Instead, the builders and the project manager have been investigating the foundations via some holes they've dug around the edge. We had suspected that the foundation under the extension would be poor, but it's unfortunately quite decent: normal strip foundations, similar to the sort we had hoped to get away with on the replacement -- bigger, heavier, two storey -- extension. It looks like that's not going to work.
We think the original house has raft foundations: a solid, thick slab of concrete going all the way down from ground level, and this is bearing up perfectly. Putting something like this under the extension will likely add in the region of 15k to the project cost. Another option, ground conditions allowing, would be to stick with the strip foundations and lighten the weight of the extension: making the upstairs walls thinner and downgrading the concrete floor we proposed to soundproof the bedroom from the kitchen.
A worse case scenario would be discovering that even a raft won't do and having to drive piles in to hold the new section up. That will be more like 60k on top.
However, it's all still speculation and investigative work will be continuing. We'll discuss all the options with the engineer next week and make a decision on how to proceed. Until then, what goes under the ground is all up in the air.
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