trees on clay

[dexterdo]

Right..

Ive had a quick look into the basic geology of the area, and yes there is a small band of Gault Clays running between the Upper Greensands and Chalks in Guildford.. as well as possible london clay and weald clay regions..

I dont think you have answered the question on how you know the soil is Gault Clay.. would be nice to know where you base this info..

Have you dug any trial holes? You could be on chalk.. so dig a 1m deep hole where you plan to put the foundation..
If soil conditions are unknown then they are assumed to be high PI and thus need the deepest depth footings the nhbc tables provide.. and as stated by others these guildlines tend to get worse and worse every year.. their latest change has increased the void depth requirements for suspended floors..

I think you have probably progressed it as far as a forum discussion can go so have to pick a path to follow:
1/ Get the footings properly designed by an SE which may require some soils testing and trial pits (unless they have done similar local projects).. and let them deal with BC on getting things approved..
2/ Propose the tree removal and shallower footings based on high plasticity clay and get written acceptance before trees are removed.. footings deeper than 1.5m will need some heave protection..

Thank you to everyone who has contributed.