Bylaws on boudaries can vary from council to council, best to check with them. Putting conifers in especially Leylandii, will nearly always upset neighbours because of the stories in the media, especially if they are close by and you haven't informed them first. It may be worth canvassing their view before you do it, if you can come up with a height that won't upset, them then all well and good.
The neighbours behind us once planted conifers and because they grow at such a rate of knots they were in excess of 10ft in 3 years or so. By that stage they were unmanageable and did cause offence.