How high can a garden retaining wall be without engineering?

In the UK, retaining walls under 1 metre high generally do not need building regulations approval for garden use. Walls over 1m or near a boundary may need structural calculations and council approval.

Do I need drainage behind a retaining wall?

Yes, always. Water pressure (hydrostatic pressure) is the main cause of retaining wall failure. Include a gravel drainage layer, perforated pipe at the base, and geotextile membrane to prevent soil clogging the drainage.

Can I use railway sleepers for a retaining wall?

Yes, railway sleepers work well for walls up to about 600mm high. Stack them horizontally, fix with rebar driven through into the ground, and ensure drainage behind. Only use new, untreated or purpose-sold sleepers for gardens.

GardenCalcs.uk

Retaining Wall Calculator

Calculate blocks, backfill, and drainage for garden retaining walls.

Wall Length

Run of the wall measured along its face, in metres

Wall Height

Height from base to top of wall

Block TypeMortared dense concrete blocks, or dry-stacked railway sleepers for walls up to ~600mm

Include BackfillInclude backfill soil behind the wall

Price per Block (optional)

Unit price for block cost estimation

How We Calculate This

Block count = (Wall Length / Block Length) x (Wall Height / Block Height), plus a waste factor for cuts. Standard UK dense concrete blocks are 440 x 215mm on the face (≈10 blocks/m² with 10mm joints).

Mortar is calibrated to manufacturer coverage, not just joint geometry — a 25kg bag (~0.012 m³) lays about 12 100mm blocks with a 10mm joint. Because the bed and perpend joints run the full block depth, mortar scales with depth, so a 215mm-deep wall needs roughly 1.8 bags/m², about double a 100mm block.

Drainage includes a 150mm gravel layer behind the wall, perforated pipe at the base, and geotextile membrane to stop soil clogging the drainage. Backfill is calculated as a 300mm depth behind the wall face.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.