Foundation Depth Calculator UK | House & Extension Foundations

The absolute minimum foundation depth in the UK is typically 450mm below finished ground level, as required by Building Regulations to ensure foundations extend below the frost line. However, this is rarely sufficient in practice. Most domestic foundations range from 600mm to 1000mm depending on soil type, with depths of 1200-2500mm common where trees affect clay soils. The minimum depth for your specific project depends on soil bearing capacity, building load, proximity to trees, and local ground conditions.

Foundation depth for a single-storey extension typically ranges from 600mm to 1000mm in the UK, depending primarily on soil type. On firm sand or gravel, 600-750mm is often adequate. On clay without trees, expect 900-1000mm. If trees are nearby, particularly on clay soil, depths can increase to 1500-2500mm. Rocky ground may allow slightly shallower foundations around 450-600mm, though frost protection minimums still apply. Always conduct trial pits and consult a structural engineer for your specific site conditions.

Two-storey house foundations in the UK typically require depths of 750mm to 1200mm on good ground, increasing substantially where clay or trees are involved. On gravel or sandy soils, 750-900mm may suffice. Clay soils generally require 1000-1200mm minimum. On clay with nearby trees, depths often reach 1800-2500mm or more. The increased loads from two storeys require not only adequate depth but also sufficient foundation width (typically 700-900mm) to distribute loads safely. Professional structural design is essential for two-storey construction.

Soil type is one of the most critical factors determining foundation depth. Rock and firm chalk offer excellent bearing capacity and can support domestic loads at 450-600mm depth. Gravel and sandy soils typically require 600-900mm. Loam and mixed soils need 750-1000mm. Clay soils, which have lower bearing capacity and are prone to moisture-related movement, generally require 900-1000mm minimum even without trees. Peat, very soft ground, and made ground usually cannot support standard strip foundations and require specialist solutions such as piled foundations or ground improvement. Soil identification through trial pits is essential for accurate foundation design.

Trees on clay soil can dramatically increase required foundation depths, often from around 900mm to 2000-2500mm or more. Trees draw moisture from clay, causing it to shrink (particularly in summer), and when trees are removed or in wet winters, the clay re-swells. This cyclical shrinkage and heave can cause severe structural damage. The depth increase depends on tree species (willows, poplars, and oaks have very high water demand), tree height (taller trees have deeper, more extensive root systems), and distance from the foundation (trees affect soil moisture up to 1.5 times their mature height). NHBC Chapter 4.2 and LABC guidance provide detailed calculation methods. Professional arboricultural and structural advice is essential for building near trees on clay.

Typical house foundation depths in the UK range from 0.6m to 2.5m, with most domestic foundations falling between 900mm and 1200mm. This range reflects the variety of UK ground conditions and building types. Simple single-storey buildings on good ground may use 600-800mm depths, while standard two-storey houses on clay often require 1000-1200mm. Where trees influence clay soils, depths routinely reach 1500-2500mm. Strip foundations are most common for houses, typically 600-750mm wide for single-storey and 700-900mm for two-storey construction. Trench fill foundations (where the trench is filled with concrete to near ground level) use the same depths but are quicker to construct and popular in the UK residential sector.

No, this calculator cannot replace a structural engineer. It provides indicative guidance only based on typical UK conditions and rules of thumb. A qualified structural engineer must design your actual foundations based on proper site investigation including trial pits, soil testing, and consideration of all site-specific factors. The engineer will produce detailed drawings and calculations that meet Building Regulations requirements and are suitable for Building Control approval. This calculator is a useful planning tool to understand approximate depths and costs, but professional engineering input is legally and practically essential for all building projects.

Foundations that are too shallow can lead to serious structural problems including differential settlement (where parts of the building sink at different rates, causing cracks), frost heave (where freezing ground lifts the foundation, cracking walls), bearing failure (where the soil cannot support the load, causing the building to sink), and tree-related movement on clay (seasonal cracking and distortion). These problems can be extremely costly to remedy, often requiring underpinning at costs of £20,000-£100,000+ for a typical house. In severe cases, buildings may become structurally unsafe. Building Control will not approve shallow foundations that don't meet regulations, and mortgage lenders may refuse to lend on properties with inadequate foundations. Always ensure foundations are designed with adequate depth and safety margins.

Yes, almost all foundation work in the UK requires Building Control approval under Building Regulations Part A (Structure). You must either submit a Building Notice or Full Plans application before starting work. Building Control inspectors will check foundation excavations before concrete is poured to verify the depth, width, and soil conditions meet the approved design. They may require changes if site conditions differ from expectations. Failing to obtain Building Control approval is illegal, will void building insurance, and can make your property impossible to sell or remortgage. Garden walls under 1m height may be exempt, but all habitable buildings, extensions, garages, and most outbuildings require approval. Always consult your local Building Control department early in the planning process.

Foundation depth increases near trees can be substantial, particularly on clay soils. Where foundations might normally be 900mm on clay without trees, nearby trees can push this to 1500-2500mm or more. The exact increase depends on a calculation involving tree species water demand (categorised as low, moderate, high, or very high), mature tree height, and distance from foundation. A typical example: a mature oak (high water demand) with height 15m located 10m from a building on clay might require foundations around 1800-2000mm deep, compared to 900-1000mm without the tree. Willows and poplars (very high water demand) close to buildings can require depths exceeding 2500mm. The NHBC Chapter 4.2 provides detailed guidance, and professional arboricultural assessment is strongly recommended for any building project near trees on clay soil.

Strip foundations and trench fill foundations use the same foundation depth - the difference is in how the excavated trench is filled. A strip foundation has a relatively thin concrete strip (typically 150-300mm thick) at the bottom of the trench, with the remainder of the trench backfilled, and blockwork built up from the concrete strip to ground level. A trench fill foundation fills the entire trench with concrete from foundation depth up to just below ground level, eliminating most of the blockwork. Both achieve the same structural result and use the same depth calculation methods. Trench fill is faster to construct and popular in the UK, though it uses more concrete. The choice between them doesn't affect the depth requirement - that depends on soil, loading, and trees as calculated by this tool or determined by your structural engineer.

Garden wall and boundary wall foundation depths typically range from 450mm to 750mm in the UK, though this varies with wall height, soil type, and tree proximity. Low walls (under 1m high) on good ground may only require the minimum 450mm depth for frost protection, with a foundation width equal to 2-3 times the wall thickness. Taller walls (1-2m) generally need 600-750mm depth and wider foundations for stability. On clay soils or near trees, depths increase significantly to match the soil movement zone. Retaining walls (holding back soil) require much deeper foundations and professional structural design. For simple decorative boundary walls under 1m high on good ground, a 450mm deep, 300-450mm wide concrete footing is often adequate, though Building Control approval may still be needed for walls over 1m or retaining walls. Always check with your local authority before constructing walls near boundaries.