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What is involved in forming a foundation for a house extension.


All building works start off with planning.

Foundations for a house extension are usually drawn up firstly an architect detailing the basic design of a foundation. But it is the structural engineer who will provide further information with regards to specific design requirements needed.

A structural engineer when designing a typical foundation for a house extension will usually consider the following factors:

  • The bearing capacity of the soil directly underneath where your extension is being built.

  • The design of the foundation will be designed in such a way it won’t settle or subside over its lifetime.

  • The rigidity of the design to minimise settlement when differential loads are placed upon it unevenly.

  • What type of materials are to be used to make the structure both practical and economical to build.

  • Calculations for dead loads, live loads, horizontal loads caused by wind loadings and earthquakes to subsurface soil.

  • The depth of the foundation design in relation to the swelling or shrinkage of the subsoil to prevent damage to the building or overturning.

  • An element of over engineering to provide enough safety against future loads for example overloading.

  • Dependent on the results from a soil investigation. It should be designed to combat various harmful chemicals if found in the subsoil such as sulphates which can attack the concrete being used and cause it to deteriorate.

  • Other environmental factors such as neighbouring buildings. The size and proximity in relation to the proposed foundation being built.

What is a Soil Investigation report

Soil investigation report will provide a structural engineer with the relevant information required to determine the type of foundation your house extension will need and assist the engineer in their design.

The investigation provides information on bearing capacities at different levels and at different locations.

They are required in areas with expansive or low strength clays. Other times a soil investigation report may be required include building where the foundation will be supported by land fill, projects on steep gradients or where a lot of grading will be carried out, locations with high ground water levels may also require a soil investigation report prior to commencement of building work activities.

Who inspects the foundation to ensure the work being carried out is satisfactory?

Engaging the building control to inspect the excavation, before and after concrete pour (they’ll advise if there is a need for further design or piling to be done dependant on the condition of the existing ground at the proposed excavation depth). If there is a requirement for piling and additional steel reinforcement. Building control will also need to inspect the piling once completed and any reinforced steel to ensure it has been installed correctly prior to any concrete being poured.

What materials are used in a typical foundation for an extension?

Typically, a foundation for an extension will consist of concrete.

However, this can vary extensively depending on many contributing factors.

Additional materials such as reinforced steel, concrete additives, heave protection can also be used depending on the results from surveys.

What happens to the existing ground which is removed to create the excavation?

Any excess spoil is generally removed from site either by skip which is filled manually or pile in a location where a grab wagon can position itself to remove it.

This can be quite a dirty work and ideally, boards and plastic sheeting should be lay on the ground to protect existing paving and driveways around the home during its removal.

For more information

For more advice and guidance on foundations for your home extension contact Gibbons Construction Management.

Gibbons Construction Management


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