In 40 seconds
Building a freestanding garden wall in the UK usually costs roughly £600–£2,500+, depending on length, height and material. As a rate, a standard brick boundary wall typically runs about £120–£300 per linear metre including footings, while a retaining wall — which holds back soil and needs a stronger structure — is more, commonly £150–£500 per square metre. Rendering a wall afterwards adds roughly £40–£80 per square metre. On planning, a wall up to 1 metre high next to a highway, or up to 2 metres elsewhere, is normally permitted development and needs no permission; above those heights, or by a listed building, you should check first. The honest answer is always a range, because it depends on your wall's length, height, footings and finish.
Most garden wall guidance is published by firms that build them, so the numbers tend to be optimistic and the height rules glossed over. The pages below give honest cost ranges for brick, block and retaining walls, explain what rendering adds, and set out the 1m and 2m planning rules — before you take a single quote.