How deep do extension foundations need to be?

What goes underneath?

Three answers come up again and again on ordinary domestic work. Which one suits a site is a ground question, not a preference.

Three answers to the same load. Trench fill trades bricklaying in a hole for concrete. A strip uses less concrete and more time. A raft spreads the load where the ground is poor or varies.

Diagram drawn for this demo. Not to scale, and not a design.

Why does depth matter?

The Building Regulations do not hand you a depth. Requirement A2 asks that swelling, shrinkage or freezing of the subsoil does not impair the stability of the building, and a depth is a way of passing that test. Compare with .

Illustrative. Not a calculation and not a design.

Depth is about what the base stands on. Clay near the surface swells when wet and shrinks when dry, and trees dry it deeper. A base in that band moves with it; one below it does not.

Approved Document A publishes minimum depths, and says in the same paragraph that they may need to be deeper.

Published minima are a floor, not an answer. What sets the real depth is the ground, the trees and what the trench turns up.

Source: Approved Document A (Structure), paragraph 2E4, including the quoted words. Guidance for England, not the requirement itself. Diagram drawn for this demo: depths to scale, the same scale as the Standards page. Not a design.

Topsoil and vegetable matter come out first, under Requirement C1(1). The simple route in the guidance stops at non-engineered fill, ground that varies under the loaded area, or softer ground below, which describes many suburban gardens. Where mature trees stand on shrinkable clay, Approved Document C asks for heave to be assessed, pointing to BRE Digest 298.

Why does width matter?

Requirement A1 asks that loads are “sustained and transmitted by it to the ground” safely, at a pressure the ground can carry. At the bottom of a trench, the part you choose is the area, and the area is the width. Try , then . The load comes down; the ground pushes back.

Illustrative. Not a calculation and not a design.

Load down the wall

Width of the base

pressure = load ÷ area

Width is about pressure. The same load on a wider base presses less hard on each part of the ground. A plain concrete strip is made at least as thick as it reaches out past the wall, so a wider one is deeper too; its underside stays where the ground puts it.

The published table shows the same thing. Approved Document A sets strip widths from the type of ground and the load per metre of wall, from 20 to 70 kN/m. On firm clay the widths run from 300 mm to 850 mm: same clay, and nearly three times the width, because the load grew.

Sources: Approved Document A, Table 10 (paragraph 2E3); the Building Regulations 2010, Schedule 1, Requirement A1, quoted in part. Crown copyright, Open Government Licence v3.0. The figure above is not wired to the table.

When does next door come into it?

If the dig goes near a neighbour’s foundation, or the work touches a shared wall, the Party Wall etc. Act 1996 puts a timetable in front of the build. Serving notice is not asking permission, and it belongs at the start of the programme, not the week the digger arrives. Where a trench needs holding up, we design that support. See the whole build in order. We do party wall surveys.

Two clocks, counted back from the start date. The dig usually starts the shorter one; cutting into a shared wall starts the longer.

Source: Party Wall etc. Act 1996, section 3 (party structure notices) and section 6 (adjacent excavation). Diagram drawn for this demo.

Nothing here sizes your foundations. That comes from your ground and a design for it, which meets the Building Regulations without concrete it does not need.

Foundations: depth, width, and next door