Ok, so you're ready to extend your growing season!  There are a few important criteria to meet when considering a new greenhouse. 

Sun-  As a general rule, the more the merrier.  If you want to grow in the spring, fall, or winter it is especially important that your greenhouse takes into account the trajectory of the sun during those seasons.  Shade cloth can always be used to dull the brightness of the sun when needed for certain plants.

Drainage- Without proper absorption and drainage through the soil in the greenhouse environment, you are doing more harm than good.  Pathogens and disease tend to arise when humidity is out of control, especially in a hot enclosed environment.  Flow is key to a healthy space, standing water invites problems. 

Wind Buffer-  Only necessary in areas prone to high-wind.  Greenhouses covered with thin plastic are vulnerable to extreme weather.  It helps to be aware of protected, more mellow sites on your land that serve as a nest and keep your greenhouse growing for many years!  Buffers can be constructed using fencing if necessary.  Balance is important- Air circulation is critical, just not so much that the structure is being compromised.  

Level land- Snow drifts can become an issue when greenhouses are built on a sloped grade.  

Clearance from structures and trees-  Falling branches can easily damage your poly greenhouse film covering, and while it might not be noticeable in the summer, trees might block the sun during the remaining seasons.  Structures like barns, garages and homes are also likely to cast a shadow over your greenhouse if you are not careful!