The most important consideration is sun exposure. Ideally, your greenhouse should receive at least 6 hours of full sun per day for optimal growing conditions. Remember that the sun is lower in the sky in winter, so if you plan to grow plants in the colder seasons, don’t place your greenhouse where it will be shaded by trees or a tall fence all winter.
The site must be level and well drained. In addition, there should be at least 2′ of clearance between the site and any structures (such as fences or buildings) to allow adequate access when installing the greenhouse.
Proximity to the water supply is helpful. If you won’t have water piped into the greenhouse, remember that you will have to carry watering cans or run a hose to the house.
Availability of power is also a consideration if you want lighting, a fan or an electric heater.
Try to place the door in a direction away from the prevailing wind; if the wind typically blows from the east, have the door facing west to minimize heat loss.
If you are near the ocean, the greenhouse is best situated on the leeward side of the house, out of prevailing winds
If your roof line points east-west, then the longer side walls will face south (and north), thus giving more sun exposure, a good thing if you want to maximize warmth in winter.
However, if the roof line points north-south, then the sun will hit the longer side walls from both east and west all summer, giving the maximum daily length of exposure in that season. Decide which season is most important to you.