Select a site for your greenhouse that gets at least 3 hours winter sunlight daily…more, if possible. If you have several suitable locations here are some suggestions to help you select the best:
An eastern or southern location is best; a western exposure usually provides adequate growing light, but requires more shade and cooling in summer. A northern exposure is suitable for philodendron, snake plants ferns and other foliage plants, African Violets and orchids.
A free-standing greenhouse can be installed anywhere that's convenient to provide the most attractive appearance in your garden.
A lean-to or attached even span greenhouse requires more care in selecting an appropriate site. Plant growth takes place most rapidly in the morning, and the ideal location should let the greenhouse face morning sun in winter.
In a greenhouse with roof sash, it isn't necessary to orient the ridge-line in any particular direction. However, if you purchase automatic roof vent equipment, install it to operate the sash on the side away from the prevailing winter wind.
In a fan-ventilated greenhouse, it's best to face the structures with the ridge parallel to prevailing summer wind; install the fan in the downwind end.
When planning an attached greenhouse, make sure it doesn't interfere with anything on the face of the building to which it will be attached: doors, windows, a chimney, or the roof line of a one-story building. Cut a stick to the greenhouse ridge-height, and holed it up against the building where the greenhouse will be installed; this way you can determine if any obstructions exist, and how they can be overcome.
If the ridge is too high, you can lower the greenhouse foundation and excavate inside for headroom. Or you can increase the height of the greenhouse foundation, to raise the ridge over existing doors or windows. If a chimney is in the way, the depth of a lean-to can be extended with a “deck” of locally purchased lumber, to build around the obstruction.
If a really tricky installation seems likely, it's best to draw the outline of the greenhouse on the building face with chalk; or construct a framework outline of the greenhouse using 1″ X 2″ lumber to hold against the building to determine that exact place the greenhouse can be located.
If a workroom or potting shed is to be attached to the greenhouse, it should be located at the north or west end to provide protection for the greenhouse from cold winter winds.
Most municipalities require a Building Permit to install a greenhouse - be sure to check your local building department, and observe local building codes to avoid unnecessary difficulties.