Quality.

    Substructure and foundations: what good looks like

    Excavation, concrete, foundation walls up to DPC.

    What good looks like

    Substructure and foundations: what good looks like

    Everything below damp-proof course level: excavation, concrete, foundation masonry and the details that stop ground moisture and gas reaching the building.

    The substructure carries every load above it and forms the first line of defence against ground moisture. Work is judged on three things: the bearing stratum matches what the design assumed, the concrete is placed and cured properly, and the damp-proofing is continuous and correctly positioned. Excavations are inspected before concrete is placed because nothing below the slab can be revisited later without breaking out. Foundation masonry below ground uses the specified mix and units, cavities below DPC are filled where the design calls for it, and service entries through the substructure are formed, not cut afterwards. The damp-proof course must sit clear of finished ground level and lap into the damp-proof membrane so there is no bridge between the ground and the wall.

    Basis in regulation

    • Part C, clause 5

      Site preparation and resistance to contaminants and moisture

      Read the source

      Checked against the live source on 20 August 2026

    Before you start

    Before you start: substructure

    Checks to clear before the first excavation, so the stage does not stall waiting on information or a failed inspection.

    Confirm the foundation design against the ground investigation and any tree or made-ground constraints. Have the setting-out checked independently against the plot layout, not just against the neighbouring plot. Agree the inspection hold points with building control and the warranty provider before excavation, because both want to see open trenches. Check that the concrete specification, reinforcement schedule and any gas or radon protection details are on site and current. Make sure drainage runs, service ducts and step positions are marked, so nothing is chased through green concrete a week later.

    Basis in regulation

    • Part C, clause 2

      Site preparation and ground conditions

      Read the source

      Checked against the live source on 20 August 2026

    Foundation depth calculator

    Work out an indicative founding depth for this plot — clay shrinkability by region, tree water demand, zone of influence, and the heave case when a tree comes out.

    Open the calculator

    Key requirements

    Key requirements: substructure

    The requirements most often picked up on inspection at foundation stage.

    Damp-proof course positioned at least 150mm above finished ground level and lapped with the floor damp-proof membrane. Excavations clean, dry and free of loose spoil before concrete is placed. Concrete placed continuously to the specified depth with no cold joints in a single pour. Cavities below DPC filled with lean mix to the level shown on the design where the detail requires it. Service entries sleeved and sealed rather than cut through after the event. Any step in the foundation formed at the designed offset, with the overlap the design specifies.

    Basis in regulation

    • Part C, clause 5.2

      Damp-proof courses and floors

      Read the source

      Checked against the live source on 20 August 2026

    Hazardous Gas Protection and Cavity Sealing

    When protecting against hazardous ground gases, such as radon or methane, specialized barriers are required, and recycled products for these applications should generally be avoided. It is essential that gas membranes are specified and installed correctly to create an effective barrier. Where caviti

    When protecting against hazardous ground gases, such as radon or methane, specialized barriers are required, and recycled products for these applications should generally be avoided. It is essential that gas membranes are specified and installed correctly to create an effective barrier. Where cavities exist and gas protection is required, it must be ensured that any associated cavity trays and DPCs continue to effectively divert water outwards, maintaining their primary function alongside gas protection.

    Basis in regulation

    Management of Compressible Soils

    When compressible soils like soft clays, disturbed ground, or peat are present, the potential effects of earthworks, including the magnitude and duration of induced settlements, must be thoroughly assessed. Significant predicted settlements may necessitate removal of the compressible material, groun

    When compressible soils like soft clays, disturbed ground, or peat are present, the potential effects of earthworks, including the magnitude and duration of induced settlements, must be thoroughly assessed. Significant predicted settlements may necessitate removal of the compressible material, ground improvement techniques such as vertical drainage, or pre-loading (surcharging) before fill placement. The impact of 'drying out' these soils due to altered groundwater levels must also be evaluated.

