A lot of greenhouse manufacturers will tell you that you will not need a permit for anything fewer than 120 square feet. I know for demande de permis de construire a fact that this is not true in our state capital. We are required to get a permit for anything over 100 square feet. So, the best thing to do is check with the permit department in your community first in order to make sure you will be able to put the greenhouse up once you have purchased it. The last thing you want to do is to return a greenhouse, especially a custom one. Some manufacturers may even have a policy that custom greenhouses are not returnable. You may also find yourself faced with restocking fees and return shipping costs.
If you are in a historical district or in a gated subdivision with covenants you should check with the appropriate authorities. Usually your homeowners association will have final say over what outbuildings are permitted. I have had to supply drawings and specifications for this. I have even had a customer with a greenhouse in a historical district that had to have a custom paint color to match their home.
Some permitting departments will not have many requirements since this is an outdoor structure and is not a residence. Some will require shop drawings, some may even request drawings stamped by a state certified engineer. Not all greenhouse companies will have this available to you, so be sure to ask when you are getting serious about a greenhouse. Some of the things they will have to certify are the snow load and or the wind load. Almost all coastal regions from Texas to the East Coast will have wind requirements due to hurricanes. And in the North you are more likely to run into snow load requirements.
Whether a greenhouse is freestanding or an attached lean-to will be important. Most permit departments will be much more stringent on attached structures than on freestanding ones. I have run into permit departments that have asked the homeowner to build a foundation similar to the homes foundation for lean-tos.
This is just an outline for some questions you may run into when approaching the appropriate authorities regarding a greenhouse. The best rule of thumb is to always check with them first, before ever making a purchase. That way you will avoid any problems you may encounter when you are trying to install your new greenhouse.