The foundation is one of the most critical parts of any house extension, as it provides the stability and strength needed to support the structure. Building a solid foundation ensures your extension stands the test of time and avoids issues like settling or cracking. In this guide, we’ll walk you through the key steps involved in building foundations for a house extension, so you know what to expect.

1. Determine the Type of Foundation

The type of foundation required for your extension will depend on several factors, including the size of the extension, soil conditions, and the existing structure of your home. The most common types of foundations for house extensions include:

  • Strip Foundations: Suitable for most small to medium-sized extensions, strip foundations involve digging a trench along the perimeter of the extension and filling it with concrete. These foundations provide excellent support and are ideal for stable ground conditions.
  • Trench Fill Foundations: Similar to strip foundations but with deeper trenches filled with more concrete. This type of foundation is often used when the soil conditions are poor or where deeper foundations are needed to reach solid ground.
  • Raft Foundations: Used when ground conditions are weak or where there is a risk of settlement. Raft foundations involve spreading a large, reinforced concrete slab across the entire area of the extension to provide even support.
  • Pile Foundations: When the soil is too unstable for shallow foundations, pile foundations are used. Piles are long, slender columns driven deep into the ground to reach stable soil layers. This type of foundation is common for larger or more complex extensions.

Your builder or structural engineer will determine the best type of foundation based on a site survey and soil tests.

2. Excavating the Ground

Once the type of foundation is decided, the next step is excavation. This involves clearing the site of any debris and digging the trenches or pits for the foundation. The depth and width of the trenches will depend on the size of the extension and the type of foundation chosen.

If the extension is close to the existing house, the foundation will need to be carefully dug to ensure the new and old foundations don’t affect each other. This may involve underpinning or strengthening the existing foundation.

During excavation, your builder will also check for any potential issues, such as tree roots or underground utilities that may need to be addressed before proceeding. Minimise structural issues with advice from our post on cracks where extensions meet the house.  To learn more, check out our blog post on crack where an extension meets house – click here.

3. Laying the Foundation

Once the excavation is complete, it’s time to pour the concrete and lay the foundation. The process varies depending on the type of foundation being used:

  • Strip and Trench Fill Foundations: Concrete is poured into the prepared trenches, ensuring it’s evenly distributed. Reinforcing steel bars may be added for extra strength.
  • Raft Foundations: A reinforced concrete slab is poured over the entire foundation area. Steel mesh is often used to reinforce the slab and prevent cracking.
  • Pile Foundations: If using pile foundations, piles are driven into the ground at strategic points. A concrete beam or slab is then poured on top to distribute the weight of the extension evenly.

It’s crucial that the foundation is level and properly cured to prevent any structural issues later on.

4. Building the Substructure

Once the concrete has set and the foundation is ready, the substructure is built. This includes the damp-proof course (DPC) and brickwork up to the floor level. The DPC is a crucial layer that prevents moisture from rising into the extension and causing damage to the structure.

At this stage, any necessary services such as drainage pipes, gas, or electricity conduits may also be installed. Proper insulation can be added to improve energy efficiency and ensure the extension meets building regulations.  Learn more about planning energy-efficient features in our designing a house extension guide. To learn more, check out our blog post on how to design a house extension – click here.

5. Inspections and Final Checks

Once the foundation and substructure are complete, a building inspector will need to check that the foundation meets local building regulations. This ensures that the work complies with safety standards and that the foundation is fit to support the rest of the extension.

The builder will arrange for the inspection before proceeding with the rest of the construction. Once approved, the project can continue with the construction of the walls, roof, and internal fittings.

Conclusion

Building a solid foundation is critical to the success of your house extension. By understanding the process and working with experienced professionals, you can ensure that your foundation is built to last. At Sawmill Contracts Ltd, we specialise in creating high-quality, durable extensions across Edinburgh and the Lothians. Explore how foundations fit into the full process of building a house extension.  To learn more, check out our blog post on how to build a house extension – click here.

If you’re ready to start your project or need expert advice on foundation options, contact us today for a consultation!