Foundation Design Requirements for UK Houses

What This Requirement Covers

Foundations transfer the loads from a building to the ground. The choice of foundation type and design depends on the ground conditions, the loads from the building, and the proximity of trees, drains, and other factors that may cause ground movement. Approved Document A provides simplified guidance for strip and trench fill foundations in houses, while more complex situations require design to the Eurocodes.

Key Requirements

Strip Foundations (Simplified Rules)

Approved Document A Table 10 provides minimum foundation widths for strip foundations based on the total load of the wall and the type of subsoil

1. Rock: Minimum width equal to the wall width
2. Gravel/sand (compact): 250-650 mm depending on load
3. Clay (firm): 300-850 mm depending on load
4. Clay (soft): Not suitable for simplified approach; requires engineering design
5. Silt/peat: Not suitable; requires specialist foundation design

Minimum depth of strip foundations

1. 450 mm below finished ground level in all cases
2. Deeper in areas of shrinkable clay or where trees are present (see below)

Trench Fill Foundations

1. Concrete poured to within 150 mm of ground level in a narrow trench
2. Minimum trench width: 450 mm (to allow concrete placement)
3. Must reach the same formation depth as a strip foundation
4. More economical for deeper foundations as it eliminates the need for blockwork below ground

Raft Foundations

1. Used on ground with variable bearing capacity, areas prone to mining subsidence, or soft clay
2. Must be designed by a structural engineer to BS EN 1997 (Eurocode 7)
3. Typically used where strip foundations would need to be excessively deep or wide

Piled Foundations

1. Required where suitable bearing strata are deep below the surface
2. Common in areas with very soft ground, former landfill sites, or where load-bearing capacity is insufficient for spread foundations
3. Must be designed by a specialist piling engineer

Trees and Shrinkable Clay

1. On shrinkable clay soils, the presence of trees can cause significant ground movement (heave and subsidence)
2. NHBC Standards Chapter 4.2 provides guidance on foundation depths near trees based on tree species, mature height, and distance from the building
3. Foundations may need to be 1.5-3.0 metres deep or more in shrinkable clay near trees
4. Where trees have been recently removed, anti-heave precautions (compressible material lining) may be required

Practical Compliance Tips

1. Always obtain a ground investigation before finalising foundation design; trial pits or boreholes reveal the actual ground conditions
2. On shrinkable clay, identify all trees within the zone of influence (typically the mature height of the tree) and design foundations accordingly
3. Check for made ground, buried obstructions, and contamination that may affect foundation design
4. Submit foundation details to Building Control before commencing excavation
5. Building Control will inspect the foundation excavation before concrete is poured; schedule this inspection in the construction programme
6. Consider ground water level; foundations below the water table may require pumping during construction and tanking for permanent works
7. Keep records of formation level, soil conditions observed, and any variations from the design for the building file