I have friends nearby who added a small, inexpensive greenhouse to jump-start their summer garden. The experiment went so well that they've built a larger greenhouse to house a winter garden. Temporary or permanent greenhouses, depending on your water or electrical needs, can be expensive and exotic, or simple and easy on the pocket. You may want to build a small greenhouse without any electricity supply to see if you're truly into growing.
Just for Starters
Florida Gardener has a straight-forward scheme for building a raised-bed, polyvinyl chloride (PVC) greenhouse that you cover with a UV-resistant greenhouse plastic skin to generate sufficient heat from the sun. It's probably not the best solution for a winter greenhouse in northern states.
You tack down bent steel rebar beams into wooden 2" by 12" bed frames and protect the interior with gopher wire. Then you connect the PVC tubes to firm up the structure.
Complete your installation by adding the plastic skin and seal against any air leaks, leaving only room for doors and vents.
You can find detailed instructions and materials lists for another version of a PVC "hoophouse" that raises daytime temperatures up to 10 degrees without electricity at the Westside Gardner site from Washington State.
Greenhouse Plans for Those Who Think Big
If you need more features and a larger greenhouse, you can build your own or call in a gardening contractor to help with planning and construction. Perhaps you're planning on installing electrical or misting systems. You can find pre-made kits that you can customize to your specific needs.
Before you start, you should have an idea of the right soils for your climate that match up well with the kinds of flowers, vegetables, or fruit you hope to raise. Cool climates require borders to protect your soils and seedlings from freezing. Unless you plan on raising mushrooms, be sure to avoid wood that rots and can provide an excellent host for spores.
For a permanent greenhouse, even the best quality plastic skins have but a three-year lifespan. So you might consider fiberglass, glass, acrylic, or polycarbonate coverings that come with the right glazing to prevent harmful solar rays.
Self-watering drip lines may be the best way to go if you don't plan on having a daily routine of gardening chores. Getting the most out of your greenhouse, however, requires your diligence in prepping soils, starting seeds, fertilizing, and weeding.