Which foundation do I need for my extension?
The foundation is the most important element of your extension. If not carried out correctly it could lead to serious issues with your project and in the worst cases it can end up being demolished.
There are 3 common types of foundation type we use, there are more but these 3 make up all of the foundation types we need to provide for a domestic extension.
This foundation is our most common. A trench is dug down, typically we will dig 1m down from the floor level. The trench is then mass filled with concrete. This area of concrete is then used to spread the load of the walls.
When the soil pressure is low on a project we may have to construct a raft foundation. A structural engineer will be required to design the steel reinforcement required to spread the load across the entire area of the new building. Concrete will be poured into the foundation and the footing to spread the loading.
If the ground is not good enough to support the structure using the other methods or when loading is such that the building needs extra support then we may have to provide a piled foundation. This is known as a deep foundation type where piles will be driven into the ground at set points of the building as instructed by a structural design. The trench on top of the piles will also typically have reinforcement and then concrete will be poured into the piles and foundation to provide deep support to the structure.
We will require a Building Control Officer to sign off the preparation of the foundation and the preparation of the footing before pouring concrete.
Issues that may arise and how to resolve
Issue 1: Not knowing the ground makeup
Sometimes an architect might specify a foundation type but then on excavation the ground stability is not good enough to suit the type specified or the existing house may be using a certain type of foundation and the new structure has to match it in order to keep up consistency. In order to resolve these issues you can carry out a trial hole.
Resolution 1: Trial holes
We can use trial holes to determine the type of foundation before the project starts. This allows the customer to check if any further costs might need to be provisioned for earlier in the project rather than at the point to which resources are committed. We will typically check the results of the trial hole with the Building Control Officer to agree a way forward.
Issue 2: Services crossing or near to the excavation area
Electric cables, gas and sewer pipes are the things we typically come across when excavating. We can usually use our judgement to work out if these services are likely to be in the excavation area. If you come across a sewer pipe that is shared with a neighbouring property then you MUST inform your local water company and obtain permission to build near or over it. If the drain only serves your property then it is owned by you and it can be adapted or built over without permission.
Apply to your water company for permission to build over or near to a sewer pipe; it is best to do this before the project starts otherwise you could experience delays. Severn Trent Website