Need some advice on foundation for a rear double storey extension.

When you step out in to the garden you are on a platform that is approx 800mm above the ground level.
There are 3 steps down to the grass level and then the ground slopes down at a small gradient.
So if i lift the floor boards inside the house there is approx 3 feet drop to the ground and the joist are supported on brick posts.

For the foundations I am assuming that i would need to go down to 800mm to the garden level then a further 1 meter or when i hit solid ground. does this sound about right?

Would i be better of with piling or a foundation slab?


  • Adam

    Hi Narinder,

    The answer to your question would be very dependant upon where you live. I’m afraid that nobody on here will be able to specify what type of foundations would be most appropriate for you without having local knowledge and experience. Your local building control officer at the council will be able to help you with any queries as could any experienced local builders or structural engineers. This is because ground conditions differ from area to area and different sub soil types require different approaches. For example, chalk is very stable and does not require deep foundations to build upon whereas clay is soft, prone to movement and requires much deeper foundations as a result.

    What you can see under your floorboards are "dwarf walls" built to support the floorboards and have nothing to do with the foundations for your property. A foundation is required to take the substantial weight of the load bearing walls of the building. This is usually the "inside skin", or the internal wall. It is this one that carries the weight (load) of the roof and the load of the internal floors; the large timbers that support the floorboards are built into this internal wall. The outer "skin", often brick or other attractive material is asthetic.

    Even when an experienced building professional (usually the structural engineer who drafts the plans) has specified the foundation requirements for your extension you might find that the plans change in response to what you find under the ground when you begin digging. Sadly these people do not have x-ray vision and cannot see what lies beneath your feet. It is one of the few "unknown" elements of an extension build. Once completed the rest of the work is "known" and more easily quantifiable as a result.

    I hope that helps! Any further questions feel free to ask. The internet is also a fantastic resource for looking into these kinds of matters.

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