Greenhouses are increasing in popularity with the trend toward home gardens and locally grown vegetables, but buying a prefabricated greenhouse can be prohibitively expensive for many of us. Kits containing all you need to build the greenhouse of your dreams abound, but the true do-it-yourselfer will eschew these in favor of a good plan, some inexpensive materials and sound advice from experts such as those at West Virginia University Extension Services or other extension offices.
A lean-to-style greenhouse attached to a south-facing house wall works well if you only need a space for starting seeds or overwintering potted plants. It is usually attached directly to the house so that the shared wall insulates against cold winds, lowering heating costs. It can be built for about half the price of similar, freestanding structures, and if placed on a wall containing an exterior door, it can communicate directly with the home's interior and function as a sunroom as well.
Freestanding greenhouses offer more growing space as well as the advantage of an open location but require more materials since they must support themselves. These can be as simple as a hoop-house--utilizing pipes and plastic (which offers ease of setup and is cheap to build) or they can be more elaborate. Many older commercial greenhouses were constructed to resemble actual houses or barns but with glass instead of wood siding. If your budget allows and you would like something more permanent, using glass and timber is a good option.
Traditional greenhouses tend to add value to property. With poured foundation walls and masonry floors that do double duty capturing sun during the day and releasing it back into the greenhouse at night, they are also more cost effective to heat. This type of greenhouse allows for a broader range of architectural styles as well, so can be matched to that of the home. It may be attached to the house--in which case it is often treated as a conservatory and made into a living as well as growing area--or it may be freestanding. This is nearly always the most expensive type of greenhouse.
For singular style with distinct advantages, the geodesic dome greenhouse is at the top of the list. By virtue of its remarkable design, it can be as large as desired without the necessity for interior support. It is among the easiest and cheapest to build and is self-leveling (as no other structure is), so makes building foundations simple. Small domes are extremely portable, and domes of all sizes withstand wind better than any other structure. The curved design also allows light to penetrate evenly as the sun follows its course across the sky, so plants benefit from the design along with the builder.
Glazing is simply glass or something having the appearance of glass, and it is the single most important component of a greenhouse. Without windows that allow your plants to receive sunlight, they will die. Scrounged materials, such as old windows or automobile windshields and shower doors made of safety glass, work well in informal or temporary greenhouses. For cheap greenhouses, use plastic film stapled or clipped to lightweight wooden or PVC pipe frames; or go a bit more expensive with the purchase of clear fiberglass panels. Acrylic or polycarbonate panels are attractive and provide superior light transmission but come with a hefty price tag to match, as does greenhouse glass. These are best for permanent greenhouses.
Free heat is best--the sun (with good insulation) is a primary source; back up with heat retaining features like solid masonry north walls, barrels of water, or under-bench compost bins. Low ceilings and double-glazing retain heat better than alternative glazing. Auxiliary heat such as wood stoves with recirculating water or air systems are good if a reliable wood source exists. Otherwise, for safety's sake, consider electricity over kerosene or propane unless the greenhouse is vented.
It is a mistake to think sun is the only consideration in placing your greenhouse--shade is equally important, especially if you live in the south.
Plants need breezes to make sturdy stems and keep them from developing fungal problems, so include adequate ventilation.
Make it as big as you can afford--nearly everyone regrets that the greenhouse is too small once he or she starts puttering around in it.