Hello again!
Thanks for explaining that : ) Im not sure of any programs available that adapt the engineering of a spacific greenhouse design, to recalibrate for new dimensions; however, I'm no expert in that feild. If they do have them, they would be the expense toys of an engineering firm. Have you found a design you have in mind? Do you know the building materials you want to work with? Once you have decided on a spacific design, you could go a little smaller, if you dont mind being over engineered. Its going bigger that would require a engineers approval to make sure minimum structual requirements were met. Depending on the greenhouse style, you could probably do the conversion yourself with information found online, especially if you have building
experience. If I'm following your post correctly, your space is 21 ft by 21 ft. You could without issue get a 20 ft by 20 ft hoop house, on top of a foundation of sorts. Going 21x21 ft is probably even do able without much issue for converting plans yourself, but the materials available would make it more efficient and cost effective to stay at least 20 ft wide, by 21 ft long, or at least using the 20 wide standard length pvc to create a 21ft wide hoop house, since you have no snow load, and extra head room from digging down. The
underground opening dimensions would of course be determined by the materials used to retain the surrounding dirt. If you want a tall fancy structure. You will need to have someone in the greenhouse feild, qualified to draw up plans with the materials, you want to use. If you want to start off with a hoop house, it would be very easy to use the same materials already in common use for your dimensions, which means you can find all that information on line. If you decide on a fancy more permanent structure out of say aluminium and polycarbonate sheeting. Well some greenhouse manufactures will custom engineer it for you, then deliver the product for assembly. But knowing the building materials you want to use, and or the style of greenhouse build, is key to pointing you in the right direction.