    Basis in regulation

    Soakaway construction and design principles

    Soakaways can be constructed as either large, engineered pits lined with permeable materials such as honeycomb brickwork or precast perforated rings surrounded by granular fill, or as smaller granular-filled holes. Large soakaways, particularly, should be designed in accordance with BRE Digest 365 t

    Soakaways can be constructed as either large, engineered pits lined with permeable materials such as honeycomb brickwork or precast perforated rings surrounded by granular fill, or as smaller granular-filled holes. Large soakaways, particularly, should be designed in accordance with BRE Digest 365 to ensure adequate storage volume and a suitable emptying time for half the stored water. A sheet or blinding should cover the top of the fill to prevent fine material from washing in.

    Basis in regulation

    Movement joints in foundations

    Foundations, particularly those formed as rafts or using piles, piers, and beams, must incorporate movement joints designed to accommodate anticipated differential settlement or thermal expansion/contraction. The design of these joints, including the selection of sealing materials and their location

    Foundations, particularly those formed as rafts or using piles, piers, and beams, must incorporate movement joints designed to accommodate anticipated differential settlement or thermal expansion/contraction. The design of these joints, including the selection of sealing materials and their location, is critical to prevent structural damage. Factors such as joint width, depth, and the sealant's movement capability should be carefully considered to ensure long-term performance.

    Basis in regulation

    Groundwater Management in Earthworks

    Groundwater conditions significantly impact the long-term performance of engineered fill and must be carefully considered in design and specification. Lowering the water table for filling can cause settlement of existing adjacent buildings, while a subsequent rise can lead to inundation settlement o

    Groundwater conditions significantly impact the long-term performance of engineered fill and must be carefully considered in design and specification. Lowering the water table for filling can cause settlement of existing adjacent buildings, while a subsequent rise can lead to inundation settlement or washout of fines within the new fill. Effective surface water management, including adequate capping and drainage, is also essential to prevent deterioration over time.

    Basis in regulation

    Engineered foundation solutions for shrinkable soils

    Foundations constructed in shrinkable soils must be specifically designed by a qualified engineer to manage potential ground movement, including both shrinkage and heave. Complex foundation types such as deep trench fill, pile and beam, pier and beam, or raft foundations are only acceptable where th

    Foundations constructed in shrinkable soils must be specifically designed by a qualified engineer to manage potential ground movement, including both shrinkage and heave. Complex foundation types such as deep trench fill, pile and beam, pier and beam, or raft foundations are only acceptable where their design rigorously accounts for all anticipated soil movements. Additionally, specific precautions must be taken for heave protection as detailed in relevant clauses.

    Basis in regulation

    Tree water demand and influence zones

    The water demand of trees varies significantly by species and size, impacting soil moisture levels and potential for shrinkage. If a tree species cannot be identified, or is known to be a high water demand species (e.g., oak, willow), a high water demand should be assumed for foundation design. The

    The water demand of trees varies significantly by species and size, impacting soil moisture levels and potential for shrinkage. If a tree species cannot be identified, or is known to be a high water demand species (e.g., oak, willow), a high water demand should be assumed for foundation design. The lateral zone of influence of a tree is determined by its species, height, and water demand, dictating the extent to which its roots will affect surrounding soil.

    Basis in regulation

    Deep trench fill foundation considerations

    When trench fill foundations exceed 2.5m in depth, specific challenges and precautions arise due to trench instability and increased interaction with the soil. The design must account for plot-specific soil desiccation and arboricultural advice, with additional measures sometimes needed for heave pr

    When trench fill foundations exceed 2.5m in depth, specific challenges and precautions arise due to trench instability and increased interaction with the soil. The design must account for plot-specific soil desiccation and arboricultural advice, with additional measures sometimes needed for heave protection. Careful control of concrete pouring is essential to avoid overspill, and compressible materials must be correctly placed to mitigate heave forces.

    Basis in regulation

    Raft foundations in shrinkable soils

    Raft foundations are an acceptable solution for shrinkable soils only when designed by a structural engineer to resist differential movements and satisfy specific criteria. The raft must be sufficiently stiff, generally rectangular with a side ratio not exceeding 2:1, and founded on granular infill

    Raft foundations are an acceptable solution for shrinkable soils only when designed by a structural engineer to resist differential movements and satisfy specific criteria. The raft must be sufficiently stiff, generally rectangular with a side ratio not exceeding 2:1, and founded on granular infill that is properly compacted in layers. Additionally, the foundation depth must be determined according to established guidelines and be less than 2.5m.

