Steve law

Thanks @Alan Ambrose that is a very helpful response. It seems to be a very difficult as if you go to architect they say you need to talk to a structural engineer but it's very hard knowing what you want to achieve in the build ie more head height in loft/first floor without blowing the build out of the realms of possible on any budget. Because when you start talking a 1.5 meter basement that's not a 1.5 meter dig that's more like 2.5 what would require so much calculation tests and technical drawings before you even break ground. It also doesn't help I have a unusual shaped plot which is long and skinny with a drive one side but only a meter the other side from boundary fence. I can easily get in to garden and use 8 ton digger and have field to dispose of waste soil. But the architect didn't really advise me very well and now I'm just trying to go down as far as I can without it getting stupid expensive. You can see by the pic the thick mortar level is the floor level and there is about 400mmdrop over the 12 meter extension so you can see that I don't want to go just 400 below ground level at start because I will be at ground level by the end of the twelve meters and achieve zero drop so nothing like the design picture

Hi I have some building experience but not a ground worker or bricklayer by any stretch of the imagination. My question is as you can see from the design, the extension is lower floor level than the original ground level to achieve more head height in the loft. I foolishly thought I could go down 1.5 meters with floor level but after consulting some people found that to have a finished floor level of 1.5 would prob need to dig down 2.5 meters causing massive costs of sheetpiling extensive soil testing and samples. Without just going ground level and putting up with very little head height in loft level is there a happy medium of depth and method of detail that I can dig footings build some kind of foundation blocks then use a liner and or tanking to go down a reasonable depth but without the need of sheet piles. The water table in this area is around 8 meters deep and and a inspection hole of 3 meters has been dug and seems to be permible clay that drains pretty well.