Retaining walls and level changes: when permission is needed

Cutting into a sloping garden, building up a terrace or putting in a retaining wall is often development, and it often needs planning permission even though nobody thinks of it as building work. Section 55 of the Town and Country Planning Act 1990 defines development to include engineering operations in, on, over or under land, and substantial earthworks fall squarely inside that. The permitted development rights that cover garden walls are narrower than most people assume, and they do not reliably cover a structure whose job is to hold back earth.

When earthworks become development

Moving soil around a garden bed is not development. Regrading a slope, excavating a terrace, or importing fill to raise a level is an engineering operation, and it is development whether or not any wall is involved.

There is no threshold written into the Act. The test applied in practice is one of degree: the volume of material moved, the change in level achieved, the permanence of the works and their effect on the appearance of the land. A 200 mm regrade to level a lawn will not interest anyone. A 1.5 m cut and fill across a rear garden to create two terraces will.

Where the works form part of a wider scheme that already needs permission, put them on the drawings and describe them in the application. Existing and proposed levels, shown as spot heights on a survey and as a section through the site, prevent an argument later about what was approved.

Height limits for walls, and why retaining walls differ

Part 2 Class A of the General Permitted Development Order 2015 allows gates, fences, walls and other means of enclosure, subject to height limits: 1 m where adjacent to a highway used by vehicles or the footway of such a highway, and 2 m elsewhere. Those rights are removed within the curtilage of a listed building, and can be removed elsewhere by an Article 4 direction or by a condition on an earlier permission.

The trap is the phrase "means of enclosure". A wall built to retain earth is doing a structural job rather than enclosing a boundary, and on that reading it is an engineering operation outside Class A altogether. Local planning authorities take different views on where enclosure ends and retention begins, particularly for a wall that does both.

Measurement adds a second trap. A retaining wall is measured from the lower ground level, not from the level of the terrace behind it, so a wall that reads as 600 mm from your side may be 1.8 m from your neighbour's. And where a fence sits on top of a retaining wall, the combined height is what counts.

A Lawful Development Certificate under section 192 of the 1990 Act is the only formal way to confirm that works are lawful without permission. On a sloping site where the answer is arguable, that certificate is worth far more than a reassuring phone call.

Raised terraces and the 300 mm rule

The GPDO defines a raised platform as a platform more than 300 mm high, and the householder rights for extensions and for outbuildings both exclude verandas, balconies and raised platforms. So a deck or patio built up more than 300 mm above the adjoining ground needs planning permission.

That single number decides a great many garden projects. Levelling a sloping garden almost always produces a platform above 300 mm somewhere, and the part of the garden nearest the house is usually where the platform is highest and the overlooking is worst.

Amenity is what the council will assess. A raised terrace changes the eye level from which a neighbouring garden is seen, and it can remove the privacy a boundary fence previously provided. Set the terrace back from the boundary, drop it where you can, and consider a planted screen rather than a taller fence, because raising the fence to solve overlooking usually creates an overbearing problem instead.

Structural evidence for a significant retaining structure

For anything beyond a low garden wall, the council will expect to see that the structure has been designed rather than guessed.

A Geotechnical Assessment supplies the ground model, and a Structural Survey is often needed where the works sit close to an existing building whose foundations are unknown. Councils expect these to be prepared by a suitably qualified author, and professional review is strongly advised before anything is submitted.

Drainage behind the wall

Retaining structures fail because of water far more often than because of load. A wall without drainage builds up hydrostatic pressure behind it, and the design load doubles or worse.

The detail that belongs on the drawing is a free-draining backfill, a perforated land drain at the base, a filter membrane to stop the drain silting up, and a positive outfall that does not discharge onto the neighbouring land. Weep holes alone are not a drainage design.

That outfall point is where planning and private law meet. Discharging collected water onto a neighbour's garden is a civil matter regardless of any permission you hold, and it is the single commonest cause of disputes after level changes. Where the site is large enough that surface water strategy becomes an issue in its own right, a Drainage Strategy sets out the approach properly.

Support for neighbouring land and private rights

Planning permission does not give you the right to remove support from your neighbour's land. The common law right of support is separate, and it survives whatever the council decides.

Excavating close to a boundary may also trigger notices under the Party Wall etc. Act 1996, which applies to excavation within defined distances and depths of a neighbouring structure. That process runs in parallel with planning and is administered privately between owners and their surveyors. It is not something the council checks, and it is not something PlanScope produces.

Restrictive covenants on the title can prohibit changes to levels or the erection of structures above a stated height. Check the title before design rather than after.

What to submit with the application

Level changes are refused more often for poor drawings than for poor design. The pack that works contains:

1. A measured topographical survey showing existing levels across the site and on both sides of each boundary.
2. Proposed levels shown as spot heights, with a clear datum.
3. At least two sections through the site, one of them cutting through the highest retained face.
4. A construction detail for each retaining structure showing foundation, drainage and finish.
5. Elevations from the neighbouring side, which is the view the case officer cares about.
6. A short written justification covering privacy, overlooking, drainage and the effect on trees.

A Planning Statement pulls those threads together and answers the amenity question before the neighbour raises it. Where mature trees sit near the works, add the root protection area to the plan, because excavation within it is usually the reason a level change gets refused outright.

This is general guidance, not legal advice. Permitted development rights vary by site and can be removed, and a Lawful Development Certificate is the only formal way to confirm that works are lawful. Check the position with your local planning authority or a qualified professional before relying on it.

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 documents a sloping-site project is likely to need.