It'll be out the back of my shed, one wall will be the back of my shed, which is made of those big 30cm long grey concrete bricks. The rest will probably be a 4x2 timber frame, unless I find a better/cheaper method. The side opposite the shed is a sudden drop into gorse over a motorway, so it'll probably get a bit of wind too, not much shelter there. The long side is pointing west, with the door pointing north, which isn't ideal for collecting winter sun, but it'll be a lot better than no greenhouse. I'm having a tree cut down so more light gets into it too.
The brick wall will be painted part black to absorb the heat of the day, which will then release it back over the night to keep the place warm. I may place some other thermal mass in there too, black painted water tanks, if there's space. On the flipside painting the wall white would reflect light back to help the plants grow. I may go with grey, or alternate black and white, not sure yet.
As the sun's high in the sky in summer when you don't really want full sun I'll probably have darker material in the roof, or some way of blocking the light in summer. The walls will be clear so in the winter as much light gets in as possible. Because light goes through best when it's perpendicular to the glass etc I'll probably angle things so in summer much of the light bounces off, but in winter most goes in... in theory.
I'm mostly interested in materials for the roof and walls. It needs to be clear and double skinned so it insulates well in winter, I need to seal it so there are no air leaks too. Can anyone recommend a suitable locally available material? The two google has found for me are
http://www.psp.co.nz/Building-Brands/Roofing-Systems/Plexiglas-Acrylic-Multiwall (or one of their other products)
Alternately I could use some kind of standard plastic, two sheets with an air gap, though I doubt "home made" double glazing would be hugely effective.
Relatively cheap is important, I'm not spending a fortune to do this, but there's no point saving $500 if it reduces the effectiveness of the structure.
I wonder if I need any planning permission for this type of thing - does anyone know? It's in a position quite visible from a motorway off-ramp.
Any other thoughts or suggestions are most welcome.