I couldn't tick one or the other of the boxes – I think the choice has to be made contingent on so many variables. In fact we have painted timber, double-glazed bifolds (with a fabulous retractable pleated fly-screen) at the end of the dining room leading out onto a terrace and a painted timber, double-glazed slider (with a timber-framed sliding fly-screen) on the other side of the kitchen (but on the same axis) opening onto a raised timber deck. Each works really well in terms of function and aesthetics but they would not be interchangeable as the different locations determined which was more suitable – but they have been made by the same highly skilled window craftsman and have given us nothing but pleasure. And on a hot, steamy evening with both open we have the most wonderful cross ventilation.

our cavity sliders don't have a track on the floor although the wall needs to be reinforced if you want to mount a towel rail on a wall with a cavity slider inside.. in 35 years we had to replace the fittings on one door (out of five cavity sliders) but you need to make sure that the fittings are good quality but the door will probably be open most of the time and a simple floor mounted door stop installed close to the wall will stop the door hitting anything without being a trip hazzard

I would go with the same style railing as the front of the house. Place the fence across (with a gate in it) after the second pillar, between the exterior light and the window if you can. You might need to make a larger "platform" with perhaps decking, to accommodate fence swing. If you use decking it might look nicer if you drop a step from the tiles. Possibly could extend your tiles along a bit and have gate sit on the tiles. Your existing wooden steps could then be put in or, do a couple of steps again, more of a platform, then more steps so you are spreading the distance down towards your back yard. If you hinge the gate behind the pillar that may look nicer and swing away from house towards the side fence and not to get in the way of the window (if window is an issue possibly a sliding gate?).

Hi Dr Retro, Thank you. I really appreciate the input and the time you've taken to explanation your point of view. I really do want to get an understanding of where the issues lie in this design. So I hope you'll forgive me digging a little deeper. I don't quite follow why you think the design would require a 6m lintel. Yes, it does require a 6m track, but the track can simply be attached to the wall. The weight of the door need not be supported by the top track. It could be supported from below. The whole point of putting the configuration on the outside of the wall is to minimize the need to make structural changes to the building. There currently is no capacity to install a 6m wide external sliding door/window unit. Likewise I don't follow why the door should be any draftier than any other external sliding doors. The cavity is on the outside of the wall, so draft is not going to get in via the cavity. Other sliding doors of the unit type have the same draft problems this configuration would and solve it by using an overlapping flange at the front end of the fixed door and the back end of the slider. The same could be applied here, where we treat the wall as the fixed door.The point at which the doors engage each other has also be solved for other door configurations. It is just a matter of having a loose tongue and groove configuration. Again this is exactly the same as a unit providing center opening sliding doors. Finally the same sliding mechanisms are used in other sliding door configurations, so I don't see why that should be any draftier here either.