Any one installing a flue outlet onto a neighbours property , or up to the boundary of that property should take due consideration that the neighbour may be entitled to build up to that boundary which can or may have an effect on the flue , basically the responsibility is with the installer & or owner of the flue !! A duty of care would be applied if any structure caused a dangerous situaation to occur with say an open flue boiler , the neighbour would have to be informed !! the end result is that the boiler would have to be moved , & or the flue altered same would apply to a rs appliance !!!