In message , Charlie
Pridham writes
If you are on clay worry, but otherwise if not showing signs of cracks
now and you have them down you should be fine. Even on clay the sooner
they are removed the less likely you are to get "heave" after.

Our neighbours have about 10 very tall ones along their side of our
nextdoor boundary. We've recently put in a single storey extension
which takes it very close to the fence. We decided to get a structural
engineer in to advise. We are on very heavy clay - so as well as
putting in *very* deep footings, we had to put special boards in to
absorb swelling and shrinkage - and a block and beam floor.

We are probably going to have to pay half the amount to get rid of the
Leylandii as the neighbours are unlikely to do it otherwise. However,
the engineer advised cutting the tops off one year to allow a gradual
change in the water saturation - then the following year take the rest.
Don't know if that has any relevance to the OP's situation.

The only benefit from the trees seems to be that a bunch of starlings
seem to be nesting there. They're not even any good for the woodburning
stove

--
regards andyw