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How deep should the foundation be for my new domestic extension?

How deep should the foundation be for my new domestic extension?

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The foundations are crucial to get right as they’ll help spread the weight of your new extension and prevent uneven settlement and structural damage to your home.

Here is a list of things your designer or builder will take into account when deciding how deep your foundations should be and what type of foundation to choose:

Soil type

Soils react in different ways to the presence of water, depending on the type of soil and how well it retains the water. Soil also needs to be able to bear the load of your extension and this ability differs in different types of soil.


Trees near your proposed structure can potentially cause problems if your foundations aren’t deep enough but LABC Warranty’s Foundation Depth Calculator can help you or your designer decide how deep to go. You’ll need to identify the species, potential height of the tree and distance from any structure.

Filled ground

You’ll probably need to dig below the level of previously excavated and filled ground into non-disturbed soil. This is also the case if you’re building on what was previously a landfill site where you may need a different type of foundation because non-disturbed ground could be far below the surface. And you will need to take mitigation measures for any contaminants or gases that are present.

For filled ground you should seek the expert advice of a structural engineer as you will generally need an engineered solution.

Adjacent buildings

If there’s a nearby building you’ll generally need to dig down to at least the level of that building. Your contractor will need to ensure that any nearby building is adequately supported when foundations are excavated.

Drains and sewers

To prevent the weight of your new structure weighing down onto existing drains and sewers and damaging them, you’ll need to dig to at least the same depth of the bottom of the drain or sewer.

If a drain passes through a foundation you will have to provide lintels to support the loading from the foundation and walls above and prevent the drain from fracturing.

The Foundation Depth Calculator

LABC Warranty have created a tool to help decide how deep a foundation should be depending on the variables mentioned above. And there's further technical information on foundations on their website.

For more advice speak to your local authority building control team before you submit your application and always make sure your budget includes a contingency for unforeseen additional expenses.