Cost of underpinning foundations for extension

[ColloquialCat23]

We have been considering either an extension on our house for an extra bedroom on top of a single story extension at the side of the house. Its an older house and ive finally deciphered the plans which say the foundations for the single dtory we would want to build on are only 600mm deep. I believe they need to be at least 1000mm for a 2 story build?

How much effort / cost would it be to rectify this (quick google has said about under pinning the foundations) or would it be easier / cheaper to nock down and start again.

I sometime think a solid 3rd option is to just move house to one with another bedroom, happily take advice on this too!!

@ColloquialCat23 You really would need a structural engineer to advise on this. You are correct that the weight of the whole extension will be transferred into the ground via the foundations. However various factors determine how deep these should be. Our single storey extension - 6ft. Clay and a tree nearby. So I’d take expert advice, but 600 is probably not good enough and the existing construction might not be capable of taking the load either. Therefore it might be that starting again gives you a much better build. When was the existing extension built? Is it double skin modern construction? Underpinning costs a lot of money and probably not the best solution.

The plans I found were 1985 😅 so quite a bit away from modern standards!!

Probably the best and most cost effective option here is to knock down the single storey and the build a brand new double storey extension. The cavity size for a modern build will need to be much wider than the existing single storey which causes added complexity and cost to adding the second storey. It is often easier and cheaper to knock down and start again unfortunately.