Lowering Cellar Floor

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.uk/faq-basement-cellar-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.
--
Cheers, Rob

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On Sunday, 21 January 2018 14:19:35 UTC, RJH wrote:

e.

If your walls go 8cm underground you can't dig at all without underpinning. There's always the compromise option to dig but leave 45 degreeish slope t o support existing walls, but that would be of little use for 45 sqft.
NT
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On 21/01/2018 14:51, snipped-for-privacy@gmail.com wrote:

The OP said '4.5 square', which I took to be 20 sqm = 200sqft.
We looked at utilising the cellar in a late-victorian building. Almost exactly the same lack of foundations, yet somehow it has survived on shrinkable clay.
The same solutions were proposed as you suggest, with the same issue of reduced space. In practice, it's not quite so bad, as you can live with what amounts to a couple of steps round the edge of a room. For example, it doesn't matter if the built-in wardrobes have a raised floor at the back of them. You could even shorten the legs on the bedstead at one end!
In the end, we got cold feet and backed out of the project. There were too many worries about flooding and the building falling down, and we felt we could manage without. Bear in mind that you lose around 3 sqm both upstairs and in the cellar for access stairs.

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On 21/01/2018 16:03, GB wrote:

Sorry, yes 4.5 m2 (4.5m x 4.5m).

Yes, it is quite a thing, sounds similar to this (1900ish). I'm not sure how thick the cellar walls are. I'd guess about 18" (4.5" brick and some sort of stone/rubble material)

yes, I could live with that. But I think they're thinking in terms of using the foot of the cellar walls as the floor level. That'd give me just over 2m.

The cellar has never flooded, but the flags change colour with the damp at various times. There's a pool of water where I've just lifted the flag and dug down about a foot. But that, I suppose is what the the underfloor drainage/sump/pump is for.
Bear in mind that you lose around 3 sqm

I'm hoping not to lose anything - just use the existing stairs/access. At worst it'll be a relatively easy to heat workshop. At best I can use it as a study. It's not as if I need the space - I'd just like what's there to be of more use.
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Cheers, Rob

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RJH wrote:

4.5m x 4.5m *is* just over 20m^2
if it was only 2.1m x 2.1m then it would be roughly 4.5m^2
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On Sunday, 21 January 2018 17:00:36 UTC, RJH wrote:

sounds more useful :)

,

d!

I don't think that's possible. You need to be fully clear about what your p lans say and make very sure they have the necessary insurance in place. Wha t they say means nothing on that point. It all needs to be in writing and c onfirmed by their insurer, and you need to check they do what they say they will. Slapdash arrangements can land you in a whole pile of hot water on a celler with shallow foundations.

a very shallow foundation old house on clay and you plan to drain the top l ayer of clay? I'm sure your structural person nows better than me.

With it being underground the floor will be reasonably insulated by the mas s of earth, though not to current BR. The thinnest practical floor would ma ximise height. Lightweight blocks laid on their side on tamped earth would give you a 4" insulated floor. A brushing of grout would solidify the top s urface and a coat of bitumen would make it dry. Much thinner is possible wi thout insulation. Whether any of that would meet BR & whether it needs to i s another matter.
NT
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On Sun, 21 Jan 2018 10:21:55 -0800 (PST) snipped-for-privacy@gmail.com wrote:

That had me wondering too.
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On 21/01/2018 17:00, RJH wrote:

2m is not much headroom. You ought to put a couple of layers of plasterboard on the ceiling, for fire and noise protection. The sump floor takes some height. Even as a man cave, rather than a living area, this could feel really cramped.
I would definitely get a 'proper' engineer to advise, not some cellar firm that won't be around if things go wrong. Bear in mind that the damp soil and flagstones help stabilise the footings of the walls, such as they are. You're about to change all that and change it into a dry structure that's much lighter.

The sump should be above the tanking. Otherwise you'll be attempting to drain the water table through your sump pump. :)

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On 22/01/2018 11:02, GB wrote:

Most of our ceilings are under 2m, and I'm 1.85... you get used to it.
Andy
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On Mon, 22 Jan 2018 21:51:23 +0000, Vir Campestris wrote:

The joys of 2.7m ceilings!
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On Monday, 22 January 2018 21:51:24 UTC, Vir Campestris wrote:

+1, perfectly workable. The main limitation is with lighting.
NT
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writes

Have you explored the cost of underpinning?
I had some concrete pads put under the 9" brick sub wall of a timber barn. The initial idea was to support a steel frame to take the weight off the walls (Victorian timber with brick infill)
We quickly found the wall did not continue far below floor level and took a decision to mine under the foundation and fill with concrete. Surprisingly the lime mortar and soft red brick did not tumble into the holes:-) I think 8 holes dug and concrete filled by one navvy cost about ?600 for labour only. Few years back of course.
I understand a full underpin would involve holes dug at 1m interval with the intermediate sections done once the first lot set.
As has been said, this sort of work requires professional advice backed by indemnity insurance.
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Tim Lamb

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Yes local newspapers are often full of partly collapsed buildings where over exuberant excavations for basements have taken place, so myself, I' not try it. Is it a single building you own or joined to others as the latter brings a whole world of pain if a partial collapse occurs. Brian
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On Sunday, 21 January 2018 14:19:35 UTC, RJH wrote:

e.

No-one can say. Test holes would have to be dug, ground water determined if any and the strength of the outer walls assessed.
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On 21/01/2018 14:19, RJH wrote:

I don't know much about this, but I do recall reading warnings about floatation. What was a permeable, slightly damp underground room is replaced by a sealed tank, and the water which made it damp now gradually builds up around this newly-built boat which will try to float.
There may have been special circumstances, I don't recall.
Cheers
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Clive

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This is not the kind of thing I would seek advice from Usenet over. Get a professional in. And not just a general builder - you need a cellar excavation specialist.
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Quite. This job is very common round here due to high house prices. Can be cheaper than moving to a larger house. It's a Victorian area, and all have to be underpinned.
Across the road had a very nice cellar conversion done. Including a lowered patio in the back garden. Cost over 200 grand. House is a very ordinary 3 bed Victorian terrace. Work took a year to do. By a local firm specialising in this sort of work.

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On 22/01/2018 11:01, Dave Plowman (News) wrote:

Around here, houses sell for about £1000 per square foot. Since building work costs £200 psf (okay, cellar conversions are more), there's a lot of extension work going on.

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Yes - and you also have to add estate agent's fees, stamp duty, and removal expenses to the equation.
Although you may not get back the full cost of any extension etc when finally selling.
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