How deep do foundations need to be in the UK?

The minimum foundation depth depends on the soil type. On rock or gravel, 450mm is the minimum. On clay soils, foundations must be at least 900mm deep (often 1m or deeper) to get below the zone affected by seasonal moisture changes. Near trees on clay, depths of 1.5-3m may be needed. Always check with Building Control for your specific site.

What is the difference between strip and trenchfill foundations?

Strip foundations use a relatively thin layer of concrete (typically 225mm) at the bottom of a trench, with blockwork built up from the concrete to ground level. Trenchfill pours concrete to near ground level, which is faster and requires less blockwork but uses more concrete. Trenchfill is preferred on clay soils and for speed.

How wide should foundations be?

For a standard domestic load-bearing wall, foundation width should be at least 450mm (for single-storey on good ground) to 600mm or more. Building Regulations Approved Document A, Table 10 provides minimum widths based on wall load and soil bearing capacity. Your Building Control officer will confirm the requirement.

Do I need Building Regulations approval for foundations?

Yes. All new foundations require Building Regulations approval. Building Control will need to inspect the excavated trenches before concrete is poured to verify the soil conditions and foundation depth. You must give them notice before covering up any foundation work.

What concrete mix should I use for foundations?

For domestic strip and trenchfill foundations, a GEN 3 or ST2 mix (C20/25) is standard. This is equivalent to a 1:2:3 site mix. For sites with sulfate-bearing soils (common in clay areas), a sulfate-resisting cement or designated mix (DC-2 or higher) may be required. Your Building Control officer can advise.

Foundation Calculator — Trench and Concrete Volumes

Calculate foundation trench volumes and concrete quantities based on soil type and UK Building Regulations guidance.

Input Method

Building Perimeter

Total external perimeter of the building

Internal Walls (if any)

Number of Internal Walls

Load-bearing internal walls needing foundations

Average Internal Wall Length

Average length of each internal wall

Soil Type

Determines minimum foundation depth per Building Regs

Trench Width

Minimum 450mm; 600mm typical for domestic

Trench Depth (override)

Leave at 0 to use recommended depth for soil type

Include wastage allowance

Adds 10% extra for uneven trench bottoms and over-excavation

Wastage Percentage

10% standard; 15-20% for rough or uneven ground

Price Per m³ (optional)

Ready-mix concrete price per cubic metre

How We Calculate This

This calculator estimates the total trench length from your building dimensions, then calculates the trench volume and concrete needed based on soil type guidance.

The formula

Concrete volume = Total trench length × Width × Depth × Wastage factor

UK Building Regulations guidance

Foundation depths are based on Approved Document A (Structure), Table 10:

Rock / Gravel / Sand: Minimum 450mm depth, 450mm width
Clay: Minimum 900mm depth (often 1m+), 600mm width
Soft clay / Soft silt: Minimum 1000mm depth, 750mm+ width

These are minimum values. Actual foundation depths depend on local conditions, proximity to trees (especially on clay soils), drainage, and the specific loads being carried. NHBC Chapter 4.2 provides additional guidance for new-build housing.

Important note

This calculator provides estimates for planning and budgeting. Foundation design must be confirmed by your Building Control body or structural engineer, who will inspect the site conditions. Always get professional sign-off before pouring concrete.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.