I am building just that , a wooden greenhouse, lean to style.
This will be sat on a dwarf brick wall, ( just got 1200 bricks, handmade circa 1830)
The timber is not quite as large as suggested above, I am using 2"x2" sawn and treated, built with housing and halflap joints, glued and screwed.
It will be in sections, sides, front etc which will be joined together as it is built up on the wall.
The wall will be on footings, with no oversite, ( oversite is the infil concrete used as a base or floor) so it will be soil inside. Probably a line of pavers or gravel down the middle and sides left clear for planting... A grapevine probably .
For glazing I decided on acrylic, clear 3 or 4mm.
The frame is rebated out to take the glazing and will be held in with a glass beading pinned on.
Glass is heavy and of course can break more easily than other glazing materials.
The doorway will be in the middle front rather than one end and the doorway will be projected forward about 30" and have a pitched roof above it that will run into the slope of the lean to roof.
The doorway will be 50" wide because I am using a set of french doors, georgian glazed so the overall look will be more a victorian summerhouse, mainly because most greenhouses detract from the looks of a garden.
Thats another reason why I am using clear acrylic , glass I have mentioned, heavy brittle and expensive, and polycarbonate twin wall is just ugly.
It will be built against a workshop gable end and so I can get power into it easily and there is a water butt nearby so I will pipe in a water feed from that.
I only got my first greenhouse 2 seasons ago and its done great, but it is only a roofing batten timber frame with polythene over and is not big enough and will not last much longer anyway , but its been enough for me to decide to build a better one.