If the greenhouse is going to be in a corner, the wind can only get at it from 2 sides and if it's in a town garden, there will most likely be other houses to break up the wind from the exposed sides. If the wind does get under and start to lift it, which will need to be pretty strong wind, it should just bobble about a bit rather than take off and land in the next street. Pegging the frame at the base may also be worth doing. Just keep the door shut if a high wind is forecast.

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