UK Foundation Depths & Footing Widths (Doc A)

The requirement

Except where you found on rock, the underside of a strip foundation should be at least 0.45 m (450 mm) deep. The width comes off Table 10, by ground type and wall load. Both rules sit in Approved Document A (Structure), Section 2E, and cover England.

Approved Document A shows one way to meet Requirement A1 of the Building Regulations. Section 2E covers plain concrete strip foundations under a normal house. Two numbers matter most on site. How deep you dig, and how wide you pour.

Depth: the 450 mm floor. Unless you build straight onto rock, dig so the underside sits at least 450 mm down. That keeps the concrete below where frost can heave the ground. The guide says this will commonly need to be deeper in areas with long spells of frost (para 2E4). So treat 450 mm as the minimum, not the plan.

Approved Document A 2013 · para 2E4 (England)

1 footing
2 reinforcement
3 ground

Deeper on shrinkable clay. Clay swells when wet and shrinks when dry, so it moves the most. Where the clay has a Modified Plasticity Index of 10% or more, para 2E4 sets deeper minimums. Here they are.

Clay shrinkage	Minimum depth to underside
Low	0.75 m
Medium	0.9 m
High	1.0 m

Near trees or hedges it may need to go deeper still. Roots dry the clay out. Which clay you’ve got, and how the trees affect it, comes from a soils report or an engineer. Not from a look in the trench.

Width: read it off Table 10. The width depends on the ground type and the total wall load, in kN per metre run. Table 10 lists the recommended minimum widths. The rows below show the ends of the range.

Ground type	Width at ≤ 20 kN/m	Width at 70 kN/m
Rock (type I)	equal to the wall width	equal to the wall width
Medium gravel/sand, stiff clay (II/III)	250 mm	650 mm
Firm clay, sandy clay (IV)	300 mm	850 mm
Loose sand (V)	400 mm	600 mm (30 kN/m cap)*
Soft silt/clay (VI)	450 mm	650 mm (30 kN/m cap)*

* On loose sand and soft silt/clay, the table only works up to 30 kN/m of wall load. Above that, an engineer has to design the footing. Very soft ground, where a finger pushes in easily, is not in the table at all. There, Table 10 sends you to specialist advice. Read your exact width off Table 10 for your ground and load.

Thickness. The strip needs a minimum thickness T of 150 mm, or the projection P, whichever is greater (para 2E2c). The projection is how far the footing sticks out past the wall face. Trench fill is an accepted alternative. That means filling the whole trench with concrete instead of a thin strip.

Work it out on site

The depth and width come off the code and the plan. The concrete volume you order is just maths. It feeds straight into the calculator. Say you’ve got a strip footing to pour. These are example numbers, not code minimums. Your real sizes come off Table 10 and your plan.

Total length of trench: 22 m.
Footing width: 0.45 m.
Footing depth (trench fill): 0.6 m.

Volume is length times width times depth.

22 × 0.45 × 0.6 = 5.94 m³.

So order about 5.9 cubic metres. Then add a bit for spillage and an uneven trench bottom. Most bricklayers tack on 5 to 10 per cent. Swap in your own sizes and the calculator does the sum for you.

Do it in 5 seconds

Part of the UK building codes hub — the pillar guide, with England, Wales, Scotland and Northern Ireland kept separate.
Setting out the stairs in the same build? See UK stair and barrier requirements.

Sources

The Building Regulations 2010, Schedule 1 Requirement A1 (Loading), satisfied via Approved Document A (Structure), 2013 edition — Section 2E “Foundations of plain concrete”, paras 2E1–2E4 and Table 10 — the 450 mm frost depth (2E4), the 0.75/0.9/1.0 m shrinkable-clay depths (2E4), the 150 mm / projection thickness rule (2E2c) and the strip-footing widths (Table 10). Free to download at gov.uk.
Adoption caveat. These figures are for England only. Approved Document A 2013 edition is still the current version in 2026 — no consolidated replacement has been issued. Wales has its own Approved Document A, and Scotland (Technical Handbooks) and Northern Ireland (Technical Booklet D) are separate regimes not covered here. Approved Documents are guidance, not the law itself — compliance can also be shown by other means, such as a full geotechnical design to BS EN 1997-1 (Eurocode 7) where the ground is variable or the loads exceed the table. Always confirm the current document and check your ground with a soils report or engineer before you dig.

Frequently asked questions

How deep do strip foundations have to be?

At least 0.45 m (450 mm) to the underside, unless you found on rock. That comes from Approved Document A, para 2E4. It keeps the concrete below the frost line. In areas with long spells of frost, the guide says it will commonly need to go deeper. So 450 mm is the floor, not a target.

How wide does the footing need to be?

Table 10 in Approved Document A lists minimum widths by ground type and wall load in kN per metre. On rock, match the wall width. Medium gravel, sand or stiff clay runs 250 mm at light loads up to 650 mm at 70 kN/m. Firmer clay runs 300 mm to 850 mm. Read yours off Table 10, don't eyeball it.

I'm digging on clay — do I need to go deeper than 450 mm?

Yes. Clays that swell and shrink with moisture (a Modified Plasticity Index of 10% or more) need a deeper dig. Para 2E4 puts the underside at 0.75 m on low-shrinkage clay, 0.9 m on medium and 1.0 m on high. Near trees or hedges it may need more. Get the clay class from a soils report or engineer.

How thick does the concrete footing have to be?

Para 2E2c sets the thickness T at 150 mm, or the projection P, whichever is greater. The projection is how far the footing sticks out past the face of the wall. P comes off Table 10 and Diagram 23. Trench fill, where you fill the dig with concrete, is an accepted alternative.

Does the Approved Document A table method cover my house?

Section 2 covers houses up to three storeys, small single-storey non-residential buildings and annexes, in traditional masonry. A family house inside those limits is fine on the table. Taller, heavier or unusual buildings fall outside it. So does variable or filled ground, or wall loads over 70 kN/m. Those need an engineer's design to Eurocode 7.