Posted: 11/10/2016 at 10:24
Aside from the roots issue
walnuts are quick growing after the first few years. The one I grew from seed about 20 years ago already extends 4 or 5 metres from the trunk. Trees don't look gook if you have to keep chopping at them to stop them hitting the house. It's a big tree, let it have the space to shine.
chuck the leylandii, ugly things, and put the walnut there. It will give you more pleasure.
Native trees will give more to wildlife (and you) than alien conifers