Greenhouses can be quite small, just large enough to satisfy the hobby gardener, or they can be extremely large. Greenhouses used for commercial purposes can be as large as structural engineering allows, or they can consist of several structures adjacent to each other. Such commercial structures are usually high-tech plant production facilities that include equipment such as heating, cooling, lighting, and watering. Much of this equipment may be automatically controlled by a computer.

Choosing an appropriate site for a greenhouse is a huge consideration. North of the equator, the side with the largest surface area should face south. In the southern hemisphere, it's the other way around. The idea is to enable the greenhouse to absorb as much sunlight as possible.

Some advantages of growing plants in a greenhouse include the following:

•     Plant production can be much more intensive i.e., more produce per unit area.

•     Gardeners can grow exotic plants i.e., plants that cannot grow in the local climate.

•     Large tracts of land are not necessary for mass crop production.

•     Plants are sheltered from sudden storms and other destructive influences.

Procedures for operating and maintaining a greenhouse can vary as greatly as the size of greenhouses and its surrounding climatic conditions. The heating requirement of a greenhouse also depends on the type of plants grown, and whether the production continues year round. Heating systems can be fueled by electricity, gas, oil, or wood.
There is one consideration that is of the highest importance, regardless of the size of a greenhouse's ventilation.
Ventilation serves the following four major purposes:

•     regulating the temperature

•     ensuring enough fresh air and sufficient CO 2 for photosynthesis

•     preventing pest infestation

•     encouraging pollination within the greenhouse

Safety concerns within a greenhouse arise primarily from lack of ventilation and the equipment used, especially heaters. The following precautions must be kept in mind:

• Unless proper ventilation takes place, a greenhouse can harbour a build-up of toxic chemicals and/or bacteria. In high concentrations,  these can be hazardous to human health.

• To prevent harmful gases from contacting plants, all gas, oil, and woodburning systems must be properly vented to the outside.

• Fresh air vents must be used to supply oxygen for burners in order to ensure complete combustion.

• Safety controls, such as safety pilots and a gas shutoff switch, should be used as required.

• Portable kerosene heaters should not be used because some plants are sensitive to gases formed when kerosene is burned.

Additional safety considerations hinge on environmental concerns:

• Water and fertilizer run-off must be managed to avoid the potential for groundwater contamination.

• Greenhouses produce a large amount of plant debris which must be composted or otherwise disposed of to prevent the spreading of plant pests and cross-contamination with various plant diseases.

The hobby gardener's greenhouse can be as simple as stretching polyethylene film over a wooden frame. The cost of building such a structure is almost insignificant, and its operational cost could be zero.
On the other hand, a fully equipped commercial year-round operation can have immense operating costs, not to mention the original cost of construction. Hundreds of thousands of dollars are involved. The off-setting factor is that a greenhouse can be a hugely productive operation which, unlike much agricultural land, can produce a bountiful harvest year round.