How far can you excavate below a cellar wall before underpinning
becomes necessary?

I've had a local specialist firm round to quote for converting the
cellar to dry space. I'd do the finishing - the work covers excavating,
laying a floor, sump/pump, a beam to replace an existing dividing wall,
and tanking with a cavity membrane. £6500 inc. for a 4.5 square space.

Anyway, I'm waiting for them to get back to me on how high the space
will be. At the moment it's 195cm to the flags. Just lifted a flag, and
the cellar wall (which I'd describe only loosely as brick) goes down
another 8cm. So that's 203cm, and I'd like a height of at least 210cm.

They'd need to take out 10cm for the Kingspan, 10cm for the floor slab,
and say 10cm for the hardcore/blinding (very clay rich soil)? So I
reckon they'd need to go down about 40cm below the existing wall. Seems
a lot to me without somehow containing the sideways force.

This site:

http://www.tracebasementsystems.co.u...ar-conversion/

says there's some flexibility, but no real guidance about how much and
under what conditions. The quote from my firm says 'as far as existing
foundations will allow without undermining them to retain as much
headroom as possible when new floor is laid'.

Any advice appreciated.


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Cheers, Rob