Hedges: thuja occidentalis smaragd soil width requirement

Hello everyone. I must first admit that I'm searching the web for an answer to a particular question and have thus stumbled upon this community, which seems lovely I must say. :)

Anyway, my problem is this. I have 15 year old (or so) thuja occidentalis which have developed nicely around my whole yard, maybe around 2.5m tall. The yard is on a 10% incline. I have now decided to flatten a part of the side yard which leaves the bottom of some of the thujas as high as 1 meter above my desired ground level. The question is-how much can I approach them from the side without ruining them.

By the way, the neighbour on the other (left) side of the yard did pretty much the same thing and came very close to them, 20 cm or so, but they have the whole of my (left)side yard to play with, so I guess that saved them. I can't count on my other neighbour(right) not to have the same idea someday, and he even suggested we take the thujas out and flatten both yards but I don't want to start over with the thujas, I'd rather save them and have a small wall on the side of the yard.

Thanks for any help, I hope it's not too confusing. :)

 

Here's how it looks at the moment, they are around 90 cm from the edge and that sidewall will be made of stone and of course perpendicular to the ground. I'd like to build the wall around 40 cm from them, 50 at worst. Hope it's a bit clearer now.

https://imagizer.imageshack.us/v2/1200x719q90/r/539/gZ6efz.jpg