I'm planning to remove a 35m-long double-planted row of very
well-established leylandii (approx 30 years old) from my front boundary
which is 4m from the front of the detached house (purchased last year),
part of which was built 100 years ago with a sizeable extension added
15 years ago.

This dense hedge was 10m tall until I cut the top off at the end of
last year in preparation for the coup de grace. It is now 4m tall. Some
of the trunks are 300mm diameter and I'm concerned about their root
systems. How far are they likely to have spread in poor-draining, clay
soil?

And what, if any, could be the risk to the foundations of the house?
Removing so many at once will presumably have a significant impact upon
the soil structure and drainage. There used to be a drainage ditch
along the front of them but this was filled in some years ago and
surface water collects rather easily in heavy rain. Without the trees
to consume a lot of it, I'm worried about drainage. Should I put in a
soakaway? TIA.

Simon