Question 1

How deep do foundations need to be in the UK?

Accepted Answer

The minimum foundation depth depends on the soil type. On rock, gravel or sand (non-cohesive ground), 450mm is the minimum to clear frost action (Approved Document A Section 2E, para 2E4). On shrinkable clay, depths vary by plasticity class: 750mm for low-shrinkage clay, 900mm for medium and 1000mm for high — these three depths are given in AD A para 2E4 itself (and echoed by LABC / NHBC Standards Chapter 4.2), measured to get below the zone affected by seasonal moisture changes. Near trees on clay, depths of 1.5-2.5m or more may be needed depending on the species and distance. Always check with Building Control for your specific site.

Question 2

What is the difference between strip and trenchfill foundations?

Accepted Answer

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.

Question 3

How wide should foundations be?

Accepted Answer

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.

Question 4

Do I need Building Regulations approval for foundations?

Accepted Answer

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.

Question 5

What concrete mix should I use for foundations?

Accepted Answer

For domestic strip and trenchfill foundations, C25/30 (RC 25/30 or FND 2 designation) is standard per BS 8500-2:2023 — the general-purpose GEN designated series (GEN0–GEN3) tops out at GEN3 and is not strong enough, so foundation concrete sits in the designated RC/FND series. This is equivalent to a 1:1.5:3 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.

Question 6

How deep should footings be for an extension?

Accepted Answer

There is no single fixed depth — it depends on the ground and, crucially, on the existing house. The Building Regulations baseline (Approved Document A 2013, para 2E4) is a minimum of 0.45m to the underside on most non-cohesive ground to clear frost, rising on shrinkable clay to 0.75m (low shrinkage), 0.9m (medium) or 1.0m (high). For an extension, the Planning Portal's official guidance adds that it is good practice to excavate at least to the same depth as the bottom of the foundation of the adjacent building, so the new structure moves in harmony with the existing one and you avoid cracking at the junction. Near trees on clay, 1.5m to 2.5m or deeper can be required. Where the new trench runs alongside the existing footing, dig and concrete in short alternating (hit-and-miss) bays so you never undermine a long run of the existing wall. Building Control will inspect the open trench and confirm the depth before you pour.

Question 7

How do I calculate how much concrete is needed for footings?

Accepted Answer

Concrete volume for a trenchfill footing is simply total trench length × width × depth, all in metres, giving cubic metres (m³). For example, a 0.6m wide trench 1.0m deep needs 0.6 m³ of concrete for every metre of trench run (0.6 × 1.0 = 0.6 m³/m), so a 40m run of trench takes about 24 m³. A traditional strip footing only fills the bottom of the trench, so you multiply by the concrete thickness (commonly 225mm = 0.225m) rather than the full trench depth. Always add a wastage allowance — 10% is typical, 15-20% on rough or over-dug ground — because trench bottoms are rarely perfectly even. Ready-mix is ordered in 0.25 m³ steps, so round your final figure up. Enter your dimensions above and the calculator does this for you, including the wastage uplift.

Question 8

What size footings do I need for a single-storey extension?

Accepted Answer

Width is set by the wall load and the ground's bearing strength, read from Approved Document A 2013 Table 10. For typical light domestic loads (around 20-30 kN/m) on medium-dense gravel/sand or stiff clay, Table 10 gives a minimum strip width of about 250-300mm, but in practice 450-600mm is normal so the footing is wide enough to dig and work in and to suit a cavity wall. The concrete must be thick enough that its thickness T is at least the projection P beyond the wall face, and never less than 150mm (AD A para 2E2c). Depth follows the soil and the existing house as above. These are minimums — your Building Control officer or engineer confirms the final size from the actual loads and a check of the ground.

Question 9

What concrete grade do foundation footings need under Building Regulations?

Accepted Answer

For plain (unreinforced) strip and trenchfill footings in non-aggressive soil, Approved Document A 2013 para 2E2b accepts grade ST2 or grade GEN1 concrete to BS 8500-2 as the minimum, or a site mix of 50kg of Portland cement to no more than 0.1 m³ (200kg) fine aggregate and 0.2 m³ (400kg) coarse aggregate. Most builders and ready-mix suppliers specify a stronger C25/30 (designated RC25/30 or FND2) for durability and consistency, which is good practice rather than a higher legal minimum. On chemically aggressive or sulfate-bearing ground (common in some clays), you must follow BS 8500-1 and BRE Special Digest 1 and use a sulfate-resisting designated FND mix instead. Your Building Control body confirms the grade for your site conditions.

Foundation & Footings Calculator — Concrete Volume

How We Calculate This

The formula

Footings for an extension

UK Building Regulations guidance

Important note

Frequently Asked Questions

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