My question is how to join our new loft extension's wall structurally to already built extension at neighbours' in terraced property? There are no brick parapet walls, just roof itself and chimney in the middle. Our neighbour's extension has slate finish on side. Shall I leave neighbour's slate tiles and construct timber frame wall with insulation, plasterboard and plywood on external side with no slates, obviously I will not have any access to install slates there... and then secure roof by applying watertight material under felt such as mastic, bonding materials etc. or maybe I should have a brick wall with insulation directly on a party wall next to slated neighbour's wall and timber frame wall at rear of my house and timber one on another side of extension? My neighbour has a felt roof and I would like it to be also made of felt. I want loft extension to be directly next to neighbour's side wall to have a continuous line. I will appreciate your help.