Footings for wall

The size of the footing and thickness of foundation wall are specified on the basis of the type of soil at the site. Garden walls are not normally controlled by building regulations unless they are part of a house or extension or provide support to the foundations of another building, if it is a retaining wall then it would be wise to get advice from a structural engineer. The width of the wall footing is generally 2-3 times the width of the wall, the depth depends on the ground, the general rule of thumb would be to dig down to solid ground which is normally clay in the uk. The minimum requirement is that the footing be at lease 150mm in depth, I hope this helps.

Garden walls aren’t typically covered by building control. It would be wise to build this to building regs which for a typical wall (not part of a house or extension and not connected to the property) would be 600mm wide and the depth of foundation is typically 600mm to 800mm deep in sand or 1.2 metres in clay.

General rule of thumb in 150mm overlap each side of the wall and atleast 6 inches deep

With a wall of that height I would make the depth 200mm whether it’s building regs or not.