Mine was purchased through Charley's Greenhouse in Washington state. The greenhouse itself was manufactured in Canada and is connected to my house but without a pass-through door from my house. Because of where I live, it had to be engineered to withstand 130mph winds (95 is the highest in 8 years though
). San Francisco has some pretty impressive windstorms too.
The biggest problem with connecting a greenhouse to your house is the permit process. An attached greenhouse becomes a room on your house and the permits are outrageously expensive with all the "rules" and inspections that come with house building. If I had built a free-standing greenhouse, it wouldn't have needed a permit at all.
In California, I did have a free-standing greenhouse (from Charley's). It wasn't commercial size (12 x 16) so didn't need any permits at all, of course I lived in a rural area. I imagine if I had built one as big as the one you are planning or if I had been in the city, the rules would have changed.
Probably the most important consideration is the floor and drainage. Greenhouses are wet messy places. My last greenhouse had a paver floor over sand and drained down into the soil below but, it didn't have a way to get rid of debris over sills. Lots of sweeping and dust pans. My new greenhouse has a concrete floor with drains but, the drainage is into a French drain so I constantly battle trying to keep junk from going into the drain and clogging it. It would have been a much better plan to have a concrete floor slightly higher than grade with drains at floor level so the water and debris could be washed outside through the drain holes.
PS: If you do need a permit, don't add the the chair until after the City/County comes to see how much they can raise your property taxes (No they don't make an appointment but will know when you are done from the inspector). You don't want to suddenly discover you built a greenhouse but are being taxed for a patio room.
