Essential shed features are as follows: (1) floor (bare earth is a no- no, so this means laying paving stones or concrete); (2) door (make sure this is wide enough for mower/wheelbarrow etc or prepare to feel very foolish and endure endless maddening repetitions of "Why don't you keep the mower/wheelbarrow etc in the shed, then?"); (3) windows (or you won't be able to see whilst pottering in the shed without a torch, which is silly. And if it is actually a potting shed, the plants won't grow unless it's light enough); (4) roof (tongue-and-groove boards are more weatherproof than plywood but leaks are likely at some point).
Even The Garden DIY Expert (Transworld pounds 5.99), normally sanguine about all kinds of scary tasks, baulks slightly at erecting one's own shed and recommends forking out to let the supplier do it for you. "Instructions can be a little puzzling and the men they send have done it before," it observes. "If you do it yourself then you will certainly need an able- bodied helper," it further adds, ominously. To able-bodied add even-tempered, steel-muscled and with GSOH, probably.
For greenhouses, take all the above and multiply. The nicest and most attractive greenhouses are in any case those that have been in situ for many years and have blended into the garden happily; these are likely to be made of brick and wood or cast iron rather than plastic and acrylic and are, says The Garden DIY Expert sternly, "Quite impractical these days," which is a shame.
And remember to check up about planning permission. Sometimes surprisingly minor buildings can require it. If putting up a shed or greenhouse (or indeed gazebo or playhouse or treehouse) is a touchy, tricky business, taking it down again, particularly involuntarily, is even worse.