Building Regulations: Foundations

Your extension relies on strong foundations to safely transfer its entire weight to the ground. Every building needs robust foundations, typically constructed from concrete, but their exact design will vary significantly depending on your specific project and the existing ground conditions.

You can choose between two main types of cast foundations: deep-fill, which fills most of the excavated trench, or shallow-fill, designed to provide just the minimum thickness needed to bear the load effectively. For unusual ground conditions, you might need entirely different foundation solutions. It's always a good idea to consult a structural engineer or your local building control team for tailored advice.

When planning your foundation, you'll need to consider several key factors:

• Soil Type: It's crucial to understand how well your soil can support the weight of the foundation and your new extension. Different soils have varying load-bearing capacities. Be especially aware of certain clay soils, which react significantly to changes in moisture, causing them to expand or contract. To avoid issues from this ground movement, foundations typically need to be deeper, around 0.75m, though this depth can change if there are trees nearby.

• Nearby Structures: You should always assess the proximity and specific type of any neighbouring buildings to your site.

• Trees: Trees extract moisture from the ground, which can lead to ground shrinkage, a particular problem in clay soils. The mature height of a tree dictates how much moisture it will draw. Therefore, foundations located near trees in clay areas may need to be considerably deeper. Be aware that removing or substantially reducing existing trees can also create problems; moisture held in their root systems will gradually release over time. In clay soils, this can cause swelling and potentially damage nearby foundations.

• Drains and Sewers: The weight from your building's foundation spreads downwards at roughly a 45-degree angle. If a drain or sewer falls within this 45-degree zone, there's a risk your foundation's load could crack it. For this reason, foundation excavations should typically extend at least as deep as the very bottom (known as the 'invert') of the deepest part of any drain, sewer, or its associated trench.

• Size and Construction of Your New Building: A two-storey extension will impose a greater load on the ground than a single-storey one. This difference significantly impacts your foundation's design, including its required depth and width. The thickness of your proposed walls also influences the foundation's width.

• Ground Conditions: Ideally, foundations are built on undisturbed ground. This means land that has not been previously built upon or backfilled, for example, over old drains or to level a site.

• Former Landfill Sites: If your property is located on a former landfill site, you might require more extensive foundation solutions, such as piling, due to the greater depth of undisturbed ground. A 'raft' foundation could be another potential alternative. A structural engineer can offer further guidance in these specific situations.

For your own health and safety, always take extreme care when working in trenches due to the inherent risk of collapse.