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How deep do the extensions need to be for a 2-storey?
We already have a 1 storey extension with a flat roof, built in the 1960's (so no plans are available).
We want to build a 2nd storey on top of this, but have been told by 1 builder that the foundations need to be 1.5m deep. However, another told us that because we already have the 1 storey in place and this rule only relates to new builds, this isn't the case.
Who is correct? Thanks!
Stufai
1 answer
The depth of foundations for new work is determined by the ground conditions and the structure they are to support, 1.00m is the general minimum depth required. With you wanting to build on top of an existing extension then it will really be down to your Local Authority Building Inspector to say whether the existing foundations are sufficient.
Providing your existing extension shows no signs of movement then the crucial question is what depth of concrete is in the foundation. Single storey works were often built on 6"/ 150mm of concrete as opposed to 9"/ 225mm for 2 storey work.
What you need to do is dig a hole at the side of the extension so that you can see how deep the foundation is and the depth of concrete. If you have only got 150mm then chances are the Building Inspector will require you to 'Underpin' (add extra concrete underneath the existing foundation). If you have 225mm and the structure shows no sign of movement then the Inspector will most likely allow you to build on top of the existing, even if the foundation is not as deep as would be required today.