I have built a greenhouse on top of my 8ft x 8ft shed. I have limited space in my garden and the only place that got the sun all day was the shed roof.
The structure is reinforced with steel and has been up for 5 months without any damage. The build is within all regulations confirmed by the local council. I got 2 old 8ft x 6ft greenhouses for free then made 1 8ft x 8ft with a sloping roof. The shed roof was taken off and replaced with wood flooring also reinforced with steel, there is an access hole in the middle of the flooring. I have been using a step ladder to get to the first floor although I am planning to fit a loft ladder for more convenience.