My greenhouse was built on top of the garage (same dimension as a standard garage) in 2001 by Florian greenhouse, we have the so-called Geneva construct.
https://www.floriangreenhouse....
It is holding up great after now past 20 years built with dual glass panels.
I would like to address the previously mentioned heating bill. The greenhouse is attached to the house, which is heated with oil, and we have 3 ducts coming from the basement into the garage and up into the greenhouse. Heating the greenhouse is a breeze, my heating bill did not go up much, actually this well insulated glass-box acts as a heat sink and on sunny Winter days the oil heat really only starts kicking in around 5 ish pm /at sun down. It heats the house!
My recommendation would always be to connect the greenhouse to the house and do it properly.
