The first thing to get right on any type of home extension project is the foundations. To do this, your project team needs to understand what’s supporting the existing house; identify the soil conditions on site; and investigate the potential for any nearby trees, services and drains to impact on the new foundations. All of these considerations can have a major impact on the foundation design. Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics to support them. Options available include:

Strip Foundations

Strip foundations are a type of shallow foundation that are used to provide a continuous, level (or sometimes stepped) strip of support for linear structures such as walls or closely-spaced rows of columns that are built on top of the foundation, placed centrally along their length.

Strip foundations are suitable for supporting linear loads in most types of subsoil, but they are most suitable where soil is of relatively good bearing capacity. They are particularly suited to light structural loadings such as those found in many low-rise or medium-rise domestic buildings - where mass concrete strip foundations can be used. In other situations, reinforced concrete may be required.

The size and position of strip foundations is typically related to the overall width of the wall they are supporting. The depth of a traditional strip foundation is generally equal to or greater than the overall wall width, and the foundation width is generally three times the width of the supported wall. This results in the load being spread at 45º from the wall base to the soil. Approved document A of Building Regulations defines minimum widths for strip footings based on the type of ground and load-bearing wall, although it is generally advisable to consult a structural engineer when designing foundations

Pad foundations

Pad foundations are generally shallow foundations, but can be deep depending on the ground conditions. They are a form of spread foundation formed by rectangular, square, or sometimes circular concrete ‘pads’ that support localised single-point loads such as structural columns, groups of columns or framed structures. This load is then spread by the pad to the bearing layer of soil or rock below. Pad foundations can also be used to support ground beams.

Raft foundations

Raft foundations are slabs that cover a wide area, often the entire footprint of a building, and are suitable where ground conditions are poor, settlement is likely, or where it may be impractical to create individual strip or pad foundations for a large number of individual loads. Raft foundations may incorporate beams or thickened areas to provide additional support for specific loads. Raft foundations are formed by reinforced concrete slabs of a uniform thickness (typically 150 mm to 300 mm) that cover a wide area, and can be considered to ‘float’ on the ground as a raft floats on water. The concrete raft tends to include steel reinforcement to prevent cracking, and may incorporate stiffening beams or thickened areas to provide additional support for specific loads.

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