I want to build a two storey extension over my driveway. The neighbor has a leylandii hedge approx 7 foot tall along the length of the driveway boundry . The hedge is 3m ( the width of the driveway) from my house but only 40cm from their single storey kitchen extension (at the rear of the house) and 90cm from their house . The trees are only about 40cm apart and have been in situ for approx 20yrs ( put in by the previous owner of the house) so it would be a bugger to try to pull it out . Would it be enough to just kill them off and cut them to ground level ? We had double gates put across the driveway and when the guys put the gate posts in, they needed to put a post a foot longer in on the hedge side. So there would appear to have been a degree of damage done to the ground level already with implications for the laying of foundations for an extension. To complicate matters further they have decking at the back right up against the leylandii .