Minimum Foundation Depth UK: Approved Document A, NHBC and Clay

Short answer: In England, Approved Document A paragraph 2E4 sets a minimum of 0.45 m to the underside of a strip foundation to avoid frost, except on rock. On shrinkable clay the minimum is 0.75 m (low shrinkage), 0.9 m (medium) or 1.0 m (high). Trees, soft ground and building control can all make it deeper.

Those four numbers (450, 750, 900 and 1,000 mm) are what most people mean by "minimum foundation depth UK". They are minimums, not targets. This post sets out where each comes from, how NHBC Standards add tree rules on top, and what changes in Wales, Northern Ireland and Scotland.

What are the minimum foundation depths by soil type?

The table below comes from Approved Document A (2004 edition incorporating 2010 and 2013 amendments), section 2E, which covers plain concrete strip foundations. NHBC Standards clause 4.3.3, Table 1 gives the same clay depths and adds the plasticity index bands.

NHBC classes a soil as shrinkable when it has more than 35% fine particles (clay and silt) and a Modified Plasticity Index of 10% or more (clause 4.3.3). The clay minimums in Table 1 only apply where existing or proposed trees and shrubs are outside the zone of influence.

Building control has the final say. Paragraph 2E4 itself says the 0.45 m depth "will commonly need to be increased" in areas with long periods of frost or to transfer loads onto satisfactory ground. The clay depths may need increasing where there are trees nearby.

Why 450 mm?

It is the frost depth. AD A 2E4 says the 0.45 m minimum is "to avoid the action of frost". NHBC clause 4.3.3 says the same for frost-susceptible soils such as chalk: at least 450 mm below finished ground level.

Two site details matter:

• Measure from the right level. If ground levels are being raised and cold weather is expected, NHBC 4.3.3 says to take the depth from the existing ground level, not the finished level, because the new fill won't be there to insulate the foundation during the winter.
• Cold-weather construction. NHBC 4.3.3 also says that when building in cold weather, the foundation should be at least 450 mm below finished ground level, or you should take precautions to stop the ground freezing. More on that in can you lay concrete in frost.

How deep should foundations be on clay?

At least 0.75 m, and often more. Shrinkable clay swells when wet and shrinks when it dries. The top metre or so moves most with the seasons, and tree roots drying the ground make it worse. AD A 2E4 says strip foundations on shrinkable clay must go deep enough that ground movement "will not impair the stability of any part of the building", taking account of vegetation and trees.

The 0.75 / 0.9 / 1.0 m split depends on the clay's plasticity index. That needs a lab test or a site investigation. You can't tell by looking at it. If you don't know the shrinkage class, NHBC 4.2.7 says to assume the worst case or consult an engineer. For more detail, see what foundation depth is needed in clay soil.

How deep should foundations be near trees?

Deeper. How much deeper depends on four things: the tree's water demand, its mature height, its distance from the foundation, and the clay's shrinkage potential. NHBC chapter 4.2 (Building near trees) is the detailed published method.

The key rules from NHBC Standards 2024:

• Use the charts, tables or app. Inside a tree's zone of influence, take the depth from the charts in clause 4.2.12, the tables in 4.2.13 or NHBC's Foundation Depth Calculator, or have it engineer-designed (4.2.7).
• Measure distance to the nearest face. D is measured from the centre of the trunk to the nearest face of the foundation (4.2.7).
• Over 2.5 m means an engineer. Foundations deeper than 2.5 m should be designed by an engineer (4.2.7).
• Removed trees still count. Felling a tree on clay can cause heave as the ground rehydrates. Heave precautions are covered in 4.2.10.
• Root barriers are not an alternative to the depth guidance (4.2.7).
• Step gradually. Where foundations step to follow tree influence, no step should be more than 0.5 m (4.2.9).
• Measure on the centre line. Depths are measured on the centre line of the excavation, from the ground level defined in 4.2.9 (4.2.6).

NHBC 4.2.7 Tables 4 and 5 also give minimum depths outside the zone of influence of trees, tied to where new planting will be allowed:

Column B matches the Approved Document A minimums. Column A lets planting come closer in exchange for a deeper foundation. Table 5 sets the no-planting zones, as multiples of mature tree height by water demand. This matters on an extension when the client plans a new garden.

What if you find roots or soft ground at formation level?

