Decide on the size of your greenhouse before you start based on how many plants you want to grow. Include room for benches and walkways.
Decide on the style for the greenhouse you are starting. As delineated by the University of Alabama extension service, "Greenhouse design styles vary widely and include Quonset, tri-penta, dome, gothic arch, slant-side, A-frame, gable roof, straight-side lean-to, curved-side lean-to, and slant-side lean-to." The style you choose will often dictate the appropriate covering, which can be flexible materials like polyethylene sheeting, or rigid ones such as glass and plastic.
Select a location for your greenhouse that provides sunshine when your plants are growing. The east side of a building allows morning sun, whereas the southwest or west side has sun later in the day. The north side of a building has the least sun and is not recommended unless you are providing supplemental light using grow lights for your plants. Freestanding greenhouses should have the majority of their windows oriented southeast to northwest.
Level the surface beneath the greenhouse location using a string or leveler. Greenhouses made of glass, fiberglass, or the double-layer rigid-plastic sheet materials should have a concrete foundation according to the University of West Virginia's extension service, whereas "Quonset greenhouses with pipe frames and a plastic cover use posts driven into the ground (for support)."
Choose framing materials. Starting your greenhouse with a manufactured kit provides the materials needed including frames and covers. If building a greenhouse from scratch, select either pressure-treated lumber for wood frames or PCV for Quonset hut styles. Be sure the frame is strong enough to withstand weather and support any objects that will be hung, such as flower baskets or grow lights.
Install flooring that provides good drainage if you are not putting a concrete foundation under the greenhouse. Several inches of pea-gravel work well and help keep down volunteer weeds.
Select a system for temperature control for your greenhouse. Heating is needed in cold weather, cooling in summer heat, and venting to provide air circulation and replace carbon dioxide. Because temperatures can fluctuate a great deal throughout the day, automated temperature control systems should be installed unless you have time to monitor your greenhouse several times a day.
Install access to water for your greenhouse plants. Handheld hoses are fine for small greenhouses. Plants in larger, self-standing greenhouses are easier to water with automatic systems.