Existing to new foundation level?

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Hi I’m building a rear extension on my 2 bed bungalow. The existing foundation is a strip foundation, and it’s a fair way below ground level, I’d say 900mm.

Because the ground is clay, we have a spec of a 600mm x 1000mm trench fill foundation. My question is, can this new foundation rise above the existing, to 225-450mm below existing DPC? Can the new foundation go against the old brickwork below ground?

thanks
 
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600 x 1000 found is a standard spec, not for clay.

Is it shrinkable clay and are there any trees or hedges within say 10 metres.

In terms of the trench, it just butts up the your existing foundation. It doesnt matterbif thats a strip and brickwork.

These days it is standard practice to do mass fimm trench as its quicker to get out the ground -esp with the advent of concrete pumps.
 
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There are no trees or hedges, not sure on shrinkable or not.

ahh okay, I just thought concrete bearing onto brickwork sounded odd to me.
 
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