It depends on what you like. Our flat garden has a fence alongside a little culvert with a large mature willow tree right the other side of it. I think it is absolutely lovely having the willow stems descend into the garden and so do most neighbours but there is someone who thinks it looks messy! We've never had any particular problems in terms of its roots - it is right by a large shed - or it obscuring light, though over a longish time roots could be a problem if right close enough/ on a clay soil (assume you are) - there are a lot of variables regarding that; 20 foot shouldn't be a problem I'd say.