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Thanks all, yes I did think of lowering the floor but when we had our shed standing patio area installed a few years ago, I was gobsmacked about just how much soil was in the small space that we had to re-distribute around the garden borders; so much in fact we had to get a skip for some of the soil in the end. Since then we have had a side extension with the only access to the back garden now through the house, so not ideal...
I also saw some of the 'walled' ones but I couldn't figure out the door; plus I'm not the best at building works! not bad at gardening, but bricks, cement etc...not great!
I just wanted a simple greenhouse installed, nothing too over the top or requiring lots of prep work...when we had a new shed a few years ago, part of the optional extras was extra height; I'm amazed that this is not the same with greenhouses!?