotextile so the fines do not wash into it.
2. A perforated land drain at the base of the backfill, laid to a fall, with a rodding point.
3. An outfall to a soakaway, a surface water drain or a suitable discharge point, never onto the neighbour's land.
4. Weep holes through the wall where a drain outfall is not practical.

The outfall is the item most often omitted, and it is also the one that causes a neighbour dispute. Discharging collected water over a boundary is a nuisance whatever the planning position, and in a garden that already floods it will be noticed within a season.

Confirming lawfulness before building

If you intend to rely on permitted development, get it confirmed. A certificate of lawfulness of proposed use or development under section 192 of the 1990 Act is the only formal way to establish that what you propose is lawful, and it costs a fraction of what removing an unlawful wall costs.

Submit with it a site plan, existing and proposed sections through the slope at two or three positions, the wall height measured from natural ground level with the datum marked, and a note on the volume of excavation. Sections are what decide these applications. Plans on their own cannot show a level change.

Where the property is in a conservation area or is listed, take advice before you demolish anything. Demolishing an existing boundary wall in a conservation area can itself require permission, and works to a wall within the curtilage of a listed building may need listed building consent, a point that arises in the same way for other historic fabric as our piece on chimney removal and stability in conservation areas explains.

This article is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it, and remember that a Lawful Development Certificate is the only formal way to confirm 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.

Enter your postcode on PlanScope to see which reports a garden level change at your property is likely to need.