While building your own greenhouse is a significant amount of work, it isn't beyond the skills of many DIY'ers. Once you've selected a suitable space and the dimensions of the greenhouse, you have to decide what to built it out of. There are several choices, depending on your budget, your needs and your skill-set.
Strong greenhouses are built using galvanized steel, aluminum or wood framing. Wood is probably the most familiar, and constructing a greenhouse from standard lumber is much like building any other structure. Of course, the wood will be subjected to high humidity on the interior, and constant exposure on the outside, so use pressure treated lumber and paint or seal it properly.
Aluminum can be fitted together similar to wood and is lighter and more durable, but it is more expensive. Steel is the strongest, most durable option, but it is also the most expensive and requires knowledge of welding and other metal working skills.
A less expensive alternative, plastic (PVC) pipes often can't provide the structural strength to stand up against wind and snow loads.
There are a wide range of possible covering materials including glass, rigid fiberglass (plexiglass), double wall plastics or plastic film.
Glass is the traditional greenhouse covering and it has the advantages of looking good and being easy to maintain. However, it does break easily and initially it's relatively expensive to install (although it may be possible to lower costs by using recycled glass--from storm doors for example). Since glass is heavy it requires a strong frame to hold it in place.
Fiberglass has the advantages of being strong and light-weight plus either clear and translucent fiberglass initially allow the same amount of light penetration as glass. However, over time (10 to 15 years) the surface layer often degrades, allowing dirt to build up that blocks light penetration.
Rigid, double layer acrylic plastic or polycarbonate sheets make a great greenhouse covering. The double layer plastic sheets provide good light penetrations plus better insulation than glass or fiberglass, so they can help minimize heating costs in cold weather climates.
Thin plastic sheets are available in a number of different grades (thicknesses) and they're light and inexpensive to install. Light penetration of all grades is similar to glass, however, no plastic sheets will last as long as the other options. For example, PVC film has a life expectancy of about five years.
Greenhouses need air circulation, ventilation, humidity control and heaters. Options for heating include propane, natural gas, oil and electricity. Your choice should depend on what is most readily available in your area and which is the most cost effective in your situation. (Electricity is a popular choice since it doesn't require venting but keep in mind in mind while electricity is efficient a power outage will mean your plants will be left in the cold).
Vents near the top of your greenhouse and fans to circulate the inside air are a requirement--both for cooling in the summer and for heat distribution in the winter. Since temperatures inside the greenhouse can be extreme in the middle of summer and winter, it's necessary to keep the air circulating so temperatures remain relatively stable, rather than having the cold air near the bottom and the hot air up near the top.
Before starting to build your new greenhouse, check with your local authorities to find out if you need a building permit. Also, ask your local utilities to mark the location of underground pipes or conduits before you do any digging on your foundation.
Finally, don't make this mistake - be sure the door to your greenhouse is wide enough to allow your wheelbarrow to pass through.