This is one of those questions that pop up over and over again. Once the hammer starts swinging, it’s a pain and the arse to take back what’s already been done. Better to place your greenhouse properly the first time than to go back and move it after you realize your Tiger Lillies aren’t growing.
For obvious reasons, you want to maximize the amount of sunlight pouring into the greenhouse. Any areas prone to shadows should be avoided. If there simply isn’t any location in your yard completely immune to shadows, opt for morning light over afternoon light. So favor the east side of any standing structures or tree groves. Never put your greenhouse to the north of shadow casting structures; this area gets the least amount of sunlight.
Remember: plants “eat” sunlight. The earlier in the day they are exposed it, the more fuel they’ll have to grow big and beautiful throughout the day.
Shading your greenhouse in the late afternoon is a valid idea if you’re worried about wilting. Any trees that lose their leaves in the fall (known as deciduous trees) are good for this because they shade in the summer but don’t impede in the winter. Birch, elm and maple are three examples.
Drainage is something else to keep in mind. You don’t want your greenhouse to flood when those April showers roll in. If there is any high ground in your yard, that’s the place to use. I’ve even digging an irrigation ditch around your greenhouse is a valid alternative.
What other things need to be considered? Water sources, heat sources, electricity, and convenience. However, access to sunlight and irrigation are the two factors you should keep in the front of your mind.