Short answer: yes. Any brick or block garden wall that you want to stay upright and crack free needs a concrete footing, even a low one at knee height. How deep and wide that footing has to be depends on the wall and, more than most people expect, on what is under your lawn, which varies a lot across Rickmansworth.
A brick wall is heavy. A single skin wall one metre high weighs roughly 200 kg per metre run, and a full brick thick wall is about double that. Sat directly on topsoil, that weight settles unevenly, and brickwork does not bend. It cracks, usually in a stepped line through the mortar joints, and then it starts to lean.
A footing spreads the load onto firm ground below the soft, organic topsoil and below the depth where frost and seasonal moisture move the ground around. The only things we would build without a concrete footing are dry laid edging, a course or two of bricks bedded as a lawn border, or timber sleepers. Anything mortared needs a proper base.
For a typical garden wall the trench is dug until it reaches firm, undisturbed subsoil, then filled with concrete to within a couple of courses of ground level so the first bricks sit just below the surface. The rule of thumb for width is two to three times the thickness of the wall.
These are the sizes we would normally start from, adjusted once we have seen the ground:
Rickmansworth sits where three river valleys meet, the Colne, the Chess and the Gade, and the ground changes within a few streets. Down on the valley floor, around the Aquadrome, Mill End and parts of Croxley, you tend to find river gravels and softer alluvial soil with a high water table. Gravel is a good base, but trenches can fill with water and soft patches have to be dug out until firm ground is reached.
Up the valley sides, towards Chorleywood, Loudwater and Moor Park, there is chalk with clay and flint over the top in places. Sound chalk makes an excellent base and footings can sometimes be shallower. Clay is the awkward one: it shrinks in dry summers and swells in wet winters, so we go deeper, typically 750 mm to a metre.
Trees matter more than anything else on clay. A mature oak, willow or poplar within several metres of the wall line will dry the ground out and can lift or drop a shallow footing. Many of the older gardens here have exactly that sort of planting, so it is the first thing we look at. Where roots are a concern, the options are a deeper footing, moving the line of the wall, or a different type of boundary.
Garden walls generally do not need Building Regulations approval, but they still have to be safe. Planning permission is normally needed for a wall over 1 metre high next to a road or footpath, or over 2 metres elsewhere. Parts of Rickmansworth are in conservation areas and some houses are listed, so check with Three Rivers District Council before you start. If the wall sits on a shared boundary, talk to your neighbour first, as the Party Wall Act can apply.
People often ask whether we can rebuild on the existing footing. Sometimes, yes. We dig a trial hole to see how deep and wide the old concrete is and whether it has cracked. If the old wall failed because the footing moved, building on it again just repeats the problem.
On cost, the footing is usually a modest share of the job. As a rough guide for this area, digging, removing spoil and concreting a straightforward footing tends to fall somewhere around £60 to £120 per metre run. Poor access, deep clay, roots, or carrying everything through the house by hand will push that up.
Only if you know what is underneath. Most patio slabs sit on a thin bed of sand or mortar with no real depth, so a mortared wall built on top will crack as the slabs settle, and it is safer to cut out a strip and dig a proper footing.
For a garden wall we normally leave the footing at least 24 to 48 hours before laying the first courses, and longer in cold or wet weather. The concrete keeps gaining strength for weeks, but it is firm enough to build on steadily after a couple of days.
Usually not if it is under 1 metre next to a road or footpath, or under 2 metres elsewhere. Conservation areas, listed buildings and some estates with their own covenants are exceptions, so check with Three Rivers District Council first.