They require a permit for a shed over 10x12. Many people put two sheds side by side to get around it. For a greenhouse, having two sections could be a bonus down the road. No engineering required.
The greenhouse in your pic is a jewell. Convective cooling, passive solar and 1/3 in ground. Looks like the UK. Year round isn't in the plan, but maybe down the road two grows is. Initially light dep will be on the list to finish earlier.
yes, used the twin wall polycarbonate sheets on the end walls of a greenhouse i built this year in MA... plastic poly over the top that is inflated with a blower fan.
the twin wall polycarb sheets are great for the endwalls sturdy and gives added protection... only reason i wouldn't cover whole house in them is the price, the plastic is way cheaper..
I second what the nino says. Twin wall is great for end walls. It's good as a primary glazing also but doesn't work on curves. Standard twin wall is about a 1.6 r value. Double layer poly is a 1.6 also at a fraction of the price and MUCH MUCH easier to install.
It's not that the twin wall polycarbonate is expensive on its own but the hardware to mount it its much more complicated and expensive then you think. H channels to fasten them, peak connectors to join at the joins. It needs to be foil taped to stop moisture from getting in. It doesn't work on steel hoop shapes. You will have to build out of pressure treated lumber or steel that has angles instead of curves. Not as easy and cost effective as it seems.