In article ,
Frank Watson wrote:

I'm still hoping someone with experience of the following will
offer some info into how much of a threat the hedge is to my
house's foundations - especially if I cut it down, and the roots
rot away. Does anyone know how far Leylandii roots spread,
radially, outwards from the trunk?

Not for a hedge, but we cut down three Leylandii type
conifers in the front last year. I dug down alongside
the stump of one of them, the one close to the road.
About 12" down there was a massive horizontal root,
about 5" diameter, heading straight under the road,
(and towards all underground services in the road).

--
Tony Williams.