Building next to a riverbank

[foxechick]

Hi, I live in a converted school around 10 m from a riverbank and want to build a single storey extension on the land between my house and the riverbank. Before I get the drawings done for planning, the architect advised my builder dig a hole to establish what we would be building on i.e. clay , rock etc. Having dug 3ft down all I can see is infill for the 1st 3ft, below that it appears to be soil. How far down would I have to go to hit clay or similar and is their a possibility there won't be clay beneath the soil? Also if I get down to 6ft and it proves to be riverbed material what kind of foundation would I need to put down?
Building Control at the council have told me to go ahead with the planning application, but I'm worried that if it is passed, then it may cost more doing the foundation than it will building the damn extension!! Can anyone point me in the right direction as I seem to be agoing round in circles!
Thanks
x

[Merlin]

hi,

A possible solution to your foundations is to use Helical screwpiles.

Regards

merlin

[welsh brickie]

check the depth of foundation on your house.
You normally dig down 1metre but that depends on ground conditions
get the building inspector in check for you, he is the best person to advise as he will have to pass the extension anyway