Janet Baraclough wrote:
The message . com
from "perrancott" contains these words:

I have a weeping willow in my half acre garden. I believe that it is
at least 12 years old. It is about 6m high

Just a baby, then.

A big mature tree in full summer leaf sucks up hundreds of gallons of
water a day, drying out the soil. IOW, when the tree grows bigger, and
especially in a warming climate, the boggy spot will dry in summer, and
those roots will spread , looking for water and nutrients elsewhere.
When your tree reaches 18 m high, you can ( very roughly) expect the
roots to travel the same distance from the trunk as its height. If
there's water uphill ( perhaps running off a house roof or patio or
driveway ), that's where they will go.

and growing in a boggy part
of the garden. An arborist from the local agricultural college told me
that its roots are not a problem because it is lower down the hill from
the building and 18m away.

Maybe, not a problem to the foundations of the house at the moment. I
wouldn't count on it staying that way, with climate warming. Buildings
insurers are getting very particular about distance from building
foundations to nearest tree.

Did he ask to see a drainplan, is it also 18 m away from all your
drains, sewer pipes or septic tank, soakaways, and those of all your
neighbours?

Janet.

If you read my post I was asking about cutting it down so most of your
response, while interesting, is irrelevant.
Are you a qualified arborist?