Go deeper, or get an engineer. NHBC 4.3.9 says to deepen soft spots to a sound bottom (or reinforce the concrete) and to remove hard spots. Where roots are visible in the bottom or sides of a trench, especially in clay, the excavation may need to go deeper or have special precautions designed by an engineer. NHBC 4.2.6 says the same about unexpected roots. Call building control before you concrete, not after.

Is the minimum foundation depth different in Wales, Scotland or Northern Ireland?

The 450 mm frost depth is common to all the documents we checked. The clay rules are where they differ.

In Wales, the Welsh Government publishes its own Approved Document A. Its paragraph 2E4 has the same 0.45 m frost rule. On clay it gives only "not less than 0.75m", noting this will commonly need to be increased. It does not have the 0.9 m and 1.0 m tiers that appear in the England edition. Since the Welsh text itself expects that depth to be increased, don't treat 0.75 m as the answer on heavy clay or near trees. See strip foundation depth in Wales.

In Northern Ireland, Technical Booklet D section 4E applies to a single family house of up to three storeys, an annex, or a small single-storey non-domestic building. It sets the minimum depth as the greater of the depth to a selected bearing stratum or 450 mm. 4E.1 says the section should not be used where there is made ground, peat, a wide variation in subsoil, or soil movement from seasonal weather changes. For the wider structural rules, see structural requirements in Northern Ireland.

In Scotland, the Domestic Technical Handbook (April 2026 edition) no longer contains the small-building structural annexes. Clause 1.0.5 points to the separately published Small Buildings Structural Guidance instead. We haven't quoted a depth for Scotland because we didn't verify it in that document.

Strip or trench fill: does the depth rule change?

No. The minimum depth is the same. What changes is how you fill the trench. Deep clay foundations are often trench fill because digging a wide trench to 1 m and building masonry back up is slow. See trench fill vs strip foundations for the trade-offs, and how deep strip foundations need to be for the short version.

A quick depth check before you dig

1. Find out the soil type from the site investigation, trial pits or building control's local knowledge.
2. If it's clay, get the shrinkage class (Modified Plasticity Index). If it's unknown, assume high.
3. List every tree, hedge and large shrub within influencing distance, including any that have been felled or are planned, with species and mature height.
4. Take the depth from NHBC 4.2 charts or tables, or from your engineer. Never go shallower than the AD A minimum.
5. Agree the depth with building control and book the excavation inspection before any concrete goes in.
6. Re-check at formation level for roots, soft spots and water, and deepen if needed.

Getting the answer on site

When the digger driver hits blue clay at 800 mm with an oak 12 m away, ask Verika "how deep do we need to go on medium shrinkage clay, oak at 12 metres?" It answers from Approved Document A and NHBC chapter 4.2, cites the clauses, and points out where the answer depends on information you don't have yet, such as the shrinkage class.

Sources

1. Approved Document A: Structure, 2004 edition incorporating 2010 and 2013 amendments, Section 2E (paragraphs 2E2–2E4, Table 10). HM Government / GOV.UK. https://www.gov.uk/government/publications/structure-approved-document-a
2. NHBC Standards 2024, Chapter 4.3 Strip and trench fill foundations: clauses 4.3.3 (Ground conditions, Table 1) and 4.3.9 (Excavations). NHBC. https://nhbc-standards.co.uk/2024/4-foundations/4-3-strip-and-trench-fill-foundations/
3. NHBC Standards 2024, Chapter 4.2 Building near trees: clauses 4.2.6, 4.2.7 (Tables 4–5), 4.2.9, 4.2.10, 4.2.12, 4.2.13. NHBC. https://nhbc-standards.co.uk/2024/4-foundations/4-2-building-near-trees/
4. "Latest edition of the NHBC Standards published and updated for 2026". NHBC, January 2026. https://www.nhbc.co.uk/insights-and-media/news/latest-edition-of-the-nhbc-standards-published-and-updated-for-2026
5. Approved Document A: Structure (for use in Wales), 2004 edition incorporating 2010 amendments, paragraph 2E4. Welsh Government. https://www.gov.wales/building-regulations-approved-documents
6. Technical Booklet D: Structure, 2012 (Section 4E, paragraphs 4E.1–4E.5). Department of Finance, Northern Ireland. https://www.finance-ni.gov.uk/publications/technical-booklet-d
7. Building Standards Technical Handbook: Domestic, April 2026 (Section 1 Structure, clause 1.0.5). Scottish Government. https://www.gov.scot/publications/building-standards-technical-handbook-domestic-april-2026-pdf/