It depends on height and position. As a general rule, a wall over one metre next to a road or footpath, or over two metres elsewhere, will need permission. A retaining wall within roughly 3.7 metres of a street also needs approval under the Highways Act. Building regulations come into it too — anything retaining a significant amount of soil, or supporting a building, driveway or path, needs proper structural design. We'll tell you which of these apply to your garden before you commit to anything.
If it's holding back roughly a metre of soil or more, or supporting anything — a driveway, a path, a building — then yes, it should be engineer-designed. That isn't caution for its own sake: retaining walls fail, and they fail suddenly, usually after heavy rain when water pressure builds up behind them. We work with structural engineers we can bring in, or we're happy to work with one you appoint.
It's the single most common reason retaining walls collapse. Water builds up in the soil behind the wall and the pressure increases dramatically — far beyond what the wall was ever designed to hold. That's why we build in weep holes, drainage pipe and free-draining backfill behind every wall. It's the part you can't see, and it's the part that determines whether the wall is still standing in twenty years. It's also why we guarantee our walls against structural failure for seven years.
