Posted: 26/04/2013 at 10:09
The sliding door extension might work. Most of the weight of sliding doors is taken by the top rail (there are usually wheels in the top of the door which run along the top door rail which comes with the kits.) All the bottom door rail does is guide the door and stops it swinging outwards, so you could even screw a piece of timber to the concrete to do that.
I would build the base and greenhouse as it is first (but no glass yet), so you can see how everything works, then cut parts out to extend the door later (an angle grinder with a metal cutting disk will easily cut through both aluminium and breeze block.) One problem may be that an extended door will hit the breeze blocks when you slide it. The way around that might be to set the the breeze blocks on that side of the door frame in a different way, so they are flush with the outside, or cut part of them away with a chisel. The most important thing is that the greenhouse frame and base is supported from below by the breeze blocks all the way around where there is glass above. If you don't do that, the weight of the glass will cause the frame/base to bend and the glass will crack. If you can't see an easy way to do that on the door end, having to use a couple of steps wouldn't be the end of the world!