UK Foundation Depths & Footing Widths (Doc A)

The requirement

Before you pour a strip foundation in England, two numbers matter most: how deep the trench goes, and how wide the footing is. Both live in Approved Document A (Structure), Section 2E “Foundations of plain concrete”. This is the guide that shows one way to meet Requirement A1 of the Building Regulations for a normal house.

Depth — the 450 mm floor. Except where you’re building straight onto rock, the underside of the strip foundation should be at least 0.45 m (450 mm) deep. That keeps it below where frost can get at the ground and heave it. The guide is clear that this depth will commonly need to go deeper in areas that get 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 that swells when wet and shrinks when dry moves the most, so it needs a deeper dig. Where the clay has a Modified Plasticity Index of 10% or more, the underside should be at least:

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

And it may need to go deeper still near trees or hedges, whose 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 a look in the trench.

Width — read it off Table 10. The footing width depends on the ground type and the total load on the wall (in kN per metre run). Table 10 gives the recommended minimum widths. Here’s the shape of it:

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)*

* Loose sand and soft silt/clay only work off the table up to 30 kN/m of wall load — above that the footing has to be designed by an engineer. Very soft ground (where a finger pushes in easily) isn’t in the table at all — Table 10 sends you to specialist advice. Read your exact width straight off Table 10 for your ground and load; the numbers above are just the ends of the range.

Thickness. The concrete strip itself has a minimum thickness T of 150 mm, or the projection P (how far it sticks out past the wall face), whichever is greater — para 2E2c. Filling the whole trench with concrete (trench fill) is an accepted alternative to a thin strip.

Work it out on site

The depth and width come off the code and the plan. The volume of concrete you order is just maths — and 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 = length × width × depth:

22 × 0.45 × 0.6 = 5.94 m³

So you’d 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?

Except where you're founding on rock, the underside of a plain concrete strip foundation should be at least 0.45 m (450 mm) deep, to keep it below the frost line — Approved Document A, para 2E4. The guide adds that this depth will commonly need to go deeper in areas that get long spells of frost. So 450 mm is the floor, not a target.

How wide does the footing need to be?

It depends on the ground and the load on the wall. Table 10 in Approved Document A gives recommended minimum widths by ground type and total wall load in kN per metre. On rock the footing just has to match the wall width. On medium gravel/sand or stiff clay it runs from 250 mm at light loads up to 650 mm at 70 kN/m. On firmer clay it runs 300 mm up to 850 mm. Read the exact width off Table 10 for your ground and load — don't eyeball it.

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

Yes. On clays that swell and shrink with moisture (a Modified Plasticity Index of 10% or more), para 2E4 says the underside should be at least 0.75 m on low-shrinkage clay, 0.9 m on medium and 1.0 m on high. It may need to go deeper still near trees or hedges. The clay class and the tree effect are a job for a soils report or an engineer, not a guess on site.

How thick does the concrete footing have to be?

Para 2E2c sets the thickness T at 150 mm, or the projection P (how far the footing sticks out past the face of the wall), whichever is the greater. P comes off Table 10 and Diagram 23. Trench fill — filling the dig with concrete instead of a thin strip — is an accepted alternative.

Does the Approved Document A table method cover my house?

Section 2 (which holds these footing rules) covers houses of not more than three storeys, small single-storey non-residential buildings and annexes, all in traditional masonry. A normal family house inside those limits is fine on the table. Taller, heavier or unusual buildings — or ground that's variable or filled, or wall loads over 70 kN/m — fall outside the table and must be designed by an engineer to Eurocode 7.