I have a single storey extension that was built before I purchased my house and is pretty old (at least 20 years).
I would like to build on top, but am pretty certain that the existing foundations won't be up to the job (naturally I am intending to dig some inspection holes to check).
However, I was wondering, assuming the foundations need strengthening, is underpinning the only option? If so, what size trench needs to be dug (it would have to be by hand as there is insufficent space for a mini-digger - the boundary fence is less that a metre away from the extension wall at its closest point)? Could I save a significant amount of the cost if I dug the trench myself?