Best level to set foundation for internal wall

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20 Feb 2013
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Gloucestershire
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United Kingdom
Hi
I'm undertaking an extension and am currently working on the internal foundations. I have a new pier to construct and have dug down about 500mm from finished floor level down to some really hard clay. this is about 200mm below the existing foundations. I will be replacing the floor in this room and will be digging out to 450mm deep with construction being 150mm hardcore 25mm sand, 100mm concrete, 100mm insulation and 75mm screed (with UFH). What is the best level to set the foundation at? I was thinking 175mm below FFL as this would tie in with the top of the concrete slab and the insulation would then sit on the slab and the bit of the foundation. However, I realise this would be a bit of a nightmare for coursing with bricks. What are your thoughts?
Thanks in advance

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Set the concrete level about 300mm down from DPC. This means you will lay a block and a brick and be up to DPC level. It is important that your new masonry aligns with that of the existing at the DPC level. All the rest is irrelevant.

You could set it less than 300mm down as long as you reduce the depth in brick sized increments, i.e. 225mm, 150mm etc. Going 300mm down means that there is no influence from the foundation concrete upon any of your over-site products.

Has building control passed the depth of your foundation dig? This is a controlled aspect and if this pier is bearing a beam etc, then it may be more than just a case of dumping a bit of concrete in a hole.
 
Hi thanks for that. I'll set it 300mm below - that makes sense.

I contacted my BCO and I explained I was going to dig 1m deep and he said it was unnecessary as the ground internally is not affected by outside climate so suggested 0.5m as he knows what my ground conditions are like. The pier will be supporting a door which is being moved by about 0.5m
 

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