    Basis in regulation

    Aggregate Selection and Assessment

    Aggregates used in concrete must be carefully selected and assessed to avoid issues such as alkali-silica reaction, excessive moisture movement, or shrinkage. Proportions of fine and coarse aggregates must be controlled to achieve a consistent concrete mix. Special precautions, as detailed in guidan

    Aggregates used in concrete must be carefully selected and assessed to avoid issues such as alkali-silica reaction, excessive moisture movement, or shrinkage. Proportions of fine and coarse aggregates must be controlled to achieve a consistent concrete mix. Special precautions, as detailed in guidance like BRE Digest 357, are necessary when using aggregates susceptible to alkali-silica reaction or other deleterious properties, particularly in aggressive sulfate ground conditions.

    Basis in regulation

    Vibratory ground improvement techniques

    Vibratory ground improvement techniques, used to enhance soil properties and support foundations, must be designed by a suitably qualified and experienced engineer in accordance with relevant British Standards and codes of practice. The application of these techniques must comply with all technical

    Vibratory ground improvement techniques, used to enhance soil properties and support foundations, must be designed by a suitably qualified and experienced engineer in accordance with relevant British Standards and codes of practice. The application of these techniques must comply with all technical requirements to ensure the long-term stability and performance of the structure. The design should specifically address the ground conditions and structural loads.

    Basis in regulation

    Foundations on shrinkable soils near vegetation

    When building on shrinkable cohesive soils, such as clay, where trees, hedgerows, or shrubs are present, special foundation design considerations are crucial. Vegetation extracts moisture, causing significant ground volume changes and potential movement that can damage structures. Foundations must b

    When building on shrinkable cohesive soils, such as clay, where trees, hedgerows, or shrubs are present, special foundation design considerations are crucial. Vegetation extracts moisture, causing significant ground volume changes and potential movement that can damage structures. Foundations must be designed to either accommodate this movement or be placed at a sufficient depth to avoid the zone of influence, mitigating the risk of structural damage.

    Basis in regulation

    Foundation design for site and home layout

    Foundation design must account for specific site conditions, including topography and ground stability, as well as the shape, size, and construction type of the proposed homes. For instance, terraced houses may require specific measures to mitigate differential settlement, and sloping sites might ne

    Foundation design must account for specific site conditions, including topography and ground stability, as well as the shape, size, and construction type of the proposed homes. For instance, terraced houses may require specific measures to mitigate differential settlement, and sloping sites might necessitate stepped foundations or suspended floor systems. The chosen foundation depth must always provide a clean, firm, and adequate bearing for the design loads.

    Basis in regulation

    Foundations for shrinkable soils near trees

    Foundations constructed in shrinkable soils, particularly those influenced by trees, hedgerows, and shrubs, must be specifically designed to mitigate the effects of soil volume changes. Key considerations for such designs include the soil classification, its potential for shrinkage and heave, the wa

    Foundations constructed in shrinkable soils, particularly those influenced by trees, hedgerows, and shrubs, must be specifically designed to mitigate the effects of soil volume changes. Key considerations for such designs include the soil classification, its potential for shrinkage and heave, the water demand and height of specific tree species, their zone of influence, and climatic factors. These elements inform the required foundation depth and type.

    Basis in regulation

    Incoming Utilities and Ground Hazards

    Incoming utilities must conform to technical standards and address potential ground hazards, such as chemical contamination or gas ingress like radon. Appropriate protective measures, including membranes and ventilation, should be implemented where buildings are constructed on land affected by these

    Incoming utilities must conform to technical standards and address potential ground hazards, such as chemical contamination or gas ingress like radon. Appropriate protective measures, including membranes and ventilation, should be implemented where buildings are constructed on land affected by these risks. Specific guidance on radon and contaminated land is available from bodies like BRE to ensure the safety and long-term integrity of the dwelling.

    Basis in regulation

    Critical dimensions at this stage

    Browse all critical dimensions
    England
    Low-rise traditional

    Original editorial guidance based on publicly available Approved Documents. Not building control approval, a warranty determination, or a substitute for the full source document.

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